research paper on paint industry

• Member Center
• All Pages News CoatingsTech Events Downloads and PDFs CT Archives Search for: -->

Perspectives on Paint and Coatings Research from Independent Laboratories

• Topic: CoatingsTech Industry Q&A , Equipment & Testing

By Leo J. Procopio, Paintology Coatings Research LLC

In the paint and coatings industry, a variety of external resources are available to assist an organization when it finds itself missing the necessary equipment, expertise, or time to address needs and opportunities.

Whether it is developing a new technology, validating performance of a new product, or investigating how to participate in a new market segment, even the largest of organizations have the occasional need for outside support. The external resources available include business and technical consultants, toll and private label manufacturers, academic and government institutions, and independent laboratories.

Third-party, independent commercial laboratories are like other vendors in that they provide a service or goods for payment but are unique because their service is supplementing the client’s own research and development activities and capabilities through experimental work.

Independent labs in the paint and coatings market may choose to offer a variety of services, which could include R&D targeted at the development of new raw materials or coatings, formulation optimization, performance testing, analytical testing, reverse engineering, failure analysis, and accelerated and natural weathering.

Not having the right knowledge and expertise in-house is sometimes a reason for seeking out an independent laboratory. The choice of a suitable laboratory as partner can afford an organization with that expertise, without the need to hire an additional employee. New equipment can be very expensive and might require personnel with special expertise to operate, and thus might not make sense to purchase and bring in-house, especially if it would be used infrequently. Here again, choosing the appropriate lab can allow access to certain equipment and testing without the capital expense.

Sometimes, an organization may have maxed out on their own resources, whether it be personnel to carry out formulating and testing, or space in a popular piece of equipment. An independent lab can assist in expanding a company’s R&D resources.

There are many benefits to seeking the assistance of a third-party, independent laboratory, and they fill an important function in our industry. CoatingsTech reached out to several laboratories to hear their perspectives on their unique role in the paint and coatings industry, why clients seek them out, and how they assist industry members in their research and development efforts. In addition, they dispel some common misconceptions about third-party labs and discuss tips for ensuring a successful partnership with an independent laboratory.

Participants in the discussion included: James E. Swope, chief commercial officer at The ChemQuest Group; Matthew McGreer, director at Atlas Weathering Services Group; Dan Marschall, president of Marschall Labs; Michael Crewdson, technical director of testing services at Q-Lab Test Services; Paul Lewis, technical manager at Univar Solutions; Ronald Obie, president of Wood Coatings Research Group; Robert Leggat, consulting and laboratory services manager at KTA Tator; Gerald Vandezande, president of 1st Source Research; Edye Fox Abrams, vice president of business development at The ChemQuest Group; and Mike Dempsey, marketing and business development at 1st Source Research.

Independent Laboratories Q&A

Answers may have been edited for clarity and length.

Q. What is the role of independent laboratories in the R&D process?

Swope, The ChemQuest Group: Multiple roles are possible. These include helping a customer in accelerating timelines to hit an aggressive deliverable and breaking a creative logjam similar to writer’s block. Other roles include characterizing new raw materials, including nano- and bioderived materials, and providing independent data either as a check on the inventor’s data or as data beyond in-house capabilities. Depending on where one is positioned in the coatings value chain, the role of our ChemQuest Technology Institute and ChemQuest Powder Coating Research labs can differ greatly. At the end of the day, our role is to help expedite product knowledge and commercialization activities.

McGreer, Atlas Weathering Services Group: The independent lab’s role depends on what the research question is. For example, is the client looking for a customized testing solution to predict service life? Are they looking to meet a material specification? Are they testing a minor revision to their product’s formulation? Each of these questions would require a different type of support from the lab. This could include consultative support, advising on an appropriate existing testing method, or simply running a common method that would be considered “spec testing.”

Marschall, Marschall Labs: The ideal outcome of a collaboration between a client and independent laboratory should be a purely unprejudiced scientific investigation that contributes to validating the performance versus cost of the finished product.

Crewdson, Q-Lab Test Services: The two main roles of independent weathering and corrosion test labs in the R&D process are the confirmation that the manufacturer’s labs have their preliminary data correct and the ability to run more complex and newer tests. Independent labs generally have the newer and more reliable test equipment, and the expertise to run them correctly. Independent labs run a much wider variety of tests, and will be accredited for them. This means that calibrations, maintenance, and training will all be at a high level.

Lewis, Univar Solutions: When coatings companies are looking to develop new products or upgrade existing products, they often do not have the expertise, test equipment, starting point formulas or other resources they need. Independent laboratories like Univar Solutions’ Solution Centers in the United States, Latin America, and Europe have experienced coatings formulators and application specialists with deep expertise and performance testing capabilities. This helps companies get to market faster.

Obie, Wood Coatings Research Group: Independent laboratories provide technology assessment and qualification through an unbiased lens. Independent laboratories can also often provide insight as to where a new technology may actually bring value, that often may not have been the original intention of the product development process.

Leggat, KTA Tator: One role of the independent laboratory is to provide independent results, which can be an advantage for both marketing and legal purposes. Also, an external verification at milestone decision points is useful before investing in scale-up and commercialization. Sometimes, independent labs can offer swing capacity, for example for customers whose in-house resources are currently tied up. Also, they can provide access to specialized equipment.

Vandezande, 1 st Source Research: The role of an independent lab varies depending upon the needs of a customer. An independent lab could be engaged at the starting point of a project, somewhere in the middle of a project, or after a project has failed and there is a need for external expertise. There can be many roles that an independent lab performs in the R&D process ranging from analytical and testing to new product development and product formulation. At 1st Source Research, we partner with customers to understand their stated needs and then conduct the appropriate work they have hired us to complete. While some independent laboratories serve a narrow focus conducting a variety of ASTM tests and providing analytical testing services, we are a little unique at 1st Source. We tend to be viewed by our customers as an extension of their R&D, marketing, and production departments, and therefore play a vital collaborative role in the growth of the customers’ company.

Q. Why do members of the paint and coatings industry seek out the help of an independent lab? What benefits can they expect?

Leggat, KTA Tator: Third-party laboratories such as KTA Tator can help in understanding and performing the testing techniques that are used in different markets. Clients can expect a different and broader perspective on various market segments and substrates.

Obie, Wood Coatings Research Group: Independent laboratories provide an objective, unbiased assessment of technology, and/or proposed ideas. Independent laboratories also provide creative and unique perspectives regarding research, development, and technology assessment. In some cases, a customer may not know or understand some of the “finer detail” requirements for a product to be successful in a given market. An independent laboratory may be able to assist with this. In other cases, a customer may not be able to fully understand the potential benefits a given technology may bring to a market or even to what market a new material might bring value. The creative capability of an independent laboratory can often “see” where a product may fit and provide value to the market, essentially helping the customer focus research and marketing efforts as well as improve product introduction into the marketplace.

Vandezande, 1 st Source Research: Customers in the industry seek us out for work that they cannot do internally, work they cannot get done due to other internal priorities, or simply if they need an external resource to offer some expertise from a company that has worked across a wider array of projects. They look to us at 1st Source for assistance with new product development, product formulation, and key insights through analytical and testing efforts. It is also important to add that customers find that using an independent lab can reduce overhead in R&D and marketing when investigating new markets.

Swope, The ChemQuest Group: At ChemQuest, we stress the knowledge component. We don’t just test properties, we recommend approaches and supporting tests that will speak to intended markets. Each market has its own set of needs, trends, drivers, and associated terminology and of course, testing protocols. The resulting output will demonstrate the suitability of the product for the intended markets.

We also service raw materials suppliers with starting point formulation, comparative testing, and formulary cost impact studies that help them determine where they might fit in the value chain and helps their customers/formulators with a head start evaluating the new raw material.

Marschall, Marschall Labs: We believe the majority of our clients find our services beneficial because of the quality of our unbiased work, as well as our ability to adapt to help solve any problem that might arise during testing. While most mid-to-large scale clients have their own in-house QA/benchmarking lab, what sets us apart, for example, is the ability to engineer custom machinery and proprietary image processing software so that we can derive objective results. Therefore, clients can expect an in-depth informative process that is ultimately more efficient and unbiased than traditional in-house benchmarking labs.

Lewis, Univar Solutions: We find there are three things that coatings companies are looking for in an independent lab: formulation assistance, testing, and analytical capability. In many cases, they can expect the independent lab to carry out these functions quicker and less expensively than if they did the work themselves.

McGreer, Atlas Weathering Services Group: Our lab focuses on the weathering of materials. While durability to the outside environment is an important part of any product development effort, our client may not necessarily be experienced on all the different testing methods that are available, or how to correctly set up those tests. Weathering seems like an easy discipline, but it is not always an exact science and there are still a lot of complicated questions that must be answered.

Crewdson, Q-Lab Test Services: Using an independent accredited test lab ensures that the test is done correctly. When a new product is being tested, there might not be the usual set of control materials that are used to verify the testing results. There is a much greater need to make sure the testing is done correctly. Another factor is that a company’s internal labs are often at capacity in their weathering and corrosion chambers with important but routine tests. This makes it difficult to conduct much experimentation with new methods or run a new product through several different methods. An independent lab can suggest alternative approaches to testing and run the tests.

Q. What are the most common services requested by your customers to assist in their innovation process?

Lewis, Univar Solutions: Because the Univar Solutions labs have extensive experience and lab equipment, we find it is a fairly even split between running tests, developing new formulas, evaluation of raw materials in specific applications, and carrying out reverse engineering. We have most of the test equipment needed in a modern coatings lab. We also have adhesives, sealants, and elastomers test and formulation capability.

Marschall, Marschall Labs: Product benchmarking is the most common function at Marschall Labs. More than 1,500 paints are tested annually for both interior, exterior and specialty applications. Exterior systems are also generally placed on exterior exposure for durability. We also provide a service for raw material suppliers by giving them an independent evaluation of their products.

Crewdson, Q-Lab Test Services: Our testing is aimed at answering the question, “How long will your product last outdoors?” We expose specimens to accelerated weathering and corrosion environments, which could also be outdoors in benchmark climates such as Florida and Arizona. We perform inspections, evaluations, and measurements to quantify the degradation changes and issue reports to our customers. Our most common services are visual evaluations, gloss readings, and instrumental color measurements.

Obie, Wood Coatings Research Group: Independent laboratories are typically involved in activities such as material property characterization, technology assessment, screening, testing, and benchmarking. From this standpoint, the goal is often the qualification of a product for a given market segment, identification of potential value and/or identification of potential market application(s). Also, there is new technology development such as raw materials for the coatings industry, coatings formulation development, and failure analysis.

McGreer, Atlas Weathering Services Group: For us, there really is not a “most common” service since we really focus on one thing, the weathering of materials. We are in sort of a niche business, doing strictly weathering/lightfastness testing and some of the related evaluations that come with it.

Fox Abrams, The ChemQuest Group: Materials characterization, defined as performance testing in different formulary conditions to understand a material’s benefits and disadvantages versus other materials. Another way to say this is to characterize how something works in combination with other materials, to gain an understanding of its benefits in use. Because our services are used for every conceivable coatings market, it’s hard to call anything we do “common.”

Mike Dempsey, 1 st Source Research: We have a wide range of customers in very diverse industries, and on any given day could be working on projects such as developing a new high performance polyurethane resin and coating formulation for maintenance or marine applications, or a light industrial PUD or WB epoxy product for commercial coatings. We might also formulate a new architectural paint and develop a latex for a caulking application. The scope of work is typically centered around a broad range of new product development projects and new product formulations that also require a suite of analytical and testing services. Customers also request our individual analytical services to meet their analytical needs.

Leggat, KTA Tator: Performance testing for specific environments is a common request, such as cyclic salt fog testing for atmospheric exposure, severe wastewater analysis testing (SWAT) for wastewater applications, and autoclave and Atlas cell testing for oil and gas applications. Customers also request customized tests to differentiate their product from their competitors’ products. For example, several years ago a client had a coating for buried pipe. The advantage of their product was that the excavated area could be backfilled with rock within minutes of coating application. They wanted us to develop a method to test that property and compare their product to the leading products in that market. Another common request is to facilitate field demonstrations or “take the laboratory into the field” with equipment such as gloss meters and field microscopes, and sampling for lab analysis.

Q. What are the types of questions that your clients ask you to solve?

McGreer, Atlas Weathering Services Group: For us, it is all about making sure that the specific type of weathering testing that we’re doing is the right one for the material’s final end-use application. Since many clients are new to weathering, educating them about their options is our most common form of communication.

Leggat, KTA Tator: Sometimes clients have a product that has a specific property that they think would be beneficial in a paint or coating and want to know what tests should be run to validate it.

Lewis, Univar Solutions: They often want to know what ingredients are in their competitors’ products, so we carry out analysis using a variety of techniques, such as FTIR, GC, HPLC, GPC, DSC, or TGA.

We have done projects for raw material suppliers who invent new molecules or products and have us test how they can be used in coatings formulas. This includes pigments, surfactants, plasticizers, resins and other materials. Because we have a large, spacious lab with a sizable amount of lab equipment we have even rented out our lab and had companies send their technical people to work here. This has been done on a short-term (hours) or long-term (months) basis. Sometimes coatings companies want accelerated weathering or corrosion resistance carried out as they don’t have the test equipment.

Vandezande, 1 st Source Research: The really tough ones. No kidding! For example, we have helped an organization develop a set of latexes and formulate them into a useful product portfolio. We have also helped that company build a latex plant to make those products, all within a short period of time.

With our background and range of skills, we are constantly entertaining all kinds of questions and requests from current and prospective customers.

Here is a small sample of just a few:

• How can I improve my product to perform in extreme conditions?
• We don’t know what to do with a certain material and don’t understand this market space. Can you help us?
• We have a new raw material. We need assistance in understanding where it might fit, and testing it in those markets?
• TiO 2 costs are soaring. What effective methods can we use to reduce formulation costs?
• How do we make our product more environmentally friendly by incorporating bio-based materials?
• How do I increase the molecular weight of my product?
• Can you develop a polymer for us that will improve our product performance?
• Can you develop new small molecule additive chemistries that will allow us to improve our resin formulation performance?

1st Source’s business model is unique, and it would be hard to boil it down to a few questions. They are dependent upon the individual customer, the market category, as well as other circumstances. And of course, there are a variety of questions that tend to range from simple to complex, and an independent lab should be staffed and equipped to address them all.

Crewdson, Q-Lab Test Services: The number one question we are asked is “how many hours in this accelerated test does it take to equal one year outdoors?” When we run an accelerated test, our customers want to know how that relates to the natural exposures. While there is no single factor that relates time in a test to time in the real world, we can offer a lot of ways that we can derive the acceleration rate for a specific comparison. We can greatly reduce the risk of using accelerated test results to predict the actual durability of a product.

Swope, The ChemQuest Group: Usually, we help our clients identify the questions they need answered. By this I mean, we are not a typical lab who performs routine testing services. Instead, our clients are often removed from the end-use markets their materials are being evaluated for, so they rely on us to help scope an appropriate and affordable customized approach from which they can gain knowledge that applies to their market. In addition to what we already mentioned, we would add trouble shooting to the list. Troubleshooting can be at the formulation stage and involve issues such as the removal of materials of concern from old formulas, formulary processes, and analyzing application issues at point of use.

Marschall, Marschall Labs: The main questions recently are related to finding or supplying offsets for raw materials because of the current supply chain crisis.

Obie, Wood Coatings Research Group: Common questions fielded by independent laboratories often revolve around failure modes that customers are seeing with their coatings, such as why is my finish cracking, lifting, delaminating, failing? Other questions focus on areas such as determining product/technology value and performance. Still other questions revolve around new product/technology development needs.

Q. What are the key challenges that an independent lab must navigate when conducting research for a customer (e.g., IP, project scope, timing, confidentiality concerns, trust, cost, etc.)?

Vandezande, 1 st Source Research: 1st Source is composed of people who have worked for many years with several multinational corporations and is equipped to reduce the complexity of the many challenges that arise. The primary aim is to fully understand the scope of the project and the relevant deliverables. You might think that is assumed, but many companies can struggle with defining them to an independent lab. Once we are past that hurdle, the key challenge is communication. Since we are an external resource, it is critical to embed excellent communication routines with our customer to ensure we are all on the same page in all phases of the project.

Swope, The ChemQuest Group: Our biggest issue is handled right up front and that is who owns the IP. If you pay for the work, you own the IP. The next challenge is constructing a scope of work that makes sense chronologically and fits within budget constraints. Not exactly hard but the most involved part of the process. We have a strong reputation in the industry, so trust and confidentiality are not barriers, and we act like everything is covered by NDA because it usually is.

Marschall, Marschall Labs: All are important. It is important that the company knows the capabilities of the laboratory and the people working there. Marschall Labs, founded in 1991, has a staff of second and third generation employees of the coatings industry. Some of our greatest challenges stem from the diversity of our work and customers’ expectations. Time management and resource allocation is one of our greatest challenges when various concurrent projects require significantly different approaches. We are able to overcome this with our unique workforce made up of seven family members each bringing their own expertise to our organization.

Lewis, Univar Solutions: We have a strict confidentiality policy and do not discuss any customer’s work with anyone else. However, some customers understandably want their technology protected by legal documents and it can sometimes take a while for the lawyers to agree on contract verbiage. Some companies know they need help but don’t know exactly what they want us to do. We have initial no-charge consultations with them to discuss the details of their need and turn that into a lab project that will help answer their questions and address their problems. Cost is rarely an issue as our lab services are very reasonably priced.

Obie, Wood Coatings Research Group: Independent laboratories must be able to navigate all those mentioned, plus more. The key to all of them is honest, open communication regarding a given project. IP and confidentiality are usually taken care of upfront before project initiation. It is important that a customer adequately defines the project, and that expectations are clear. It is not unusual that a customer is seeking assistance with project definition from the independent laboratory as the independent laboratory is often considered an expert in the field.

Leggat, KTA Tator: The issue of intellectual property needs to be carefully considered while developing the scope of work. Client confidence in confidentiality is key. For example, KTA Tator has intentionally not developed the capabilities to reverse engineer formulations because we want our clients to be comfortable submitting samples to us knowing that their intellectual property is safe.

McGreer, Atlas Weathering Services Group: In our specific business, the weathering testing commonly takes a long time. Even accelerated weathering tests can take several months to complete. So timing is really one of the biggest challenges. While we can accelerate the weathering process, we must always be concerned about whether that accelerated test will actually correlate with real-world results. This is where things can get quite complicated.

Crewdson, Q-Lab Test Services: The key challenge is to make certain that we are running the best test method for the intended objectives. This means asking a lot of questions about the product such as how and where it will be used, what failure modes are likely, and what is the expected durability. There are many variables to consider when setting up an accelerated weathering or corrosion test, and it is our responsibility to make sure we are running the test that will give the most reliable results.

Other items such as confidentiality are generally not a challenge since most independent labs sign NDAs and much of the actual testing is blind. The lab only knows the specimen ID numbers, never the formulation or even the material type. Trust is key for an independent lab. The customer must be assured the lab is performing everything correctly. The good third-party labs will have ISO 17025 accreditation, with a specific limited scope that defines the test methods they are capable of running correctly.

Q. Is an independent lab typically engaged earlier in the product development process (e.g., proof of concept), or more in the later stages (e.g., optimization of formulation, product introduction)? How does your participation change based on the stage?

Marschall, Marschall Labs: Marschall Labs is involved in both early and later stages in the development process. We have assisted clients in the early stages by testing their products versus competitive control samples. The results help fine tune the client’s product to achieve greater performance.

Crewdson, Q-Lab Test Services: We are more likely to be involved in the latter stages of product development, when the new or improved product is getting close to release. This is when the results from an independent lab are used to confirm the data. The last stages in the development are to pass a specification. This is set by a company to establish minimum performance requirements. A report from an independent lab is generally required as proof of meeting the specification.

Leggat, KTA Tator: Different labs specialize in different areas. We can prepare substrate, apply coating, conduct compositional tests and evaluate performance. Different clients have different needs, depending on their own capabilities.

McGreer, Atlas Weathering Services Group: While it could be anywhere, for our testing discipline we are typically engaged later in the process. In fact, sometimes we are contacted because a product has been launched, only to soon find that there are some unexpected concerns with product durability. I believe the level or type of participation required goes back to my answer to the first question, and that is, what the research question is in the first place.

Dempsey, 1 st Source Research: We can only speak to 1st Source’s customer experiences. Over the past 11 years, it has been split evenly between the two stages. Some companies engage us much earlier in the process and require us to partner with them from proof of concept all the way through to final production. Others bring us in when they have developed the project to a point where they need external expertise to advance the project.

Obie, Wood Coatings Research Group: The answer to this question depends on the particular customer/situation. It is not unusual for an independent laboratory to be engaged in proof of concept research for new and unusual materials, or even to suggest proof of concept ideas to a customer/partner. In many cases, the customer has developed a product or material and is seeking product optimization, qualification services, market direction, or benchmarking services, i.e., later stage participation.

Swope, The ChemQuest Group: This is very customer dependent. Our participation occurs at any level of the development process, but sometimes we are frustrated because someone has wasted time and money pursuing the wrong path. We could have been more helpful if involved earlier, but that is a common issue in knowledge pursuit.

Lewis, Univar Solutions: We have done a significant amount of both proof of concept and late-stage formula optimization. Certain customers request that we evaluate and screen coating or raw material prototypes. We have also had customers send us a series of blind-labeled test panels or formulated coatings and ask us to test them to help identify a final formula. We handle both approaches essentially the same: we discuss the customer’s needs with them, turn this into actionable lab work, then send them a quotation.

Q. What are the key trends guiding innovation in the paint industry and how do they steer your research efforts?

Leggat, KTA Tator: We see a lot of coating manufacturers developing their own internal methods to validate their products. The end-users need a standard method for comparison between products. Ultimately, multiple labs need to be able to reproduce the standard method. For protective coatings, this is evident in areas such as fire protection and corrosion under insulation.

Lewis, Univar Solutions: There are a number of trends that influence what coatings and raw material suppliers have us evaluate. Regulations are changing, so customers need help with, for instance, developing a lower VOC version of a coating they already have. Right now there are significant raw material supply constraints, so customers want to reformulate their products to use ingredients that are more available. There are also coatings companies who are buying already formulated coatings and want to be able to offer a formula of their own so they can control their own destinies. Sometimes these requests can be challenging, and we might formulate several prototypes and give them options to consider.

Swope, The ChemQuest Group: As mentioned earlier, bio-derived and nano materials are prevalent, and their use is driven by environmental and performance trends. Removing “bad” actors from the equation is probably the biggest challenge for our customers. That means things like high VOC components, fugitive materials of all kinds, and anything affecting health and safety. Many current bad actors were workhorse components in the recent past.

Crewdson, Q-Lab Test Services: The key trends in the coatings industry have been to increase the durability of the product while reducing the cost and to compensate for the removal of environmentally harmful ingredients. We witnessed an increase in testing when heavy metals were banned from paints, and when water-based coatings took over from oil-based paint. These new formulations had to show the same durability. In the future, we might see an increase again as more biobased coatings are introduced, and environmental regulations look at other harmful ingredients.

Obie, Wood Coatings Research Group: I think key drivers are still related to regulatory requirements for the most part. Another important area is applicability of environmentally friendly systems. Both drivers also require a competitive cost profile.

Marschall, Marschall Labs: Product sustainability appears to be the major trend in today’s marketplace. Biobased materials are being offered to our industry on almost a daily basis.

Dempsey, 1 st Source Research: Some of the key trends in the paint industry include lowering or virtually eliminating VOCs, obtaining alternate suppliers for raw materials (due to supply chain issues), minimizing water soluble ingredients in coatings that can leach to the surface and mar the paint surface over time, and as always seeking improved product performance at a lower cost, while meeting regulatory compliance trends both locally and globally, among many others. As an independent external resource, we continually stay engaged with the latest trends and respective research to effectively support our customers. We spend a good bit of time internally on monitoring trends and reviewing the latest literature from industry and academic papers to provide a level of innovation that our customers require.

McGreer, Atlas Weathering Services Group: In weathering testing, the two words most used are “acceleration” and “correlation.” As the world moves at a faster pace, product development times are shorter, but product life expectations are longer. This conundrum is something that we are trying to deal with all the time. The interesting point is that there are currently ways in which testing times can be shortened with improved correlation. However, this may mean that the client must do testing in a significantly different way. But testing differently may mean that all previous research is no longer comparable (or no longer directly meeting the requirements of a particular material specification), and so they keep doing the same (slower) tests over and over.

Q. Do you ever perform internally driven research (versus client-driven)? If so, why and what are your key goals?

Obie, Wood Coatings Research Group: We conduct significant self-funded research. Independent research offers the opportunity to move the industry forward by helping to answer “unsolvable” industry problems or by providing research, methodologies, and/or techniques that assist the industry to better test and quantify. Providing new tools and/or better, more reproducible testing methods, allow the industry to develop new products faster, with a greater degree of success, and in a timelier manner.

Some key issues that we address in our research include test method development, coating and adhesive application, flow and leveling, drying, curing, coalescence, and structure/property relationships. In the coatings industry, results can sometimes be subjective in nature or lack reproducibility, or even lack ability to statistically characterize a result. It is important that material testing data is numerically relevant and characterized statistically as customers seek to “engineer” material properties. We believe independent research helps provide customers access to partners with enhanced creativity and who are up to date with current industry trends, and that these will be applied to their project.

Independent research also provides customers with testing and measurement methodologies to numerically access material properties of their products and materials. A specific example of this is a method called Evaporative Dynamic Oscillation Technique (EDOT). The EDOT method allows the characterization of material rheology, drying, curing, and material property development, from the time of film application through consolidation, in situ, both in terms of air dry as well as for a given oven cycle profile. Further, DMTA characterization of the coating or material film may be characterized in situ at a given timestamp. EDOT allows formulation variables as well as structure/properties relationships to be analytically characterized.

Crewdson, Q-Lab Test Services: We do conduct our own internal research to improve the methods that are being used. We are mainly driven by the published standards that our customers’ request, but there are many instances when new test cycles are needed to better simulate an end-use environment, or a specific failure mode. The goal for Q-Lab is to create test methods that give the most accurate results. For accelerated testing this means a faster and more reliable prediction of the real-world outcome.

Lewis, Univar Solutions: The vast majority of the lab work we carry out is directly for coatings companies or raw material suppliers, so we don’t spend much time with internally driven projects. But sometimes a new raw material will come our way and we will investigate novel ways of using it, then present these ideas to the supplier, which they greatly appreciate.

McGreer, Atlas Weathering Services Group: At Atlas, we are in sort of a unique situation where we actually manufacture nearly all of the instruments used in our testing facility. So, our accelerated testing laboratory can serve as an actual customer to the manufacturing side of the business. In fact, our new generation of Weather-Ometer, the Ci4400, has user-convenience features included and some of them definitely came from doing “market research” by interviewing our laboratory manager and the technicians who actually use the equipment. Atlas also participates in a number of different standards organizations and committees. Sometimes, there is test development work that is necessary, and our independent laboratory supports much of those efforts as well.

Marschall, Marschall Labs: In general, our projects are client-driven, but we do some internally driven research to keep up with new products that are offered to the coatings industry. Most of our internally driven research comes out of a desire to enhance quality and reproducibility in our testing.

Vandezande, 1 st Source Research: At 1st Source, we anticipate trends in the marketplace and try to stay ahead of the curve. We constantly scan the literature for new technologies we can apply to future projects. Our analytical department takes time to optimize our analytical tools and tweak them for faster throughput and more precise results. We also spend time updating our safety protocols and Standard Operating Procedures (SOPs). In addition, we perform internally driven research around emerging markets, new technologies or innovative trends in order to have the capabilities to meet our current or future customer needs.

Leggat, KTA Tator: As a commercial lab, our research tends to be client-driven, but we do develop new methods to address needs in the industry. Because we work with different clients and see the collective needs, we sometimes make investments to develop capabilities that an individual client would not.

Swope, The ChemQuest Group: We won’t get too specific, but one thing we often do is satisfy our own curiosity and add to our knowledge. This might be working with a new raw material just to see if it does what the inventor claims. Another area we pursue involves understanding the interplay of chemistry and equipment in solving problems. We are blessed to have both a flat line and hang line to replicate manufacturing, as well as a full array of Heraeus UV curing and Plasmatreat plasma pretreatment equipment to complement traditional ovens and application equipment.

Q. What would you like potential industry partners to know about having a successful partnership with an independent lab? Are there any misconceptions about independent laboratories that you would like to dispel?

Fox Abrams, The ChemQuest Group: There are many kinds of independent labs from national labs to universities to private testing facilities. Understanding who owns the work product upon completion is crucial to a successful project. ChemQuest Technology Institute and ChemQuest Powder Coating Research are unique in their foundation as knowledge centers staffed by industry experts with many years of experience running the same types of organizations as our clients. We are not “sample in–data out.” Our most successful client partners are eager to learn and collaborate with us. They understand the parameters around a defined project may evolve during the project because of the knowledge we provide as feedback throughout the project. These companies approach their work with us without fear of “NIH—not invented here.” In fact, they embrace the opportunity to expand their team by having access to ours.

Vandezande, 1 st Source Research: There are a few common misconceptions about utilizing an external resource. I must admit that when I was working with a few major corporations before starting with 1st Source Research, I also had some of these misconceptions. However, they simply are not real. Some key misconceptions include:

• External R&D takes longer and is more expensive than internal R&D.
• External R&D should only be used as a last resort.
• The ramp up and ramp down time for external R&D is excessive and unwieldy.

In reality, external R&D/process expertise can breathe fresh air into an organization. The right independent lab can work seamlessly with sales, marketing, and R&D providing new insights to an organization that has a difficult time breaking into new markets.

Leggat, KTA Tator: It is key for the client and the lab to understand the scope of work and what gaps will be filled. The lab needs to fill the client’s needs without trying to upsell. As an independent lab, we work with many clients who may be competitors with one another; however, it is imperative that the lab maintain the confidentiality of all client information. We can bring the knowledge and experience gained from working with multiple clients in different industries while maintaining the confidentiality of each client’s data.

Lewis, Univar Solutions: I think one of the big misconceptions is that using independent labs is expensive. We have found we can carry out testing or analysis or new product development at a fraction of the cost of hiring a new chemist. It is often very time consuming for companies to hire a new person and that person may need training to bring them up to speed with the work the company needs them to carry out. As an independent lab, we can do focused projects for companies as and when they need them, usually quicker than they can do themselves, and for a fraction of the cost.

McGreer, Atlas Weathering Services Group: First and foremost, communication is key and must be a two-way street. The independent lab needs to be proactive in communicating any concerns or asking for clarity about specific testing instructions. And our clients should always be willing to ask questions about specific testing methodologies. Speaking for our own laboratory, I think it is important to work with a lab that you can trust and has the expertise in their specific field of testing. I have many colleagues that participate in standards meeting or have years of experience doing consulting work for clients. While we can’t be expected to be experts on every specific material or product that we receive for testing, we can collaborate with our clients to combine our knowledge from the testing side with the knowledge from the material side to ensure that the testing we are doing is correct for the application, and that we can provide insight into the interpretation and analysis of the results.

Crewdson, Q-Lab Test Services: Independent labs are a great way for companies to expand their expertise and resources on an ad hoc basis if needed. The independent lab fills a need that the customer may not be able to maintain themselves. The lab can be used as needed, for example when a new paint or coating is being developed, or a new market is being explored. A good independent lab is generally easy to deal with, they are customer friendly, and they have a great deal of experience they are willing to share.

Marschall, Marschall Labs: Knowing the lab you are working with is very important. It is also important to have a trust in confidentiality. A possible misconception of testing labs is the cost involved. Individual testing labs, most times, can work much more efficiently than larger company labs because there is less bureaucracy.

Q. Do you have any other comments you think would be helpful for readers to better understand how independent laboratories are involved in the innovation process?

Marschall, Marschall Labs: Independent labs offer a unique perspective of the industry. The variety of challenges we see, along with our inherent objectivity, allows us to bring a different approach to the innovation process.

Crewdson, Q-Lab Test Services: We are asked about which tests are the most reliable, and how we can design exposure cycles to recreate a specific climate or a particular end-use environment. Laboratory accelerated tests are simulations and their success is based on how close they are to the real world. A great accelerated test is one that provides the same result as the real world but in a much shorter time. Our clients are relying on our experience to answer the question of which is the best test method for their specific requirements. Independent labs generally have a very wide knowledge base and the experience to help their customers innovate.

Fox Abrams, The ChemQuest Group: The right independent lab should bring you expertise and/or equipment you do not have in-house so you can learn faster than if you tried to hire that talent into your organization. Working with an outside partner on innovation projects allows your team to concentrate on their day-to-day activities while someone else de-risks the pursuit of innovation.

McGreer, Atlas Weathering Services Group: I always feel that an educated client is the best client for the lab. Those clients will then better understand how to interpret results. There are lots of ways to learn more about the specific testing details, whether it be online webinars, recorded video presentations, or hopefully soon, back to face-to-face meetings and symposia where ideas can be shared.

Vandezande, 1 st Source Research: To maximize your investment, choose a lab that you can trust and enjoy collaborating with, that will focus on your needs, provide solutions that are applicable to a range of technologies and products, and has all the necessary equipment, knowledge, and skills to meet your company’s project goals.

Lewis, Univar Solutions: My advice to coatings companies and raw material suppliers is to be open to using independent labs instead of trying to do everything in-house. Seek out test labs and interview them to see if they meet your needs. Ask them to give you a virtual tour of their facility so you can see their capability, rather than just viewing a brochure or a website.

About the Author

Leo J. Procopio, Ph.D., is the president and owner of Paintology Coatings Research LLC. He may be contacted at lprocopio123 (at) yahoo.com.

CoatingsTech January 2022 |  Vol. 19, no. 1

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

• Open access
• Published: 07 June 2021

After the paint has dried: a review of testing techniques for studying the mechanical properties of artists’ paint

• Gwen dePolo 1 ,
• Marc Walton 1 ,
• Katrien Keune 2 , 3 &
• Kenneth R. Shull   ORCID: orcid.org/0000-0002-8027-900X 1  

Heritage Science volume  9 , Article number:  68 ( 2021 ) Cite this article

8788 Accesses

13 Altmetric

Metrics details

While the chemistry of artists’ paints has previously been studied and reviewed, these studies only capture a portion of the properties affecting the response of paint materials. The mechanical properties of artists’ paints relate to the deformation response of these materials when a stress is applied. This response is dependent on many factors, such as paint composition, pigment to binder ratio, temperature, relative humidity, and solvent exposure. Here, thirty years of tensile testing data have been compiled into a single dataset, along with the testing conditions, to provide future researchers with easy access to these data as well some general discussion of their trends. Alongside the more commonly used techniques of tensile testing and dynamic mechanical analysis, new techniques have been developed to more fully investigate the mechanical properties, and are discussed along with salient results. The techniques have been divided into two categories: those that are restricted to use on model systems and those that are applicable to historic samples. Techniques applied to model systems (tensile testing, dynamic mechanic analysis, quartz crystal microbalance, vibration studies) require too large of a sample to be taken from art objects or focus on the mechanical properties of the liquid state (shear rheometry). Techniques applied to historic samples incorporate the use of small sample sizes (nanoindentation), optical techniques (laser shearography), computational simulations (finite element analysis), and non-invasive comparative mechanical properties (single-sided nuclear magnetic resonance) to investigate and predict the mechanical properties of paints.

Introduction

Paint is a ubiquitous component of art objects and a research subject that has received prominent attention for many years in both industry and cultural heritage. A great amount of research has focused on understanding the chemical changes of the curing processes of paints, but they only partially capture the overall stability of a paint film [ 1 , 2 , 3 , 4 ]. The mechanical properties play a key role in assessing its stability over the lifetime, providing information about the stiffness, toughness, and the likelihood of crack formation, among other things. The most recent review paper discussing the mechanical properties of paint was published thirty years ago, emphasizing the role that temperature (T) and relative humidity (RH) play on brittleness of these films [ 5 ]. In the present review our aim is to compile the innovative mechanical testing techniques that have been developed in the last two decades, together with the data derived from these experiments. It is shown how this area of research has grown and begun to provide possibilities for contributing to improved conservation practices.

Paints are typically comprised of a pigment for coloration, a binding medium, and other additives that can adjust the optical properties and workability of the paint film. All paint materials have several properties that change over time, initially being a liquid-like film that evolves into a solid-like paint film. This transition from liquid- to solid-like behavior can occur two ways: through a drying process, where solvents added to the system evaporate out of the paint film, or through curing, where crosslinking and other chemical reactions occur within the pain film to increase the stiffness. The timescale over which these drying and/or curing processes occur can vary greatly, from a few hours (acrylics) to months (alkyds) to decades (oil-based paints) [ 6 , 7 ]. As the paint transitions from liquid-like to solid-like, the stiffness of the paint film can typically increase several orders of magnitude. For oil-based paints, the mechanical properties can continue to change over the long periods of time that the curing process is ongoing, motivating active research into the mechanical properties of these paints. Alkyd and acrylic paints, since they are modern paints that have been in use for less than 100 years, have also been a strong focus of research efforts in the last few decades. Other historic paints, such as distemper or egg tempera, are known to have a short drying phase and to form stiff, brittle films [ 6 , 8 ]. Once tempera paints solidify, the mechanical properties remain fairly stable, so they have not been the focus of more recent research efforts. Distemper paintings are rare to find in a pristine condition, as many have been subjected to a varnish treatment, which alters the original mechanical properties of the film [ 9 ].

The mechanical properties of a paint film are affected by the paint composition, interactions between paint layers for a composite painting structure, and the environmental conditions (temperature, relative humidity, transportation, etc.) to which a paint is subjected. Each of these contributions will be discussed in further detail in the following sections, followed by an overview of the techniques discussed in the review and at what point during the lifetime of a painting they are relevant for use.

Pigment to binder ratio

A simplified definition of a paint film is a composite material comprised of rigid pigment particles suspended in a more flexible binding medium. The concentration of pigment or additive present tends to have a strong contribution to the mechanical properties of the paint. The concentration can also vary widely depending on the desired properties and particular combination of pigments used in the paint. These relative concentrations can provide insights about the expected stiffness of paints since the additives function as rigid fillers within the binding medium matrix. The added filler is typically much stiffer than the corresponding binding medium matrix, and thus contributes to an increased stiffness of the overall paint [ 10 , 11 ]. A challenge for researchers is accurately determining the pigment to binder ratio of fully cured historic paints, which provides relevant information for understanding the mechanical properties of the paint and for understanding an artist’s technique by making paint reconstructions. Two approaches to determine pigment to binder ratio that have been previously used various imaging and spectroscopic techniques to analyze these cross sections. The first approach used a sequence of \(\upmu\) X-ray diffraction ( \(\upmu\) XRD), particle induced X-ray emission (PIXE) and backscattering spectrometry (BS) to determine the light and heavy elements present in a historic paint cross section, using the techniques together to identify the pigment(s) and estimate of the ratio of pigment to binder in the sample [ 12 ]. Alternatively, researchers have looked into the quantification of the ratio between pigment and binder on stable, liquid-like paint systems using Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy [ 13 ]. The use of these spectroscopic techniques becomes difficult on dried historic paints where the pigment, binder, or both are not stable over long periods of time, since the particular chemical peaks tracked in this study can change quite a bit over longer periods of time. Some more recent work has used X-ray micro- or nano-tomography to determine the pigment to binder ratio for the full volume of a paint sample [ 14 ].

Some studies focused on the mechanical properties of paint have used a measurement of pigment content based on volume, pigment volume concentration (PVC), instead of pigment-to-binder ratio, which is a weight-based measurement [ 15 , 16 , 17 ]. Mathematical models have also been developed to describe the effects of pigment particle concentration and shape on the stiffness of the sample and have been explored in some of the studies that will be highlighted in Section  2 [ 18 , 19 ].

Paintings as multi-layer structures

The complexity of paint mechanical properties goes beyond the effect of a rigid filler on the stiffness of the paint. Multiple layers of paint ranging from tens of microns to 1 mm in thickness are generally utilized in the creation of a painting. These layers are placed on a support, forming a multi-layer structure where the mechanical properties of a given paint layer can be affected not only by the environmental fluctuations or conservation treatments, but also by changes in the surrounding layers (Fig.  1 ) [ 20 ].

The multi-layered structure of a traditional oil painting showing the support, glue, ground, preparatory, paint/glaze, and varnish layers. The complexity of the layered structure of the painting provides an added level of difficulty for researchers when studying the mechanical properties of paints and how they affect the overall stability of the painting. Painting information: Mary Cassatt, The Child’s Bath, 1893, Oil on canvas. The Art Institute of Chicago, Robert A. Waller Fund. Adapted from a figure in [ 20 ].

Obtaining information about this complex multi-layer structure presents challenges to researchers, especially understanding how each layer contributes to an overall response of the object and how the material/chemical composition influences that response.The range in size of single paint layers to entire paintings also provides a challenge for determining which length scale (e.g., micron, millimeter, or meter) and therefore which technique is most relevant for understanding the mechanical response of an object. Any expansion or swelling of a given layer (such as the swelling of a canvas in response to increased RH) or significance differences in stiffness between layers (i.e. a much stiffer ground layer below a paint layer) can lead to small applied strains on the surrounding layers, causing stresses that increase the potential for cracks to form and propagate through the painting. Paint layers can also be subject to issues of adhesion, which can lead to delamination or paint loss, which is also problematic. Understanding the mechanical properties of these layers the mechanical response to neighboring layers or environmental fluctuations are relevant when determining the overall structural stability of a painting and will be discussed within this review.

One of the main concerns for conservators is the formation or propagation of new cracks within the painting due to fluctuations in environmental conditions or as a function of transporting a painting. The craquelure present in a painting is acknowledged as an inherent part of the drying process of the paint, but the formation of new cracks from an impact, transportation, or a drastic change in environmental conditions can increase the chances for paint delamination and paint loss. The development of craquelure in paint has been an area that has been discussed fairly extensively in a qualitative nature, but has experienced difficulties in developing a standardized metric for characterizing the craquelure [ 21 , 22 , 23 , 24 ]. Some models have been discussed with regards to how ground layers may develop cracks as a result of environmental fluctuations [ 25 ]. Research on the craquelure formed using model colloidal systems, which can be tuned to have similar particle size and distribution as historic paint layers, has provided some information about the distinctions between drying and aging cracks, as well as the insight that as paintings age, the degree of cracking increases [ 26 ]. The research was also extended to discuss how craquelure on paintings can relate to the mechanical properties of the visible and underlying paint layers, as well as the implications of these craquelure in the routine conservation work performed on paintings [ 27 ]. These colloidal system studies provide some experimental data to correlate with the qualitative descriptions of craquelure patterns, but it can still be difficult to understand the underlying mechanisms influencing crack formation and propagation within the paint.

Two distinct length scales need to be considered when discussing crack formation in a paint layer. The first of these is the macroscopic scale of the entire painting, which determines the stresses that the paint layer is exposed to. In fracture mechanics terms these stresses define the driving force for continued growth of a crack, typically expressed in terms of an energetic driving force, \({\mathcal {G}}.\) This quantity has units of energy per unit area, and quantifies the energy that is recovered by the overall system as the crack grows. Crack growth occurs when \({\mathcal {G}}\) exceeds a critical value, \({\mathcal {G}}_{c}\) , that is characteristic of the paint. Growth of the crack alters the stress field around the crack itself, affecting it’s own trajectory and the ability of neighboring cracks to grow. These interactions determine the craquelure patterns that are observed in actual paintings, and motivate the need to understand the system behavior at the length scale of the cracks themselves. Because of the complexities of the experimental geometries in real paintings, numerical finite element methods have an important role to play at both of these length scales, although considerable insight can also be obtained from appropriate analytic expressions.[ 27 ]. The main point we want to make here is that it is generally not sufficient to understand the properties of the paint in absence of an understanding of the relevant driving forces for crack propagation. These driving forces originate from a combination of external forces imposed on the object and internal forces generated by the varying response of the different layers to environmental conditions.

Relevant techniques

The techniques covered within this paper, along with applicability to either model systems or historic samples and when they can be used during the lifetime of a paint, are shown Table  1 . Deciding when to use a given technique depends on the desired mechanical property and the goal of the research program, whether it’s to address questions from art history (i.e., how an artist achieved a specific painting effect), to monitor or assess the condition of historic objects, or to understand the fundamental responses of mockup paint samples that can be deformed and subjected to damage more readily than a historic painting. Since there can be some challenges in comparing the results obtained from model systems with historic samples (typically due to age and composition differences), these techniques have often been paired with other analytical techniques to study the chemical response of a paint and provide a more direct comparison.

For each of the techniques in Table  1 , we will discuss the advancements and significant results as well as future directions for their optimization and use to study paints. To help explain the types of data and the significance of results, there will first be a brief overview of the mechanical properties investigated with each technique.

Tensile resting

Static mechanics overview.

In typical statics measurements of paint systems, a force (stress, \(\sigma\) ) is applied to a system to cause a displacement (strain, e ). The stresses can be applied in either a normal or a shear direction to the sample. For paint materials, the stresses are commonly applied through the geometries shown in Fig.  2 . These sample geometries take into consideration some of the structural aspects of the material, such as whether it is a membrane system (e.g., an individual paint layer or painting on a canvas), is attached to a rigid support (e.g, painting on a wood panel), or is still in a liquid form (e.g., reproductions of historic recipes). Uniaxial tensile testing (Fig.  2 a) applies force on opposing ends of a paint film, providing material properties that will be described shortly. Biaxial tensile testing (Fig.  2 b) applies tensile forces in two directions, which can be useful when looking at the composite structure of a paint layer on a support (i.e. a linen canvas). However, the composite nature of these systems can make it difficult to accurately define the individual material contributions to the overall behavior. Four point bending (Fig.  2 c) applies forces on either side of a sample in such a way as to cause both compressive and tensile forces at the center of the sample. If there is an adhesive or filler material being used on a wooden object, these tests are useful for determining the potential failure conditions of the conservation materials.

Schematics of a uniaxial tensile, b biaxial tensile, and c 4-point bending experimental geometries. The grey regions in a indicate where clamps are attached to the sample. The arrows indicate the direction of applied force in each experimental geometry

The stress-strain behavior for a material can exhibit a range of phenomena, depending on the temperature and timescale of the measurement. In polymeric materials there is a general equivalence between time and temperature, with similar behavior obtained either by decreasing the temperature or increasing the time scale of the measurement. For artist paints, the properties also evolve with time, adding another dimension of complexity. Generic results obtained from a uniaxial tensile test are shown in Fig.  3 . While not all of these behaviors are necessarily observed in the same material, the following general regimes can often be identified, based on 4 different temperature regimes ( \(T_{1}\) , \(T_{2}\) , \(T_{3}\) and \(T_{4}\) ).

T 1 : Brittle behavior of a glassy material. This is generally observed at sufficiently low temperatures.

T 2 : Ductile behavior followed by localized thinning of the sample and failure.

T 3 : Ductile behavior with substantial localized deformation after yield.

T 4 : uniform deformation of a very soft, rubber material.

Typical generic temperature behavior at different temperatures shown with a qualitative load (stress) vs. strain plot. As the temperature decreases from \(T_{4}\) to \(T_{1}\) , the sample exhibits more brittle behavior. This trend is also equivalent to increasing the strain rate (or decreasing the measurement time) used for the tensile test

Note that Fig.  3 is highly qualitative and does not accurately capture the detailed changes that are observed. As one follows the blue arrow from \(T_{4}\) to \(T_{1}\) , the polymer sample would pass through the glass transition temperature, \(T_{g}\) , which is the point when a polymer transitions from a rubbery, more flexible response to a glassy, more brittle response. A more quantitative example of a tensile test is the stress-strain curve for the 8-year-old lead white sample shown in Fig.  4 . These curves provide several results that help the researcher understand the overall mechanical behavior of the paints. The elastic modulus ( \(\mathrm {E}\) ) (a.k.a. Young’s modulus) can be determined from the from the slope of the curve at low strains and is shown by the line extended from the stress strain curve. When the paint film begins to yield, the slope of the stress strain curve decreases. The stress at which this yield point occurs is known as the yield stress ( \(\sigma _{\mathrm {Y}}\) ). Not all paint samples exhibit a yield stress either due to the softness of the material or to the viscoelastic behavior of the film. For cases where a yield stress is not present in the data, a secant modulus ( \(\mathrm {E}_{\mathrm {s}}\) ) can be calculated using the following equation:

where common values of e are less than or equal to 0.05. The end of a tensile test usually occurs when there is a failure such as a fracture in the paint sample. The stress and strain values at this point, the ultimate tensile strength ( \(\sigma _{\mathrm {T}}\) ) and strain to fracture ( \(e_{f}\) ) respectively, are useful material properties for characterizing the limits of a sample.

Sample stress (MPa) vs. strain plot for an 8-year-old lead white paint film in acid refined linseed oil. The relevant mechanical properties that can be determined using these curves are also displayed on the graph. Data replotted from [ 28 ]

These static tensile measurements require larger samples, making them impractical for measurement of historic paints and requiring the use of either model systems or sacrificial collection samples. The sample geometry for tensile tests also requires solid samples, which means the paints have to dry for at least 1 week (acrylic) to 1 month (oil) before measurements can be taken. The instruments used for these measurements can be modified to include environmental chambers that control T and RH, some of the more important practical parameters for understanding the material properties of paints.

Uniaxial extension

Uniaxial tensile testing of paint samples has been one of the most commonly used techniques for investigating the mechanical properties of paints in cultural heritage over the last few decades. A summary of the references that have used tensile testing to measure the mechanical properties of paint materials is shown in Table  2 . A summary of the data from these references are available as supplementary information in PDF (Additional file 1 ) and Excel (Additional file 2 ) files Footnote 1 .

Plot showing elastic modulus (MPa) vs aging time (years) from uniaxial tensile tests of oil, alkyd, and acrylic paints. The graph does not account for testing variables such as T, RH, solvent exposure, or pigment/particle type used in the paint medium, contributing for the wide spread of the modulus data. Data compiled in this graph are from [ 15 , 18 , 19 , 28 , 30 , 33 , 34 , 35 , 36 , 37 , 39 , 41 , 42 , 43 , 46 , 47 , 48 , 49 , 50 , 51 , 54 , 55 , 58 , 60 , 61 ]

From the references in Table  2 , Fig. 5 provides an overview of the range of \(\mathrm {E}\) from the samples tested as a function of age (associated with the curing time) of the samples, which spans four orders of magnitude. While this graph does not show a strong trend in the data, it does highlight one main takeaway about analyzing these data: it is difficult to analyze trends in the data using only a single variable from these uniaxial tensile tests. As discussed above, the type of pigment used will impact the pigment to binder ratio of a given sample. Inorganic pigments also tend to be much stiffer than the binding media, also leading to a significant increase in the stiffness of the sample. Many of the studies focused on the effects of temperature [ 18 , 29 , 30 , 36 , 37 , 41 , 43 , 46 , 50 , 54 , 57 ] and relative humidity [ 15 , 29 , 30 , 34 , 36 , 37 , 41 , 43 , 44 , 46 , 52 , 57 , 59 ] on the stiffness of the sample. There is a general trend across all three binding media classes of increasing stiffness as the temperature of a paint decreases, shown in Fig.  6 , which would indicate that the increase in stiffness is controlled more by the properties of the binding medium than the particular pigment used. For all these general classes of binding media, \(-10^{\circ }\mathrm {C}\) is below their \(T_{g}\) , which leads to a glassier, more brittle response from the samples. When the RH is increased for a paint system, researchers observed a general softening phenomenon occur since water can act as a plasticizer in most paint films, leading to a decrease in the stiffness that can range from 30-90%. Footnote 2 [ 15 , 29 , 30 , 34 , 36 , 37 , 41 , 43 , 44 , 46 , 52 , 57 , 59 ]. When the RH is decreased, the paints become stiffer as less ambient water is present, showing anywhere from 40-300% increase in the stiffness.

Plot showing the elastic modulus (MPa) vs temperature ( \(^{\circ }\mathrm {C}\) ) from uniaxial tensile tests of oil, alkyd, and acrylic paints. The graph does not account for other testing variables such as age, RH, solvent exposure, or pigment/particle type used in the paint medium. Data compiled in this graph are from [ 15 , 18 , 19 , 28 , 30 , 33 , 34 , 35 , 36 , 37 , 39 , 41 , 42 , 43 , 46 , 47 , 48 , 49 , 50 , 51 , 54 , 55 , 58 , 60 , 61 ]

Another common variable for the material response of paint materials is solvent exposure to assess the effects of solvents commonly used for cleaning paintings [ 18 , 28 , 36 , 37 , 42 , 48 , 49 , 62 ]. The most common trend after exposure to volatile solvents was that the paint films would become embrittled, exhibiting stiffer, more brittle responses (anywhere from 100-900% increase in the stiffness) due to the leaching and volatilization of low molecular weight components. Yet, exposure to non-volatile solvents resulted in increased plasticity of the paint films (up to a 74% decrease in the stiffness) [ 28 ]. Results from uniaxial tensile tests are also dependent on the strain rate of the test, or how quickly the sample is deformed. As the strain rate is increased, the paint film has less time to respond to the deformation taking place, leading to a stiffer, glassier response. Hagan et al. varied the strain rate during their research on acrylic and oil-based paint systems to apply time temperature superposition (explained more fully in Sect.  3 ) to their results and observe the material response over a wider range of testing strain rates [ 18 , 19 , 46 , 50 , 54 ]. The studies also considered the effects of PVC as well as the shape of the pigment particles on the overall response of the pigment/binding medium composite sample using mathematical models focused on filler effects in a composite, highlighting the importance of understanding the PVC of a paint sample [ 19 ]. There is a challenge of the composition of the binding medium and PVC not being readily available for commercial artists’ paints, but with enough time and access to analytical tools they can be determined. Not all the responses are explained through pigment type or filler concentration, though, because the alkyd paints continue to increase in stiffness as a function of time regardless of pigment, shown in Fig.  7 , whereas the oil and acrylic paint classes do not show any trends as a function of age due to convolution from other testing parameters.

Plot showing the elastic modulus (MPa) vs age (years) for the alkyd paints. The wide spread in the value of the elastic modulus for Hansa Yellow is due to solvent exposure. The spread in the data for Titanium White is due to temperature variations. Data compiled in this graph are from [ 30 , 35 , 42 , 48 , 54 , 58 ]

One of the main drawbacks to using model systems for paints is their relatively young age; they are typically 30 years old at their oldest. While this age range is more relevant for the newer alkyd or acrylic paint classes, it becomes difficult to draw conclusions about the expected material responses of oil paints that are 50 years or older. While accelerated aging may be a way to address this issue, previous research has shown that without careful tailoring of the study, accelerated aging of the paint film enables different chemical reaction pathways than natural aging for oil paints, which can make comparisons difficult [ 63 ]. Some theoretical extrapolations can be made about the response by observing the changes in stress at constant strains. Over longer times, the stress begins to follow a linear trend when plotted against the natural log of time, which allows researchers to create theoretical stress-strain curves that can be more predictive [ 36 , 38 ]. This analysis method provides a good first approximation for some brittle paints, but it has not been applied to softer paint systems or to paint systems where the stiffness remains constant, but the flexibility of the paint film decreases.

Some of the studies shown in Table  2 focus on other materials found in the multi-layered structure of a painting than paint. Researchers have studied the response of grounds to RH to determine ideal storage conditions and the effects of drying shrinkage on the formation of cracks within the ground [ 52 , 56 ]. The stiffness of canvas as a function of the orientation of the weave of the sample has also been explored using uniaxial extension, demonstrating that the stiffness is strongly dependent on the orientation that the load is applied to the canvas [ 53 ]. Samples comprised of multiple layers ( i . e ., paint-paint or canvas-paint) can provide some insights into the response of the composite to changes in temperature or relative humidity, but it becomes difficult to determine which aspects of the composite structure are contributing to those changes [ 40 , 57 , 59 , 64 ].

Uniaxial tensile testing provides researchers with the ability to investigate the effects of T, RH, solvent exposure, and PVC on the mechanical properties of paint, but these investigations can only be performed on model systems due to the relatively large amounts of sample required for measurements.

Biaxial extension

When we consider the composite nature of paintings, those mounted on stretchers experience stresses to the paint layers from two directions, which can be difficult to capture using only the uniaxial extension test. The way that paintings are mounted to stretchers is very similar to how they would be studied using biaxial extension (Fig.  2 b), making this technique a relevant method for studying these model systems. To address the complexity of observing stresses in two directions, Young developed a biaxial tensile testing system that would be compatible with painting mockups [ 65 ]. An electronic speckle pattern interferometer was incorporated into the instrumentation to help determine the amount of strain occurring in each direction during the tensile tests [ 66 ]. The biaxial setup has been used to assess the behavior and interaction of glue, oil, and flour paste grounds when they are applied to canvases [ 45 ]. The glue and oil grounds demonstrated stronger changes as a function of RH than the flour paste grounds, but the flour paste grounds were not as stiff as the other two grounds [ 45 ]. Using the biaxial geometry to investigate canvases and paint layers allows researchers to investigate how the mechanical response of a canvas-paint composite sample will be affected by environmental changes since the response of the composite system will be different dependent on the canvas weave direction. Due to the isotropic nature of paint samples, when using this geometry both E and Poisson’s ratio ( \(\nu ),\) the lateral strain that occurs perpendicular to the applied stress, can be determined. One important aspect to keep in mind is that the corners of the sample will act as stress concentrators when placed under tension (see Sect.  7.2 ), which can cause issues during data analysis. A workaround can be to remove the corners of the model samples or to use computational simulations to study the impact of the corners of a sample more fully. From a materials characterization standpoint biaxial testing provides information that is comparable to what is obtained from uniaxial testing. It’s primary use as described above is in studying the mechanical response of the overall composite painting structure under realistic loading conditions.

Bending geometries

As was mentioned earlier, a bending geometry allows one to investigate both compressive and tensile forces applied to a material, especially if those are potential forces that need to be accounted for in a rigid art object. Young et al. performed a study of adhesives and fillers that are typically used in the conservation of panel paintings, which can have a large range of stresses applied as a function of environmental shifts (typically T or RH) [ 67 ]. With the bending geometry, it was easier to determine whether the failure of the joints studied was due to the adhesive or to the wooden support [ 67 ]. The load curves from this research were compressive rather than tensile, generating stiffness measurements that are not as directly comparable to the tensile data shown earlier. These more rigid paint composite systems can also be more thoroughly studied through computational simulation, which will be discussed more in Sect.  7.2 . This geometry had targeted benefits of exploring the response of paints, adhesives, and fillers on rigid substrates, but is not as useful beyond this targeted application.

• Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) is a technique ideally suited for studying viscoelastic samples such as paints. Most DMA studies of artists' paints have used a similar sample geometry and force application as uniaxial tensile testing (see Fig. 2 ). Unlike tensile testing, which applies a fixed stress or strain rate, DMA samples undergo oscillatory loading conditions to look at the frequency dependent response common to polymer-based systems, including artists’ paints. Quantifying this behavior is important when planning storage, travel, or exhibition conditions and conservation treatments for a work of art. In DMA experiments, the stress is applied as a function of time ( \(\sigma (t)\) ), which leads to a time dependent expression for the elastic modulus, E ( t ), which for an oscillatory stress and strain can also be notated as a complex function, \(E^{*}\) :

where \(E^{\prime }\) is the storage modulus that describes the solid-like behavior of the sample, \(E^{\prime \prime }\) is the loss modulus that describes the liquid-like behavior, \(\left| E^{*}\right|\) is the magnitude of the complex elastic modulus, and \(\phi\) is the viscoelastic phase angle, which describes the phase difference between the oscillatory stress and strain. The two right sides of Equation  2 can be related through these two equations:

We can combine Eqs.  3 and  4 to get the following expression for \(\tan \phi\) (also known as tan \(\delta\) in literature), commonly referred to simply as the loss tangent:

For samples that are thermorheologically simple, or have the relaxation times present in the sample that are all affected the same way by temperature , frequency sweep experiments can be performed at various temperatures and used to generate a master curve of both \(E^{*}\) and tan \(\phi\) using time-temperature superposition [ 68 ]. From these master curves of the complex elastic modulus versus shifted frequency, the glassy and rubbery regimes of a sample are visible. The peak of a tan \(\phi\) versus temperature plot is a measure of the glass transition temperature, \(T_{g}\) , where the sample transitions from rubbery behavior above the \(T_{g}\) to a more brittle, glassy response below \(T_{g}\) . Due to the nature of polymer relaxation times, the \(T_{g}\) is also partially dependent on the frequency of the measurement. In a typical amorphous polymer, increasing the frequency of measurement by an order of magnitude increases \(T_{g}\) by about 3 \(^{\circ }\) C, which is important to remember when comparing data between different techniques that use a different base frequency, such as will be discussed in Sect.  5 . Shift factors ( \(a_{\mathrm {T}}\) ) are used to align the \(E^{*}\) and tan \(\phi\) to the values for a given reference temperature by shifting the data horizontally along the frequency axis, using the underlying assumption for time-temperature-superposition to apply these shift factors. Figure  8 provides example master curves of \(\left| E^{*}\right|\) as a function of frequency and of tan \(\phi\) and \(a_{\mathrm {T}}\) as a function of temperature for an alkyd binding medium commercially made by Galkyd and aged for 159 days before testing. In Fig.  8 a, \(|E^{*}|\) spans over 15 decades of frequency, showing the rubbery-like behavior on the lower left of the graph and the glassy behavior on the upper right. The \(\mathrm {tan}\phi\) curve shows a peak around \(26\,^{\circ }\mathrm {C}\) , which would be a rough estimate of the \(T_{g}\) for this alkyd binding medium. Mathematical models are often used to describe the shift factors and predict the response of the polymers [ 68 ], but are not discussed in this review.

Plots showing master curves for the a complex elastic modulus ( \(|E^{*}|\) ) versus frequency, b \(\tan \phi\) versus temperature, and c the shift factors ( \(a_{\mathrm {T}}\) ) versus temperature obtained from a DMA experiment on a commercial Galkyd alkyd binding medium that has been aged for 159 days. The complex elastic modulus shows the frequency dependent behavior of the Galkyd binding medium, \(\mathrm {tan}\phi\) provides an estimate of the glass transition temperature for this system, and the shift factors can provide insights into the overall polymer behavior of the Galkyd system. Data reproduced from [ 69 ]

Hedley et al. presented some of the earliest foundational work using DMA on artists’ paints to assess changes in mechanical responses of lead white (“medium lean”) and burnt sienna (“medium rich”) samples after exposure to solvents typically used for cleaning [ 70 ]. Using a shear stress geometry and temperature range from \(5-100^{\circ }\mathrm {C}\) , an increase of \(G^{\prime }\) (loss shear modulus, defined in a similar way to the \(E^{\prime }\) ) and significant reduction in \(\tan (\delta )\) for the burnt sienna samples indicated leaching and embrittlement after solvent exposure; the lead white samples showed minimal change in response after solvent exposure [ 70 ]. Samples of the same pigments aged 15 years were explored over a range of relative humidities (54–94%) and compared to composite samples of canvas, primer, and a white oil paint layer from a rear primed painting, which demonstrated a softening behavior with an increase in relative humidity [ 71 ]. Foster et al. modified a DMA system to work with a controlled RH chamber in order to study how the RH and curing process of polyester/melamine paint systems affect the glass transition temperature [ 72 ]. The analysis performed on \(E^{\prime }\) and \(\tan \phi\) master curves is a good example and illustrates just how much information is obtained through these experiments.

Ormsby et al. focused on acrylic paints, specifically investigating the effects of solvent exposure on the chemical, optical, and physical properties of the samples [ 73 ]. Comparisons between acrylic brands and exposure methods of swabbing or immersion were considered, showing a strong effect on the dimensional extension of samples after being immersed in the solvents, especially more polar solvents. The \(T_{g}\) of the samples would increase with extended immersed solvent exposure, showing an opposite trend than previous data on oil paints, most likely due to embrittlement of the acrylic paints as a result of the solvent exposure. The samples that were swabbed experienced significantly smaller changes in the DMA data, leading to the conclusion that using swabs for solvent exposure will reduce the overall changes to the bulk properties of the paint compared to full immersion of the paint film, even if some surfactants are lost at the surface of the film [ 73 ]. Titanium white acrylic paints exposed to temperature and RH sweeps showed typical responses corresponding to an embrittlement/decrease in \(T_{g}\) with a decrease in RH, indicating a strong relationship between the water content of the paint and the mechanical properties [ 74 ]. Some samples that were thermally aged exhibited smaller \(\tan (\delta )\) , peaks, indicating stiffer films than the naturally aged films and potentially better coalescence of the paint films. The onset of a brittle response from alkyd and acrylic based grounds was investigated using the \(T_{g}\) from DMA to help confirm the ductile to brittle transition as temperatures were dropped from \(20^{\circ }\mathrm {C}\) to \(-10^{\circ }\mathrm {C}\) [ 41 ]. Phenix performed an extensive survey of \(T_{g}\) values of oil paints and how they were affected by the type of pigment and the age of the paint sample (1–16 years), finding that the range of \(T_{g}\) s varied widely with sample composition [ 75 ]. Most samples in the study demonstrated an increase in \(T_{g}\) with age, indicating an embrittlement effect that could pose potential long term issues for paintings stored in a museum environment.

The application of time-temperature superposition to DMA data collected over a range of temperatures allows researchers to access a much wider range of measurement frequencies than typically accessed with other methods. Sturdy et al. used DMA to understand the material response of a commercially produced alkyd based binding medium (Galkyd) as a function of curing time and filler content of zinc oxide, providing a survey of the effects of filler content and the compatibility between DMA, the quartz crystal microbalance (QCM), and nanoindentation for studying paint materials [ 17 , 69 ]. These studies also compared the material responses measured using DMA and the QCM, demonstrating that both techniques are able to capture the glassy material response well. Verifying the general response regime of a viscoelastic system using the \(T_{g}\) is also fairly common; a linseed oil with zinc/lead white ionomer system determined the \(T_{g}\) of samples with DMA to inform diffusion models for solvent exposure of the ionomer systems, which are used to study the mechanisms controlling the formation of metal carboxylates, or metal soaps, within a paint film [ 76 ]. More recently, the mechanical properties of oil paints aged 6 years with several additives including fatty acids (both with and without a metal base), linseed oil, and alumina hydrate were observed and quantified using the DMA to studying commercial Winsor & Newton oil paints, commonly used by contemporary artists [ 77 ]. With more linseed oil, the \(T_{g}\) of paints decreased, while alumina hydrate increased the \(T_{g}\) ; the added fatty acids did not have a consistent effect on the \(T_{g}\) [ 77 ]. Quantification of the effects of solvent exposure (immersion and sponge) for a water-sensitive yellow ochre paint using DMA and nanoindentation indicated that full immersion had the higher chance of embrittling the paint films than the sponge cleaning, highlighting the importance of choosing the appropriate method and solvent when preparing to clean a specific paint [ 78 ]. A study that focused on the material response of composite sample was Bridarolli et al. , which assessed the change in \(E^{\prime }\) of consolidants that are commonly used as treatments on degraded canvases as a function of relative humidity to determine their effectiveness and viability for use in conservation treatments [ 59 ].

Most research on paint samples using DMA have emphasized the importance of \(T_{g}\) in assessing the overall response of the paint as well as providing a metric for understanding the type of response (i.e. more glassy/brittle or rubbery/ductile) to expect from paint samples. \(T_{g}\) of a paint is also an important parameter to know when adding an infill to a painting to ensure there are not added stresses created within the paint layers through a mismatch of mechanical properties. Due to the amount and geometry of the sample necessary to perform these measurements, model systems are typically used, introducing challenges when drawing conclusions about the behavior of “young” paint samples (1–15 years) and using them to predict the behavior of older paint samples (100+ years).When time-temperature superposition is used for a DMA experiment, one can obtain more information about a single paint film than from a typical tensile testing experiment. There are some nuances to be aware of when determining the \(T_{g}\) of the paint sample, but the papers in this section provide guidance towards understanding those nuances.

• Shear rheometry

Shear rheometry can be viewed as a version of DMA that is used in a shear geometry and well suited for samples with a liquid character. It has been used to study historic paint formulations, varnish, and glaze recipes, with an emphasis on understanding on how the recipe influenced the overall texture and visual presentation of paintings today. These experiments are typically performed using either the parallel plate geometry, shown in Fig.  9 a, or the cone and plate geometry, shown in Fig.  9 b. Samples studied using these geometries experience shear forces, resulting in the measurement of a complex shear modulus, \(G^{*}\) , analogous to the complex elastic modulus, \(E^{*}\) defined in Sect.  3 . For isotropic materials, which have the same bulk properties in all directions, the shear modulus can be related to elastic modulus through the following expression that also involves Poisson’s ratio, \(\nu\) ,

For liquids and relatively soft polymers at temperatures above their glass transition temperature, \(\nu \approx 0.5\) and \(E\approx 3G\) . In an oscillatory experiment where the complex shear modulus, \(G^{*}\) , is obtained, \(E^{*}\approx 3G^{*}\) , \(E^{\prime }\approx 3G^{\prime }\) and \(E^{\prime \prime }\approx 3G^{\prime \prime }\) =, where \(G^{\prime }\) and \(G^{\prime \prime }\) are the storage and loss shear moduli, defined as described in Sect.  3 , but with \(G^{\prime }\) , \(G^{\prime \prime }\) and \(G^{*}\) replacing \(E^{\prime }\) , \(E^{\prime \prime }\) and \(E^{*}\) .

Schematic of the cone and plate geometry for shear rheometry. The blue portion is the sample, the purple arm is the base plate, and the orange portion is the top plate, which is either a flat plate or a cone. The angle \(\theta\) indicates the angle of the cone, which is typically between 0.25 and \(4^{\circ }\) . The arrows indicate the direction of force applied to the sample. The figure is modified from an image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported

Originally developed for use in industry, shear rheometry has been used for about 70 years to understand the mechanical properties of paints. Due to the longstanding use of the technique, best practices for measuring the mechanical properties of historic recipes have been described in some detail [ 79 , 80 ]. One of the earliest reported discussions about the composition of the paint affecting the rheological behavior of the paint examined the state of white impastos used by Rembrandt, concluding that the proportions of pigment and driers resulted in a thixotropic behavior of the paint, with the viscosity decreasing with increasing strain rate. Not only did such insights provide a greater understanding of Rembrandt’s paint structures, it was shown that these data could be used to relate the artist’s technique to the measured yield stress of the paint [ 81 ]. Further surveys of the effects of adding resins, solvents, and pigments on the viscosity and resulting yield stress of paints, glazes, and varnishes has broadened the knowledge base for thinking about how these historic recipes influenced the response and characteristic appearance that we associate with these materials [ 82 , 83 , 84 ]. More recent research has focused on studying the rheological properties of gumtion, an experimental paint material that added resin to the oils to decrease the drying time of the paint, and the effects of fillers such as calcite on the workability of lead white paints [ 85 , 86 ]. For instance, the amount of pigment Van Gogh needed to create the impasto effects in his paintings was connected to the relationship between the yield stress and pigment concentration [ 16 ]. Rheology has also been used to characterize plant-oil based inks (similar recipes to oil-based paints) to optimize their properties as well as adhesive combinations to maximize their effectiveness as consolidant materials [ 87 , 88 , 89 ]. Thixotropic behavior and the effects of pigment concentrations on the flow of the paint through rheology provides both important conservation data explaining causes of sagging, drippings, or flaking of paintings as well as art historical insights into how historical recipes can be correlated to artist technique.

• Quartz crystal microbalance

Originally used as a technique for studying small mass changes, the QCM has been developed to study the viscoelastic properties of biological and polymer-based systems [ 90 ]. Many of the advancements in determining the viscoelastic properties over the past decade have been developed by the Johannsmann group [ 91 , 92 ]. The QCM measures the impedance spectra of a quartz crystal when an oscillating voltage is applied to electrodes on either surface of the crystal as shown in Fig.  10 . When a sample film is deposited on one surface of the crystal, the resonant frequencies are shifted, allowing one to determine both the mass and mechanical properties of the sample.

Schematic of a quartz crystal used on the QCM. The grey is the AT-cut quartz crystal, which has a resonant frequency of 5 MHz. The gold layers are the electrodes which conduct the oscillating voltage, shown in the circle at the lower right of the diagram. The purple layer is a sample that has been deposited onto the crystal and typically has a thickness less than 10 \(\mu \mathrm {m}\)

Resonant frequencies of quartz crystals are typically in the megahertz (MHz) regime, which means the QCM probes material responses in the “glassy regime” for most polymer and paint systems. Sample preparation is important because the thickness of the sample plays a critical role in the ability to obtain meaningful data. A broader discussion about the range of material properties probed by the QCM and the required thicknesses to detect those properties has been previously published by the Shull group [ 93 , 94 ]. In brief, polymer samples prepared to target viscoelastic properties can range from hundreds of nanometers to 10 \(\mu \mathrm {m}\) in thickness. Such small sample sizes can effectively be treated as a skin layer since there is minimal diffusion required before a sample is equilibrated with its environment. Having an exposed surface of the sample also facilitates measurements of temperature, humidity, and solvated atmospheres, and the use of non-invasive molecular characterization techniques ( e . g ., FTIR, Raman). For paint model systems, data collection can begin within a few minutes of casting, rather than having to wait for months before the sample becomes touch dry, allowing the study of paint model systems through the entire curing process.

Representative QCM data during the drying and curing of a commercial Galkyd alkyd binding medium sample showing a normalized mass, b the complex shear modulus of the third harmonic multiplied by density ( \(|G_{3}^{*}|\rho\) ), and c) the viscoelastic phase angle ( \(\phi\) ) as a function of time. The three regions shown on the plot are representative of the I) solvent evaporation, II) oxygen uptake, and III) long term curing behavior of the alkyd samples. Data reproduced from [ 69 ]

The curing behavior of a commercial alkyd binding medium was measured over the course of three years, as well as the effects of temperature and the filler effects when zinc oxide was added [ 17 , 69 , 95 ]. Analyzing both the mass and the mechanical properties of a sample can provide a clearer picture about how the material responses of paint films change as a function of their environment as demonstrated in Fig.  11 . From these data, Sturdy et al. determined three distinct regions during the curing process of the alkyd paint system: solvent evaporation (region I) taking place in the first few hours of data collection, short term curing where oxygen uptake happens (region II) during the first day, and long term curing over the following three years (region III). It is possible to see the impact of these mass fluctuations on the mechanical properties, with the short term curing region showing the steepest rise in stiffness (shown by the sharp increase in \(|G_{3}^{*}|\rho\) (product of the magnitude of the complex shear modulus at 15 MHz and the film density) and decrease in \(\phi\) (viscoelastic phase angle at 15 MHz) of Fig.  11 b and c). Even after three years, the complex shear modulus of the alkyd samples continued to increase, supporting the understanding that paint systems continue to experience chemical changes years after they are cast onto the quartz crystal. The QCM is versatile enough to incorporate solvent exposure for understanding the swelling behavior of paints, providing the capability to study the effects of plasticization on the overall stability of a paint sample. Samples can also be exposed to UV light to understand the effects on the mechanical properties of paints with photosensitive pigments, and the temperature-dependent response of the the sample can also be probed [ 93 ]. These temperature-dependent measurements are a particular focus of ongoing research because they probe molecular relaxations in the material that are likely coupled to the ductile/brittle transition in the material, complementing information obtained from more traditional measurements of the glass transition temperature. While these samples can be exposed to a wide range of environmental conditions, it is dependent on good contact between the sample surface and the quartz crystal, making it difficult to use the QCM to study historic paint samples directly. These studies provide initial steps for being able to understand more fully the initial curing stages of binding media, how they affect the overall paint material response, and begin to investigate more targeted questions related to curing and degradation processes in paints.

• Nanoindentation

Technique background

All the techniques described up to this point in the review require the use of model systems in order to obtain the mechanical properties of paints. These techniques provide useful information and can allow a wider range of experiments to be performed (especially those that are more destructive in nature), but there can be difficulties relating these results back to actual historic paints. One technique well suited to bridge the gap between model systems and historic paint samples is nanoindentation. The small sample size required to obtain data about the modulus, stiffness, and hardness of a paint sample is well suited for use on cross sections from paintings. Typically, the indentations are ≈ 2-5 μm wide, making them nearly invisible to the naked eye. Oliver and Pharr have outlined the methodology for contact mechanics of elastic-plastic materials, which has been widely incorporated into nanoindentation analysis [ 96 ]. Oyen and Cook summarized how contact mechanics analysis can be expanded to include a wider range of tip shapes, two of which are shown in Fig.  12 , and accommodate material responses that are viscous, expanding contact mechanics to use on biological and polymeric based materials [ 97 ].

Geometry of a a Berkovich tip and b a spherical tip commonly used in indentation experiments on paint materials. The angle, \(\alpha\) , is \(65.35^{\circ }\) for a standard Berkovich tip. The figure is based on images available in the public domain through Wikimedia Commons

Typical load-displacement curve for indentation of the polyester resin used to embed the paint samples, labeled to illustrate the values of \(P_{max}\) , \(\delta _{max}\) and S

Material properties are obtained from load-displacement curves, such as the one shown in Fig.  13 . Note that a displacement of zero is defined as the point where initial contact is made between the indenter and the material. Because most paint materials have a time-dependent response, the detailed protocol control for controlling the displacement will affect the measurement. The load-displacement curve shown in Fig.  13 shows an experimental protocol where the displacement increased at a constant rate from zero to a maximum value of \(\delta _{max}\) during a loading time of \(\approx\) 1s. The displacement is then fixed at this maximum value for \(\approx 1\) minute, and is then decreased at the same rate as that used for the loading portion of the experiment. It is also common to have the holding condition be based on a fixed load. The elastic modulus is obtained from the unloading stiffness, S , defined as the slope of the initial portion of the unloading curve (see Fig.  13 ). For a parabolic indenter where the contact between the indenter and the material has a radius of a , the following expression for S is used to extract the elastic properties:

Here \(E_{r}\) is the reduced modulus obtained from plane strain geometries like the indentation experiments of interest here. It involves the following combination of the elastic modulus and Poisson’s ratio:

The quantity \(a_{max}\) is the contact radius radius established for \(\delta =\delta _{max}\) , which for a parabolic indenter is given by the following expression:

where R is the radius of curvature of the indenter at its tip.

For non-parabolic indenters and those for situations where material yielding needs to be accounted for (as with the Berkovich indenters shown in Fig.  12 a, a generalization of Eqs. 7 and  9 needs to be utilized. The most commonly used procedure was developed by Oliver and Pharr [ 96 ], and involves the following:

Replacement of \(a_{max}\) with an effective radius, \(a_{eff}\) . The area of a circle with this radius is the same as the actual maximum contact area between the indenter and the material:

Definition of a contact depth, \(\delta _{c}\) , which accounts for deformation of the material outside the contact zone. The Oliver and Pharr expression for \(\delta _{c}\) is:

Use of a relationship between \(A_{max}\) and \(\delta _{max}\) which depends only on the indenter geometry. For a Berkovich indenter this relationship is:

For a parabolic indenter of with tip radius of curvature R , this relationship is:

These equations enable the maximum contact radius (or the effective maximum contact radius, \(a_{eff}\) ) to be obtained from load-displacement relations directly without direct visualization of the contact radius, a determination of the reduced modulus, \(E/\left( 1-\nu ^{2}\right) .\)

The other property that is often obtained from a nanoindentation experiment is the hardness, H , given by the ratio of the load to the projected contact area:

While a value of H can be obtained from Eq.  14 for any tip shape, it is usually used with tips with sharp edges, like the Berkovich tip. In these cases the material deforms plastically in the regions near the indenter edges, and the hardness is taken as a measure of the yield stress of the material. In these cases indentation necessarily involves irreversible deformation of the material, and limits measurements of historic samples to regions of the paint that are not visible (small portions taken from underneath the frame, for example). Indentation probes a volume of the sample with dimensions controlled by  \(a_{max},\)  and this length scale controls both the depth that is sampled in the measurement and the lateral resolution that is obtained. This length scale is in turn controlled by the size of the indenter tip and the depth of the indent in the sample, and is much smaller for atomic force microscopy (AFM) than for nanoindentation.

Advancements for artists’ paints

In cultural heritage research, nanoindentation has been used to correlate changes of mechanical properties with optical and chemical changes as a function of aging or light exposure for natural and synthetic binding media [ 98 ]. Comparisons between reconstructed and historic paint samples of white pigments used by Van Gogh demonstrated that the age of the sample had a direct effect on the resulting reduced modulus (with the modulus of historic samples being up to an order of magnitude higher than the reconstructed samples), highlighting one of the main issues for studying model systems [ 99 ]. However, the authors of this study also concluded that it can be difficult to determine an accurate modulus for a paint due to the filler behavior of pigment particles in the paint samples. The presence of these stiffer pigment particles causes significant increases in the reduced modulus near the particles. More recent studies using nanoindentation have incorporated comparisons of the data from nanoindentation with data from DMA and dynamic vapor sorption to understand how each technique showed the changes in mechanical properties when subjected to variations in RH [ 100 ]. Measurements on the same zinc oxide filled alkyd samples with DMA, QCM and nanoindentation have confirmed the compatibility of the three techniques when the frequency of the measurement is appropriately taken into account [ 17 ]. Fujisawa et al. add a compliance term to their determination of mechanical properties to account for edge effects of the stiffer embedding resin on a paint sample [ 101 ]. A more systematic study with nanoindentation focused on understanding edge effects for cross sections mounted in resin, showing how these effects impact the data obtained from the experiments [ 102 ]. Since paint samples are typically mounted in resin materials that can exhibit a higher modulus, Freeman et al. explored the minimum size required of cross sections to be able to minimize an artificial stiffening affect from the resin substrate. Initial steps have been taken towards mapping the mechanical properties of an entire painting cross section, which will be an important method for being able to probe the mechanical properties of historic paints more readily [ 103 ]. More work remains to be done in order to refine this mapping technique and adapt it to paintings. Resolving effects of solvent cleaning for a water sensitive oil paint were studied with nanoindentation as well as DMA to determine how much surface and bulk mechanical properties would be effected, where most of the data for sponge cleaned samples exhibited statistically insignificant changes [ 78 ].

In tandem with expanding the use of nanoindentation through mapping and optimizing the analysis of cross sections, atomic force microscopy has the potential to be applied to painting cross sections to obtain mechanical properties. Since the AFM tip is smaller than the tip of a nanoindenter, it can provide a higher resolution of the surface mechanical property data across the paint sample. One issue that needs to be considered with the AFM is that for very small contact dimensions the effect of adhesive interactions become more important. When these effects are appropriately accounted for, the AFM can be used to quantify the viscoelastic properties of a material. The approach has been validated for an acrylic system [ 104 , 105 ], and similar approaches can in principle be used to investigate the properties of aged paint systems. A variety of complications exist when applying the technique to heterogeneous systems like actual artist’s paints [ 106 ], but the availability of commercial AFM instrumentation with the ability to obtain load/displacement curves over a small region of the sample may prove to be useful in the field of cultural heritage.

Developing research directions

• Laser shearography

One technique developed in the last couple of decades to observe changes in the mechanical properties of paintings is laser shearography. As shown in the schematic in Fig.  14 , interference patterns of a painting are collected before and after a slight thermal, acoustic, or vibrational excitation caused by the diffuse light sources in the image. The interference patterns are used to create a phase map of the painting surface, which can be unwrapped to generate a displacement gradient map, also known as a strain map.

Schematic of a shearography setup for measuring slight thermal changes in paintings as a result of a diffuse light source. The laser light from the optical head is used to obtain the phase maps of the surface before and after the thermal loading of the painting. The phase shifting is achieved using a piezoelectric transducer driver (PZT driver) and an analog output card (DAQ). Modeled after and adapted from Klausmeyer et al. [ 107 ]

Kalms et al. focused on developing a setup that would be mobile enough to easily set up in a museum environment and be stable enough to provide interferograms that were sensitive to changes in the painting after thermal loading, but not sensitive to slight environmental changes in the surrounding conditions [ 108 , 109 ]. The sensitivity of the phase maps obtained through laser shearography helps researchers observe defects such as cracks, paint losses, and delaminations more easily, especially if the defects are subsurface. The maps are also capable of serving as a diagnostic tool for cultural heritage, guiding attention to areas that need more immediate attention during a conservation survey. The sensitivity of laser shearography to detect delaminations, impacts on a painting surface, and the propagation of cracks through paint layers was successfully demonstrated with mockup samples [ 110 ]. The use of strain maps to show the potential areas of concern and highlight areas of potential degradation in the paintings have been optimized for ease of interpretation by conservators [ 111 ]. Shearography has also been paired with other imaging techniques such as terahertz imaging, fringe projections to observe 3D topographic maps, and reflectance transformation imaging to provide more complete datasets for diagnosing defects and changes in a painting after environmental fluctuations or transport [ 107 , 112 , 113 ]. Some optimization of strain maps from the displacement gradients was also performed on a test painting to quantify the strain experienced by a painting as a result of cycling lights in a museum setting [ 107 , 114 ]. Comparing the phase maps of defects in paintings to simulated phase maps generated using finite element analysis (see Section 7.2), allow researchers to understand the nature of defects within a painting and to vary the parameters of the defect until the simulated map matches the experimentally observed map, which can be useful for understanding how different defects/areas of degradation within a painting can affect the resulting phase map [ 115 ].

Laser shearography shows great promise as a diagnosis and monitoring tool for performing preliminary conservation surveys and tracking the state of a painting, especially after transport for an exhibition. The mobility of the setup allows it to be fairly cost effective for a museum setup. While it takes some time to gain the experience to interpret the data from the phase maps, the strain maps are a useful tool for understanding the mechanical response of the painting to slight fluctuations in its environment.

• Finite element analysis

When a paint layer is considered in the context of a painting, there can be range of complex stresses applied to the layer by the neighboring layers and quantifying the stresses can be difficult using free standing films. One way to look at understanding these stresses is to use finite element analysis (FEA), which is capable of modeling the field responses (such as stresses or strains) for complicated objects. A brief description of the process for using FEA is that the geometry is discretized into smaller elements, a simplified material model is chosen that resembles the expected response from the object, boundary and load conditions are applied, then the solutions are calculated to determine the field responses to the given load. When the geometry is defined, material properties need to be inputted, which typically include the elastic modulus, Poisson’s ratio, and hygric or thermal expansion coefficients, depending on the type of loading the object will experience. These material properties are usually informed from experimental data using one of the techniques previously discussed in this review. How the material will respond to a load also needs to be defined; the most common responses are isotropic (the same response in all directions) for paints or grounds and orthotropic (some directional dependence) for wood and some canvas substrates. The ability to break down the object geometry into smaller elements allows easier comparison of the stress/strain response on the micro and macroscopic scale, which is one of the strengths of this technique. Selection of a material model is determined by the expected response from a material. The two models that have been applied to cultural heritage objects are a linear elastic model, assuming a fairly elastic response of the material, or a visco-hyperelastic model, which allows for more time dependent deformation of the object [ 123 ]. The boundary and load conditions are used to constrain the geometry and explicitly define the scenario being modeled, for example, a strain being applied as a displacement along one axis of the model. The results from a simulation are typically displayed as field maps, such as the stress field depicting the horizontal stresses in a gesso ground layer shown in Fig.  15 . For this particular example, the highest areas of stress are immediately next to the crack tip, which is functioning as a stress concentrator within this layer. From these stress fields, the energy release rate can be determined by integrating the area around the crack tip. Comparing the value of the energy release rate to values of \({\mathcal {G}}_{c}\) obtained from measurements on model materials will indicate if the crack would continue to propagate under these conditions.

A stress field surrounding a crack tip depicting the horizontal stresses in a gesso ground layer, indicating that the highest stresses in the ground layer are occurring at the crack tip, which is functioning as a stress concentrator. The FEA simulation used to create this stress field is based on the simulations used in Bratasz et al. [ 116 ]

FEA is a very powerful method and allows the comparison of a range of material properties and scenarios in a relatively short time compared to preparing that range of experiments, there are also some things to keep in mind when reviewing the results of a simulation. The outputs from a simulation are heavily dependent on the material properties inputted, which can make it challenging to extend the use of a simulation beyond its initial goal. The results from a FEA model need to be validated against either experimental values (e.g. the expected results from the experiment that provided the material property inputs) or against analytical models. A balance also needs to be struck between the computational expense of running the model with its overall accuracy, which can make it difficult to model some non-linear behavior that would be expected from paints and paintings.

Application to cultural heritage

An area of cultural heritage research that has begun using FEA more often focuses on understanding craquelure development and the mechanisms governing crack opening or delamination. For the papers discussed in this section, all use a linear elastic model as their material model unless otherwise specified. Mecklenburg’s work using FEA focused on the understanding of the response of paintings to T or RH fluctuations, out of plane bending of the canvas due to vibrations from transport, and an impact force on the side and corner of a painting to simulate a painting being dropped, comparing the resulting simulated craquelure patterns to ones observed in painting mock ups [ 29 , 61 , 117 ]. Further studies of vibrations affecting canvas paintings have been carried out to assess the vibrational modes that are most relevant to consider [ 118 ]. Bratasz et al. used a more targeted FEA approach to study the behavior of gesso ground layers on a wood substrate, focusing on how the response of the ground layer translates to the development of a craquelure pattern that is less susceptible to environmental fluctuations [ 116 , 119 ]. Eumelen et al. have investigated the growth behavior of metal soaps combined the chemical information related to the diffusion of metal soaps into aggregates with estimated mechanical properties for a paint layer to both understand the directionality of metal soap formation and the most likely regions for cracks to form within the model from the stresses caused by the aggregates [ 120 ]. More recent investigations from Bosco et al. have been focused on understanding the underlying deformation mechanisms that can lead to crack opening or delamination when a paint layer is exposed to flexural stresses as a result of increased moisture [ 121 , 122 ].

An additional area where FEA can be applied for cultural heritage research is to simulate long term fatigue behavior. The simulations can study the potential for cyclic fatigue that would cause failure or delamination of a paint layer, which was performed using a visco-hyperelastic material model since the timescales of fatigue responses tend to be more viscoelastic in nature [ 123 ]. Computational simulations studying fatigue behavior in paintings provide the flexibility to test multiple parameters and draw preliminary conclusions on a faster timescale than can be done with experiments, which is useful for informing conservator practice on a wide range of paintings varying in composition and structural support in a relatively short time. FEA can also be used to generate displacement fields as a result of thermal excitation, as was shown by Buchta et al. when observing defects using laser shearography [ 115 ].

While being able to test a wide range of parameters using FEA simulations is a strong benefit of the technique, there is a major caveat: one needs to insure the results are physically relevant. Accurate determination of the critical energy release rate, \({\mathcal {G}}_{c}\) , mentioned above, is one example. Informing the models using material properties for the paints or paintings of interest is a key step towards validating and extending the model.

• Vibration studies

Another potential source of damage to paintings is from vibrations causing flaking or crack formation, whether from transport conditions, acute scenarios such as a construction project, or more diffuse ambient noise in or around a museum. When these vibrations occur, they tend to apply an oscillating stress over a longer period of time, which can lead to longer term fatigue responses of the painting. Conservative limits for vibrations during a construction along with a proposed plan for assessment of a construction project was outlined thoroughly by Johnson et al. to determine a vibration range of 1.5–4.0 mm/s depending on the frequency of the vibration [ 124 ]. Assessing these sources of vibration and understanding the effects on the longevity of objects in the museum has been assessed through the detection of background vibrations in museum as well as exposure of model systems to sustained periods of vibration to determine thresholds for damage to objects. Wei et al. assessed the affects of these background vibrations within a museum setting to understand some thresholds of safety for paintings and objects, determining it to be somewhat lower than expected [ 125 ]. A particular case study where vibrations are more problematic are pastel paintings, or paintings where powdery pigment layers are applied to the support. Due to the low level of adhesion between the paint layers and support, pastel paintings are a useful case study for assessing the potential limit of material loss due to vibrations before the painting is noticeably damaged [ 126 ].

The a schematic of a vibration fatigue stage setup for mock up pastel paintings and b a modified Wöhler curve showing the vibration level versus the number of cycles of vibrations. The dashed curve is the fatigue limit for the mock up, above which (and emphasized by the shaded region of the plot) the pastel mock up would be damaged by the vibrations. The schematic of the setup is based off the setup used in [ 127 ]

Shown in Fig.  16 , a biaxial tension setup with mockup canvases is used to generate modified Wöhler curves (displacement of material versus number of cycles) to correlate the amount of vibration sustained by an object with the overall loss of material and minimum amount of acceptable loss from pastel paintings as determined by imaging and conservator observation [ 126 ]. Within this setup, a laser interferometer is used to track the amplitude of the vibrations and the camera is used to monitor the overall surface damage of the canvas. These fatigue tests can provide useful information about the amount of vibration that can be sustained before damage, which when paired with information about display and travel of an object, can help curators and conservators determine the best lifetime decisions to maintain the best conditions for a pastel painting as long as possible. While pastel paintings are more sensitive to damage by vibration than other paintings, these types of fatigue tests can be used to assess the chances of new craquelure formation in an object, especially after travel or acute exposure to sustained vibrations. Efforts in this area have focused on understanding the effects of pigment size on the stability of the pastel painting, incorporating high speed imaging to understand the resonant vibration modes of a canvas more fully, and working on more directly relating the data from mockup samples to actual museum objects. The latter point would allow the use of Wöhler curves as a predictive tool for determining if moving a pastel is within a safe limit.

• Single-sided nuclear magnetic resonance

Paintings that undergo solvent exposure for cleaning treatments tend to experience embrittlement of the surface paint layers. A more thorough review discussing the chemical changes and implications has been recently published [ 128 ]. A fairly novel way of studying paintings has been explored by researchers to correlate the mechanical properties with chemical changes in paint is single-sided nuclear magnetic resonance (NMR), or NMR relaxometry [ 129 ]. This technique uses a NMR-MOUSE (Mobile Universal Surface Explorer) to look at the relaxation times within a layer using a locally generated magnetic field and detector. Transverse relaxation times ( \(T_{2}\) ), related to spin-spin interactions of the material being studied, can be recorded in depth profiles and can be related to the stiffness of a material. If a material has reduced intermolecular motion (such as a cross-linked paint material), \(T_{2}\) will decrease as the stiffness increases [ 129 ]. Single-sided NMR has been employed to look at both PVC and the effects of solvent on painting materials. Model systems of traditional oil paints, water-miscible oil paints, and acrylic paints with varying PVC values were studied to determine if the comparative results could be quantified as a function of increasing PVC [ 14 , 129 , 130 , 131 , 132 ]. One of these studies looked to extend their findings with young model systems to see if they could develop a protocol for determining the age of paint materials based on the response of the relaxations times, which was unfortunately inconclusive [ 131 ]. Single-sided NMR has also been used to study the effects of solvent exposure on the stiffness of paints. The first study assesses previous solvent treatments of a 17th century painting through comparison with another similar painting from the Pipenpoyse Wedding Set , demonstrating that a cleaning treatment performed on one of the paintings had a statistically significant effect on the stiffness, most likely due to a redistribution of low molecular weight molecules through the paint layer.[ 133 ]. A more recent study used both single-sided NMR and solid phase microextraction to investigate the effectiveness of swab or gel cleaning of more green solvents for painting cleaning and is able to demonstrate that the gel method produces a reduced paint response and showed less penetration into the paint layer than the swab method [ 134 ]. While this technique does not provide absolute information about the stiffness of the paint layer, single-sided NMR can provide relevant information about stiffness changes over the depth profile of a paint layer, which can be especially useful when investigating the solvent response of a paint material.

Conclusions

When considering the mechanical properties of paints, there are quite a few factors that need to be considered. It is important to understand the material properties of both the binding medium and the pigment, which can be partially informed from chemical analysis, as well as the pigment concentration and how it affects the overall properties of the paint. Since paintings are multi-layered structures, there also have to be considerations for the interactions (both chemical and mechanical) between layers. Beyond the paint composition and structure, the environmental factors (e.g., T, RH, vibrations, and solvent exposure) can affect the response of the paint and need to be accounted for when considering a conservation treatment.

Several techniques have been developed over recent decades to help provide this information. Tensile testing, DMA, rheology, and the QCM can provide information about the mechanical properties of paints from the liquid to the solid state, creating a data set that is more comprehensive and complete than any dataset provided by a single technique. These techniques are limited to model systems, but with the correct assumptions and comparisons to historic samples, the data can be used to inform conservation treatments. Techniques that are directly applicable to historic objects have been developed and adapted from industrial applications for cultural heritage, which expands the breadth of knowledge accessible to conservators. Nanoindentation can provide an estimate of the stiffness of a paint cross section and allow a conservator to explore the temperature and solvent response of the cross section before beginning a treatment. Laser shearography, since it is both portable and non-invasive, is a technique that will be most useful for both initial surveys of a new painting in the collection (pinpointing areas most in need of deeper investigation) and for a painting traveling on exhibition (to track the amount of damage accumulated as a result of travel). Computational studies using FEA can allow researchers to consider both the macroscopic and microscopic length scales affecting the mechanical properties of a paint simultaneously as well as explore a wide parameter space more rapidly than experiments, given that the models are well informed and verified via theories or experimental comparison. Studies analyzing the vibration response of model systems and generating predictive curves that can be used to assess objects in a museum, especially more sensitive objects such as pastel paintings, give some insights into how much travel and exhibition an object can endure before it needs restoration or to be retired. Single-sided NMR is emerging as a non-invasive technique that can provide useful depth profiles of comparative stiffness measurements, which can provide information about PVC orprevious solvent exposure of a painting sample.

To continue progressing the field and to provide relevant information for conservators, research efforts expanding the breadth of pigments investigated with these techniques will provide data to compare to when determining a conservation treatment for an object. Information about paint samples can also be expanded by combining any of the techniques discussed in the paper, requiring an understanding of how the techniques can complement each other. Extrapolating expected material responses of historic paints from relatively young model systems continues to be a challenge for researchers and requires systematic efforts to look at aging trends and see if they can be extrapolated to longer times for a wider range of paint materials. Expanding the efforts of performing optical or in situ measurements of paintings to obtain mechanical properties is very appealing, especially when surveying objects for conservation treatment [ 135 ]. One potential way to push this direction forward is to collaborate with industry, adapting their existing technology to work for paintings [ 136 ]. The choice of which techniques are most relevant to use is also two potential research aims: to answer technical art history questions ( e.g. , such as determining an artist’s historic formulation) and to expand the knowledge base of relevant materials in ways that benefit the conservation community. The purpose of this review has been to describe past work while highlighting newer capabilities that we expect to be useful in addressing these aims, in hope that the interdisciplinary collaborations enabling success would continue to flourish.

Availability of data and materials

The uniaxial tensile testing datasets analyzed during the current study are available through supplementary information as both a PDF file (Additional file 1 ) and an Excel file (Additional file 2 ). All other data sets presented and the codes used to generate figures are available from the authors upon request.

To help understand and look at overarching trends in the mechanical properties of paints studied using uniaxial testing, we compiled a summary of the reported mechanical property data from studies focusing on oil, alkyd, and acrylic paints as well as some paint consolidants and grounds. See Additional files 1 and 2 in the Supplementary Information.

For all of the ranges presented for changes in stiffness as a result of a change in a testing parameter, the range has been estimated across all binding media and sample compositions in an attempt to show the maximum impact a particular parameter can have on the stiffness.

Abbreviations

Temperature

Relative humidity

\(\mu\) -X-ray diffraction

Particle induced X-ray emission

Backscattering spectrometry

Fourier transform infrared spectroscopy

Pigment volume concentration

Atomic force microscopy

Nuclear magnetic resonance

Nuclear magnetic resonance-mobile universal surface explorer

van Gorkum R, Bouwman E. The oxidative drying of alkyd paint catalysed by metal complexes. Coord Chem Rev. 2005;249(17):1709–1728. http://www.sciencedirect.com/science/article/pii/S0010854505000342 .

Juita DBZ, Kennedy EM, Mackie JC. Low temperature oxidation of linseed oil: a review. Fire Sci Rev. 2012;1(1):3. https://doi.org/10.1186/2193-0414-1-3 .

Article   CAS   Google Scholar  

Soucek MD, Khattab T, Wu J. Review of autoxidation and driers. Prog Org Coat. 2012;73(4):435–454. https://linkinghub.elsevier.com/retrieve/pii/S0300944011002529 .

Honzíček J. Curing of air-drying paints: a critical review | industrial and engineering chemistry research. Ind Eng Chem Res. 2019;58(28):12485–505. https://doi.org/10.1021/acs.iecr.9b02567 .

Michalski SW. Paintings—their response to temperature, relative humidity, shock, and vibration. In: Mecklenburg M, editor. Art in transit: studies in the transport of paintings. Washington, D.C.: National Gallery of Art; 1991. https://www.academia.edu/741944/1991_Paintings_-_Their_Response_to_Temperature_Relative_Humidity_Shock_and_Vibration .

van Loon A, Noble P, Burnstock A. Ageing and deterioration of traditional oil and tempera paints. In: Conservation of easel paintings. 1st edn. Routledge series in conservation and museology. London/New York: Routledge; 2012. p. 214–41.

Learner T. Modern paints. In: Conservation of easel paintings. 1st edn. Routledge series in conservation and museology. London/New York: Routledge; 2012. p. 242–251.

Hermens E, Townsend JH. Binding media in western easel painting. In: Conservation of easel paintings. 1st edn. Routledge series in conservation and museology. London/New York: Routledge; 2012. p. 207–213.

Rothe A. Andrea Mantega’s “Adoration of the Magi”. In: Historical painting techniques, materials, and studio practice. Preprints from the Symposium at the University of Leiden, the Netherlands. The Getty Conservation Institute; 1995. p. 111–116.

Sato K. The mechanical properties of filled polymers. Prog Org Coat. 1976;4(4):271–302. http://www.sciencedirect.com/science/article/pii/0300944076800100 .

Perera DY. Effect of pigmentation on organic coating characteristics. Prog Org Coat. 2004;50(4):247–262. http://www.sciencedirect.com/science/article/pii/S0300944004000694 .

de Viguerie L, Beck L, Salomon J, Pichon L, Walter P. Composition of renaissance paint layers: simultaneous particle induced X-ray emission and backscattering spectrometry. Anal Chem. 2009;81(19):7960–6. https://doi.org/10.1021/ac901141v .

Wiesinger R, Pagnin L, Anghelone M, Moretto LM, Orsega EF, Schreiner M. Pigment and binder concentrations in modern paint samples determined by IR and Raman spectroscopy. Angewandte Chemie Int Edn. 2018;57(25):7401–7. https://doi.org/10.1002/anie.201713413 .

Liu X, Di Tullio V, Lin YC, De Andrade V, Zhao C, Lin CH, et al. Nano- to microscale three-dimensional morphology relevant to transport properties in reactive porous composite paint films. Sci Rep. 2020;10(1):18320. https://www.nature.com/articles/s41598-020-75040-6 .

Hagan E, Charalambides M, Learner TJS, Murray A, Young CT. Factors affecting the mechanical properties of modern paints. In: Modern paints uncovered: proceedings from the modern paints uncovered symposium. Los Angeles: Getty Conservation Institute; 2007. p. 227–235.

Salvant Plisson J, de Viguerie L, Tahroucht L, Menu M, Ducouret G. Rheology of white paints: how van gogh achieved his famous impasto. Colloids Surf A Physicochem Eng Aspects. 2014;458:134–141. http://www.sciencedirect.com/science/article/pii/S092777571400212X .

Sturdy LF, Wright MS, Yee A, Casadio F, Faber KT, Shull KR. Effects of zinc oxide filler on the curing and mechanical response of alkyd coatings. Polymer. 2020;191:122222. http://www.sciencedirect.com/science/article/pii/S0032386120300641 .

Hagan EWS, Charalambides MN, Young CRT, Learner TJS, Hackney S. Viscoelastic properties of latex paint films in tension: influence of the inorganic phase and surfactants. Prog Org Coat. 2010;69(1):73–81. http://www.sciencedirect.com/science/article/pii/S0300944010001566 .

Hagan EWS, Charalambides MN, Young CRT, Learner TJS, Hackney S. Micromechanics models for predicting tensile properties of latex paint films. In: Proulx T, editor. Time dependent constitutive behavior and fracture/failure processes, Volume 3. Conference proceedings of the society for experimental mechanics series. New York, NY: Springer; 2011. p. 297–306.

Leonardi R. Nuclear physics and painting: sub-topic of the wide and fascinating field of science and art. Nucl Phys A. 2005;752:659–674. http://www.sciencedirect.com/science/article/pii/S0375947405002241 .

Keck S. Mechanical alteration of the paint film. Stud Conserv. 1969;14(1):9–30.

Google Scholar  

Bucklow S. The description of craquelure patterns. Stud Conserv. 1997;42(3):129–40. https://doi.org/10.1179/sic.1997.42.3.129 .

Article   Google Scholar  

Bucklow SL. A stylometric analysis of craquelure. Comput Humanit. 1998;31:503–21.

Bucklow S. Classification of craquelure. In: Hill Stoner J, Rushfield R, editors. Conservation of easel paintings: principles and practice. London/New York: Butterowrth-Heinemann; 2012. p. 285–90.

Michalski SW. Crack mechanisms in gilding. In: Gilded wood: conservation and history. Madison, Connecticut: Sound View Press; 1991. p. 171–181. https://www.academia.edu/741952/1991_Crack_mechanisms_in_gilding .

Giorgiutti-Dauphiné F, Pauchard L. Painting cracks: a way to investigate the pictorial matter. J Appl Phys. 2016;120(6):065107. http://aip.scitation.org/doi/10.1063/1.4960438 .

Pauchard L, Giorgiutti-Dauphiné F. Craquelures and pictorial matter. J Cult Herit. 2020;46:361–373. https://www.sciencedirect.com/science/article/pii/S1296207420304398 .

Tumosa CS, Millard J, Erhardt D, Mecklenburg MF. Effects of solvents on the physical properties of paint films. Preprints, ICOM Committee for Conservation, 12th Triennial Meeting, Lyon. 1999;1:347–352. http://repository.si.edu/xmlui/handle/10088/35962 .

Mecklenburg MF, Tumosa CS. An introduction into the mechanical behavior of paintings under rapid loading conditions. In: Art in transit: studies in the transport of paintings. Washington D.C.: National Gallery of Art; 1991. p. 137–171.

Erlebacher JD, Brown E, Mecklenburg MF, Tumosa CS. The effects of temperature and relative humidity on the mechanical properties of modern painting materials. In: Materials issues in art and archaeology III. vol. 267. San Francisco: Materials Research Society; 1992. p. 359–370. http://repository.si.edu/xmlui/handle/10088/35967 .

Mecklenburg MF, Tumosa CS, Erlebacher JD. Mechanical behavior of artist’s acrylic paints under equilibrium conditions. In: Division of polymer chemistry. vol. 35. Washington, D.C.: American Chemical Society; 1994. p. 297–298. http://repository.si.edu/xmlui/handle/10088/10305 .

Whitmore PM, Colaluca VG. The natural and accelerated aging of an acrylic artists’ medium. Stud Conserv. 1995;40(1):51–64. https://www.jstor.org/stable/1506611 .

Mecklenburg MF, Tumosa CS. Traditional oil paints: the effects of long-term chemical and mechanical properties on restoration efforts. MRS Bull. 2001;26(01):51–54. http://www.journals.cambridge.org/abstract_S0883769400023058 .

Carr DJ, Young CRT, Phenix A, Hibberd RD. Development of a physical model of a typical nineteenth-century English canvas painting. Stud Conserv. 2003;48(3):145–54. https://doi.org/10.1179/sic.2003.48.3.145 .

Mirone G, Marton B, Vancso GJ. Elastic modulus profiles in the cross sections of drying alkyd coating films: modelling and experiments. Eur Polym J. 2004;40(3):549–560. https://linkinghub.elsevier.com/retrieve/pii/S0014305703002726 .

Mecklenburg MF, Tumosa CS, Erhardt D. The changing mechanical properties of aging oil paints. In: Vandiver PB, Mass JM, Murray A, editors. Materials issues in art and archeology VII. Warrendale, PA: Materials Research Society; 2004. p. 13–24. http://repository.si.edu/xmlui/handle/10088/36019 .

Hagan E, Murray A. Effects of water exposure on the mechanical properties of early artists’ acrylic paints. In: Materials issues in art and archaeology VII. vol. 852. Boston: Materials Research Society; 2004. p. 41–47.

Erhardt D, Tumosa CS, Mecklenburg MF. Long-term chemical and physical processes in oil paint films. Stud Conserv. 2005;50(2):143–50. https://doi.org/10.1179/sic.2005.50.2.143 .

Tumosa C, Erhardt D, Mecklenburg M, Su X. Linseed oil paint as ionomer: synthesis and characterization. In: Materials issues in art and archaeology VII. vol. 852. Boston: Materials Research Society; 2005. p. 25–31.

Young C. interfacial interactions of modern paint layers. In: Modern paints uncovered: proceedings from the modern paints uncovered symposium. Los Angeles: Getty Conservation Institute; 2007. p. 247–256.

Young C, Hagan E. Cold temperature effects of modern paints used for priming flexible supports. In: Preparation for painting: the artists’ choice and its consequences. London: Archetype Publications; 2008. p. 172–179.

Fuesers O, Zumbühl S. the influence of organic solvents on the mechanical properties of alkyd and oil paint. In: IR, NIR, VIS UV Raman. Jerusalem, Israel; 2008. p. 1–14.

Ormsby B, Hagan E, Smithen P, Learner TJS. Comparing contemporary titanium white-based acrylic emulsion grounds and paints: characterisation, properties and conservation. In: preparation for painting: the artists’ choice and its consequences. London: Archetype Publications; 2008. p. 163–171.

Fuster-López L, Mecklenburg M, Castell-Agusti M, Guerola-Blay V. Filling materials for easel paintings: when the ground reintegration becomes a structural concern. In: Preparation for painting: the artists’ choice and its consequences. London: Archetype Publications; 2008. p. 180–186.

Carlyle L, Young C, Jardine S. The mechanical response of flour-paste grounds. In: Preparation for painting: the artist’s choice and its consequences. London: Archetype Publications; 2008. p. 123–131.

Hagan EWS, Charalambides MN, Young CT, Learner TJS, Hackney S. Tensile properties of latex paint films with TiO2 pigment. Mech Time Depend Mater. 2009;13(2):149–61. https://doi.org/10.1007/s11043-009-9076-y .

Tumosa CS, Mecklenburg MF. Oil paints: the chemistry of drying oils and the potential for solvent disruption. In: New insights into the cleaning of paintings: proceedings from the cleaning 2010 international conference, Universidad Politecnica de Valencia and Museum Conservation Institute. No. 3 in Smithsonian Contributions to Museum Conservation. Washington, D.C.: Smithsonian Scholarly Press; 2013. p. 51–58.

Mecklenburg M, Tumosa CS, Vicenzi E. The influence of pigments and ion migration on the durability of drying oil and alkyd paints. In: New insights into the cleaning of paintings: proceedings from the cleaning 2010 international conference, Universidad Politecnica de Valencia and Museum Conservation Institute. No. 3 in Smithsonian Contributions to Museum Conservation. Washington, D.C.: Smithsonian Scholarly Press; 2013. p. 59–67.

Doménech-Carbó MT, Silva MF, Aura-Castro E, Doménech-Carbó A, Fuster-López L, Gimeno-Adelantado JV, et al. Multitechnique approach to evaluate cleaning treatments for acrylic and polyvinyl acetate paints. In: New insights into the cleaning of paintings: proceedings from the cleaning 2010 international conference, Universidad Politecnica de Valencia and Museum Conservation Institute. No. 3 in Smithsonian Contributions to Museum Conservation. Washington, D.C.: Smithsonian Scholarly Press; 2013. p. 125–134.

Hagan EWS, Charalambides MN, Young CRT, Learner TJS. The effects of strain rate and temperature on commercial acrylic artist paints aged one year to decades. Appl Phys A. 2015;121(3):823–35. https://doi.org/10.1007/s00339-015-9423-6 .

Fuster-López L, Izzo FC, Piovesan M, Yusá-Marco DJ, Sperni L, Zendri E. Study of the chemical composition and the mechanical behaviour of 20th century commercial artists’ oil paints containing manganese-based pigments. Microchem J. 2016;124:962–973. http://www.sciencedirect.com/science/article/pii/S0026265X15001964 .

Krzemień L, Łukomski M, Bratasz Ł, Kozłowski R, Mecklenburg MF. Mechanism of craquelure pattern formation on panel paintings. Stud Conserv. 2016;61(6):324–30. https://doi.org/10.1080/00393630.2016.1140428 .

Penava Ž, Penava DŠ, Tkalec M. Experimental analysis of the tensile properties of painting canvas. Autex Research Journal. 2016;16(4):182–195. https://content.sciendo.com/view/journals/aut/16/4/article-p182.xml .

Hagan EWS. Thermo-mechanical properties of white oil and acrylic artist paints. Prog Org Coat. 2017 Mar;104:28–33. http://www.sciencedirect.com/science/article/pii/S0300944016302363 .

Fuster-López L, Mecklenburg M. A look into some factors influencing the film forming properties of oil paint films in copper paintings and the effects of environment in their structural behavior. In: Fuster-López L, Chulia Blanco I, Sarrio Martin MF, Vasquez de Agredos Pascual ML, Carlyle L, Wadum J, editors. Paintings on copper and other metal plates: production, degradation, and conservation issues. Valencia: Universidad Politecnica de Valencia; 2017. p. 95–102.

Doutre M, Murray A, Fuster-López L. Effects of humidity on gessoes for easel paintings. MRS Proc. 2017;1656:167–171. http://link.springer.com/10.1557/opl.2014.828 .

Roche A, Soldano A. The effect of changes in environmental conditions on the mechanical behaviour of selected paint systems. Stud Conserv. 2018;63(sup1):216–21. https://doi.org/10.1080/00393630.2018.1504434 .

Fuster-López L, Izzo FC, Damato V, Yusà-Marco DJ, Zendri E. An insight into the mechanical properties of selected commercial oil and alkyd paint films containing cobalt blue. J Cult Herit. 2019;35:225–234. https://www.sciencedirect.com/science/article/pii/S1296207418301031 .

Bridarolli A, Nualart-Torroja A, Chevalier A, Odlyha M, Bozec L. Systematic mechanical assessment of consolidants for canvas reinforcement under controlled environment. Herit Sci. 2020;8(1):52. https://doi.org/10.1186/s40494-020-00396-x .

Mecklenburg MF, Institute CC, Institution S, National Gallery of Art (U S ), Gallery T, editors. Art in transit: studies in the transport of paintings. Washington: National Gallery of Art; 1991.

Mecklenburg MF, McCormick-Goodhart M, Tumosa CS. Investigation into the deterioration of paintings and photographs using computerized modeling of stress development. J Am Inst Conserv. 1994;33(2):153–170. https://www.jstor.org/stable/3179424 .

Zumbuhl S, Attanasio F, Scherrer NC, Muller W, Fenners N, Caseri W. Solvent action on dispersion paint systems and the influence on the morphology—changes and destruction of the latex microstructure. In: Modern paints uncovered: proceedings from the modern paints uncovered symposium. Los Angeles: Getty Conservation Institute; 2007. p. 257–268.

Erhardt D, Tumosa CS, Mecklenburg MF. Natural and accelerated thermal aging of oil paint films. Stud Conserv 2000;45(sup1):65–69. http://www.tandfonline.com/doi/full/10.1179/sic.2000.45.Supplement-1.65 .

Rogala D, DePriest P, Charola AE, Koestler R. The mechanics of art materials and its future in heritage science. No. 10 in Smithsonian contributions to museum conservation. Washington D.C.: Smithsonian Scholarly Press; 2019.

Young CRT, Hibberd RD. Biaxial tensile testing of paintings on canvas. Stud Conserv. 1999;44(2):129–141. https://www.jstor.org/stable/1506725 .

Young C. Measurement of the biaxial properties of nineteenth century canvas primings using electronic speckle pattern interferometry. Opt Lasers Eng 1999;31(2):163–170. http://www.sciencedirect.com/science/article/pii/S014381669900007X .

Young C, Ackroyd P, Hibberd R, Gritt S. The mechanical behaviour of adhesives and gap fillers for re-joining panel paintings. In: National gallery technical bulletin. vol. 23. London: National Gallery Publications; 2002. p. 23–96.

Brinson HF, Brinson LC. Polymer engineering science and viscoelasticity: an introduction. 2nd ed. New York: Springer; 2015.

Book   Google Scholar  

Sturdy LF, Yee A, Casadio F, Shull KR. Quantitative characterization of alkyd cure kinetics with the quartz crystal microbalance. Polymer. 2016;103:387–396. http://www.sciencedirect.com/science/article/pii/S0032386116308631 .

Hedley G, Odlyha M, Burnstock A, Tillinghast J, Husband C. A study of the mechanical and surface properties of oil paint films treated with organic solvents and water. J Therm Anal. 1991;37(9):2067–88. https://doi.org/10.1007/BF01905579 .

Odlyha M, Chan TYA, Pages O. Evaluation of relative humidity effects on fabric-supported paintings by dynamic mechanical and dielectric analysis. Thermochimica Acta. 1995;263:7–21. http://www.sciencedirect.com/science/article/pii/0040603194023874 .

Foster GM, Ritchie S, Lowe C. Controlled temperature and relative humidity dynamic mechanical analysis of paint films. J Therm Anal Calorim. 2003;73(1):119–26. https://doi.org/10.1023/A:1025133508109 .

Ormsby B, Learner TJS, Foster GM, Druzik JR, Schilling M. Wet cleaning acrylic emulsion paint films: an evaluation of physical, chemical, and optical changes. In: Modern paints uncovered: proceedings from the modern paints uncovered symposium. Los Angeles: Getty Conservation Institute; 2007. p. 189–200.

Ormsby B, Foster G, Learner T, Ritchie S, Schilling M. Improved controlled relative humidity dynamic mechanical analysis of artists’ acrylic emulsion paints: Part II. General properties and accelerated ageing. J Therm Anal Calorim. 2007;90(2):503–508. http://link.springer.com/10.1007/s10973-006-7725-9 .

Phenix A. Thermal mechanical transitions in artists’ oil paints and selected conservation materials: a study by dynamic mechanical analysis (DMA). In: The AIC paintings specialty group. vol. 22. Los Angeles: American Institute for Conservation and Historic Works; 2009. p. 72–89.

Baij L, Hermans JJ, Keune K, Iedema PD. Time-dependent ATR-FTIR spectroscopic studies on solvent diffusion and film swelling in oil paint model systems. Macromolecules. 2018;51(18):7134–7144. http://pubs.acs.org/doi/10.1021/acs.macromol.8b00890 .

Andersen CK, Freeman A, Mortensen MN, Beltran V, Łukomski M, Phenix A. Mechanical and moisture sorption properties of commercial artists’ oil paint by dynamic mechanical thermal analysis (DMA), nanoindentation, and dynamic vapour sorption (DVS). In: van den Berg KJ, Bonaduce I, Burnstock A, Ormsby B, Scharff M, Carlyle L, et al., editors. Conservation of modern oil paintings. Cham: Springer International Publishing; 2019. p. 403–418. https://doi.org/10.1007/978-3-030-19254-9_32 .

Freeman AA, Lee J, Andersen CK, Fujisawa N, Łukomski M, Ormsby B. A pilot study of solvent-based cleaning of yellow ochre oil paint: effect on mechanical properties. Herit Sci. 2021;9(1):28. https://doi.org/10.1186/s40494-021-00501-8 .

Fischer EK. Rheological properties of commercial paints. J Colloid Sci. 1950;5(3):271–281. http://www.sciencedirect.com/science/article/pii/0095852250900304 .

Eley RR. Applied rheology and architectural coating performance. J Coat Technol Res. 2019;16(2):263–305. https://doi.org/10.1007/s11998-019-00187-5 .

Groen K. Investigation of the use of the binding medium by rembrandt. Zeitschrift fur Kunsttechnologie und Konservierung. 1997;2:207–227. https://pure.tue.nl/ws/files/1370067/617823.pdf .

de Viguerie L, Ducouret G, Cotte M, Lequeux F, Walter P. New insights on the glaze technique through reconstruction of old glaze medium formulations. Colloids Surf A: Physicochem Eng Aspects. 2008;331(1):119–125. http://www.sciencedirect.com/science/article/pii/S0927775708004809 .

de Viguerie L, Ducouret G, Lequeux F, Moutard-Martin T, Walter P. Historical evolution of oil painting media: a rheological study. Comptes Rendus Physique. 2009;10(7):612–621. http://www.sciencedirect.com/science/article/pii/S163107050900111X .

Tirat S, Echard JP, Lattuati-Derieux A, Le Huerou JY, Serfaty S. Reconstructing historical recipes of linseed oil/colophony varnishes: influence of preparation processes on application properties. J Cult Herit. 2017;27, Supplement(Wooden Musical Instruments):S34–S43. https://reader.elsevier.com/reader/sd/pii/S1296207417305654 .

de Viguerie L, Jaber M, Pasco H, Lalevée J, Morlet-Savary F, Ducouret G, et al. A 19th century “ideal” oil paint medium: a complex hybrid organic–inorganic gel. Angewandte Chemie Int Edn. 2017;56(6):1619–1623. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201611136 .

de Viguerie L, Glanville H, Ducouret G, Jacquemot P, Dang PA, Walter P. Re-interpretation of the old masters’ practices through optical and rheological investigation: the presence of calcite. Comptes Rendus Physique. 2018;19(7):543–552. http://www.sciencedirect.com/science/article/pii/S1631070518301142 .

Blayo A, Gandini A, Le Nest JF. Chemical and rheological characterizations of some vegetable oils derivatives commonly used in printing inks. Ind Crops Products. 2001;14(2):155–167. http://www.sciencedirect.com/science/article/pii/S0926669001000796 .

Belgacem MN, Blayo A, Gandini A. organosolv lignin as a filler in inks, varnishes and paints. Ind Crops Products. 2003;18(2):145–153. http://www.sciencedirect.com/science/article/pii/S0926669003000426 .

Cimino D, Chiantore O, de La Rie ER, McGlinchey CW, Ploeger R, Poli T, et al. Binary mixtures of ethylene containing copolymers and low molecular weight resins: a new approach towards specifically tuned art conservation products. Int J Adhes Adhes. 2016;67:54–62. http://www.sciencedirect.com/science/article/pii/S0143749615002298 .

Marx KA. Quartz crystal microbalance: a useful tool for studying thin polymer films and complex biomolecular systems at the solution-surface interface. Biomacromolecules. 2003;4(5):1099–120. https://doi.org/10.1021/bm020116i .

Johannsmann D. Viscoelastic, mechanical, and dielectric measurements on complex samples with the quartz crystal microbalance. Phys Chem Chem Phys. 2008;10(31):4516. http://xlink.rsc.org/?DOI=b803960g .

Johannsmann D. The quartz crystal microbalance in soft matter research. Soft Biol Matter. Cham: Springer International Publishing; 2015. http://link.springer.com/10.1007/978-3-319-07836-6 .

Shull KR, Taghon M, Wang Q. Investigations of the high-frequency dynamic properties of polymeric systems with quartz crystal resonators. Biointerphases. 2020;QCM2020(1):021012. https://avs.scitation.org/doi/full/10.1116/1.5142762%40bip.2020.QCM2020.issue-1 .

dePolo GE, Schafer E, Sadman K, Rivnay J, Shull KR. Sample preparation in quartz crystal microbalance measurements of protein adsorption and polymer mechanics. JoVE J Vis Exp. 2020;(155):e60584. https://www.jove.com/video/60584 .

Sturdy L, Casadio F, Kokkori M, Muir K, Shull KR. Quartz Crystal Rheometry: A quantitative technique for studying curing and aging in artists’ paints. Polym Degrad Stabil. 2014;107:348–355. http://linkinghub.elsevier.com/retrieve/pii/S0141391014000573 .

Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res. 1992;7(06):1564–1583. http://www.journals.cambridge.org/abstract_S0884291400017039 .

Oyen ML, Cook RF. A practical guide for analysis of nanoindentation data. J Mech Behav Biomed Mater. 2009;2(4):396–407. http://www.sciencedirect.com/science/article/pii/S1751616108000805 .

Chiantore O, Scalarone D. The macro- and microassessment of physical and aging properties in modern paints. In: Modern paints uncovered: proceedings from the modern paints uncovered symposium. Los Angeles: Getty Conservation Institute; 2007. p. 96–104.

Salvant J, Barthel E, Menu M. Nanoindentation and the micromechanics of van gogh oil paints. Appl Phys A. 2011;104(2):509–515. http://link.springer.com/10.1007/s00339-011-6486-x .

Andersen CK, Freeman A, Mortensen MN, Beltran V, Łukomski M, Phenix A. Mechanical and moisture sorption properties of commercial artists’ oil paints by dynamic mechanical thermal analysis (DMA), nanoindentation, and dynamic vapour sorption (DVS). In: Conservation of modern oil paintings. Springer; 2019. p. 403–417.

Fujisawa N, Łukomski M. Nanoindentation near the Edge of a Viscoelastic Solid with a Rough Surface. Materials & Design. 2019 Dec;184:108174. Available from: https://www.sciencedirect.com/science/article/pii/S0264127519306124 .

Freeman A, Łukomski M, Beltran V. Mechanical characterization of a cross-sectional TiO2 acrylic-based paint by nano-indentation. J Am Inst Conserv. 2020;59(1):27–39. https://doi.org/10.1080/01971360.2019.1603062 .

Tiennot M, Paardekam E, Iannuzzi D, Hermens E. Mapping the mechanical properties of paintings via nanoindentation: a new approach for cultural heritage studies. Sci Rep. 2020;10(1):7924. https://www.nature.com/articles/s41598-020-64892-7 .

Chyasnavichyus M, Young SL, Tsukruk VV. Probing of polymer surfaces in the viscoelastic regime. Langmuir. 2014;30(35):10566–82. https://doi.org/10.1021/la404925h .

Kolluru PV, Eaton MD, Collinson DW, Cheng X, Delgado DE, Shull KR, et al. AFM-based dynamic scanning indentation (DSI) method for fast, high-resolution spatial mapping of local viscoelastic properties in soft materials. Macromolecules. 2018;51(21):8964–8978. https://pubs.acs.org/doi/10.1021/acs.macromol.8b01426 .

Collinson DW, Eaton MD, Shull KR, Brinson LC. Deconvolution of stress interaction effects from atomic force spectroscopy data across polymer-particle interfaces. Macromolecules. 2019. https://pubs.acs.org/doi/10.1021/acs.macromol.9b01378 .

Klausmeyer P, Cushman M, Dobrev I, Khaleghi M, Harrington EJ, Chen X, et al. Quantifying and mapping induced strain in canvas paintings uising laser shearography. In: The noninvasive analysis of painted surfaces: scientific impact and conservation practice. No. 5 in Smithsonian Contributions to Museum Conservation. Washington D.C.: Smithsonian Scholarly Press; 2016. p. 1–13.

Kalms MK, Osten W, Jueptner WPO. Advanced shearographic system for nondestructive testing of industrial and artwork components. In: Deng S, Okada T, Behler K, Wang X, editors. Photonics Asia 2002. Shanghai, China; 2002. p. 34. http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.482923 .

Kalms MK, Jueptner WPO. A mobile shearography system for non-destructive testing of industrial and artwork components. Key Energy Mater. 2005;295–296:165–70.

Groves RM, Osten W, Doulgeridis M, Kouloumpi E, Green T, Hackney S, et al. Shearography as part of a multi-functional sensor for the detection of signature features in movable cultural heritaged. In: Fotakis C, Pezzati L, Salimbeni R, editors. Optical metrology. Munich, Germany; 2007. p. 661810. http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.727497 .

Groves RM, Li A, Liu X, Hackney S, Peng X, Osten W. 2.5D Virtual reality visualisation of shearography strain data from a canvas painting. In: Pezzati L, Salimbeni R, editors. SPIE Europe Optical Metrology. Munich, Germany; 2009. p. 739109. http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.827519 .

Groves RM, Pradarutti B, Kouloumpi E, Osten W, Notni G. 2D and 3D non-destructive evaluation of a wooden panel painting using shearography and terahertz imaging. NDT & E International. 2009;42(6):543–549. http://www.sciencedirect.com/science/article/pii/S0963869509000619 .

Morawitz M, Hein N, Alexeenko I, Wilke M, Pedrini G, Krekel C, et al. Optical methods for the assessment of transport and age induced defects of artwork. In: Osten W, editor., et al., Fringe 2013. Berlin, Heidelberg: Springer; 2014. p. 951–5.

Chapter   Google Scholar  

Meybodi MK, Dobrev I, Klausmeyer P, Harrington EJ, Furlong C. Investigation of thermomechanical effects of lighting conditions on canvas paintings by laser shearography. In: Furlong C, Gorecki C, Novak EL, editors. SPIE Optical Engineering + Applications. San Diego, California, USA; 2012. p. 84940A. http://proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.958089 .

Buchta D, Heinemann C, Pedrini G, Krekel C, Osten W. Combination of FEM simulations and shearography for defect detection on artwork. Strain. 2018;54(3):12269. https://onlinelibrary.wiley.com/doi/abs/10.1111/str.12269 .

Bratasz Ł, Akoglu KG, Kékicheff P. Fracture saturation in paintings makes them less vulnerable to environmental variations in museums. Herit Sci. 2020;8(1):11. https://doi.org/10.1186/s40494-020-0352-0 .

Mecklenburg M, Tumosa C, McCormick-Goodhardt M. A general method for determining the mechanical properties needed for the computer analysis of polymeric structures subjected to changes in temperature and relative humidity. In: Materials research society symposium. vol. 267. San Francisco, CA: Materials Research Society; 1992. p. 337–358.

Chiriboga Arroyo PG. Finite element modeling of vibrations in canvas paintings [Doctoral]. Technische Universiteit Delft. Delft, Netherlands; 2013. https://repository.tudelft.nl/islandora/object/uuid%3Aa30b358e-d0de-4a81-92f3-e9f255443043

Bratasz Ł, Sereshk MRV. Crack saturation as a mechanism of acclimatization of panel paintings to unstable environments. Stud Conserv. 2018;63(sup1):22–7. https://doi.org/10.1080/00393630.2018.1504433 .

Eumelen GJAM, Bosco E, Suiker ASJ, van Loon A, Iedema PD. A computational model for chemo-mechanical degradation of historical oil paintings due to metal soap formation. J Mech Phys Solids. 2019;132:103683. http://www.sciencedirect.com/science/article/pii/S0022509619302182 .

Bosco E, Suiker ASJ, Fleck NA. Crack channelling mechanisms in brittle coating systems under moisture or temperature gradients. Int J Fract. 2020;225(1):1–30. https://doi.org/10.1007/s10704-020-00461-3 .

Bosco E, Suiker ASJ, Fleck NA. Moisture-induced cracking in a flexural bilayer with application to historical paintings. Theor Appl Fract Mech. 2021;112:102779. https://www.sciencedirect.com/science/article/pii/S0167844220303554 .

Tantideeravit S, Charalambides MN, Balint DS, Young CRT. Prediction of Delamination in multilayer artist paints under low amplitude fatigue loading. Eng Fract Mech. 2013;112-113:41–57. http://www.sciencedirect.com/science/article/pii/S0013794413003172 .

Johnson AP, Robert Hannen W, Zuccari F. Vibration control during museum construction projects. J Am Inst Conserv. 2013;52(1):30–47. http://search.ebscohost.com/login.aspx?direct=true&db=asu&AN=86141021&site=ehost-live .

Wei W, Sauvage L, Wölk J. Baseline limits for allowable vibrations for objects. In: Preventive conservation. vol. 1516. Paris: International Council of Museums; 2014. p. 7.

Sauvage L, Wei WB, Martinez M. When conservation meets engineering: predicting the damaging effects of vibrations on pastel paintings. Stud Conserv. 2018;63(sup1):418–20. https://doi.org/10.1080/00393630.2018.1504444 .

Sauvage L, Stratis H, Harrison L. Pastel roundtable. J Paper Conserv. 2018;19(1):33–5. https://doi.org/10.1080/18680860.2019.1580005 .

Baij L, Hermans J, Ormsby B, Noble P, Iedema P, Keune K. A review of solvent action on oil paint. Herit Sci. 2020;8(1):43. https://doi.org/10.1186/s40494-020-00388-x .

Udell NA, Hodgkins RE, Berrie BH, Meldrum T. Physical and chemical properties of traditional and water-mixable oil paints assessed using single-sided NMR. Microchem J. 2017;133:31–36. https://www.sciencedirect.com/science/article/pii/S0026265X16306038 .

Rooney M. Effect of pigment volume concentration on physical and chemical properties of acrylic emulsion paints assessed using single-sided Nmr [Master of Science]. College of William and Mary. Virginia; 2018. https://scholarworks.wm.edu/etd//1530192815 .

Busse F, Rehorn C, Küppers M, Ruiz N, Stege H, Blümich B. NMR relaxometry of oil paint binders. Magn Reson Chem. 2020;58(9):830–839. https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/abs/10.1002/mrc.5020 .

Rooney M, Meldrum T. Effect of pigment concentration on nmr relaxometry in acrylic paints. Magn Reson Chem. 2020;58(9):880–888. https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/abs/10.1002/mrc.5053 .

Fife GR, Stabik B, Kelley AE, King JN, Blümich B, Hoppenbrouwers R, et al. Characterization of aging and solvent treatments of painted surfaces using single-sided NMR. Magn Reson Chem. 2015;53(1):58–63. https://onlinelibrary.wiley.com/doi/abs/10.1002/mrc.4164 .

Prati S, Sciutto G, Volpi F, Rehorn C, Vurro R, Blümich B, et al. Cleaning oil paintings: nmr relaxometry and SPME to evaluate the effects of green solvents and innovative green gels. New J Chem. 2019;43(21):8229–8238. https://pubs.rsc.org/en/content/articlelanding/2019/nj/c9nj00186g .

Walter P, de Viguerie L. Materials science challenges in paintings. Nat Mater. 2018;17(2):106–109. https://www.nature.com/articles/nmat5070 .

Shull K. Future trends in studies of the mechanics of artists’ paints. In: The mechanics of art materials and its future in heritage science. No. 10 in Smithsonian Contributions to Museum Conservation. Washington D.C.: Smithsonian Scholarly Press; 2019. p. 87–92.

Download references

Acknowledgements

The authors would like to thank Alan Phenix and Arnaud Lesaine for helpful conversations surrounding the references selected for this review paper and Marc Vermeulen for his time proofreading this manuscript.

This work was supported by the NSF (OISE-1743748) and is also part of NU-ACCESS’s broad portfolio of activities, which is made possibly by generous support of the Andrew W. Mellon Foundation.

Author information

Authors and affiliations.

Department of Materials Science and Engineering, Northwestern University, Evanston, USA

Gwen dePolo, Marc Walton & Kenneth R. Shull

Van’t Hoff Institute for Molecular Science, University of Amsterdam, Science Park, Amsterdam, The Netherlands

Katrien Keune

Conservation and Science, Rijksmuseum Amsterdam, Amsterdam, The Netherlands

You can also search for this author in PubMed   Google Scholar

Contributions

GD—writing, data compilation, and data analysis, MW—conceptualization and editing, KK—review and editing, KRS—data analysis, conceptualization, and editing. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Kenneth R. Shull .

Ethics declarations

Competing interests.

Not applicable

Additional information

Publisher's note.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

Additional file 1..

Appendix: Tensile testing tables.

Additional file 2.

 Excel spreadsheet containing the tensile testing data shown in Additional file 1.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/ ) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and permissions

About this article

Cite this article.

dePolo, G., Walton, M., Keune, K. et al. After the paint has dried: a review of testing techniques for studying the mechanical properties of artists’ paint. Herit Sci 9 , 68 (2021). https://doi.org/10.1186/s40494-021-00529-w

Download citation

Received : 18 December 2020

Accepted : 05 May 2021

Published : 07 June 2021

DOI : https://doi.org/10.1186/s40494-021-00529-w

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

• Mechanical properties
• Tensile testing

Academia.edu no longer supports Internet Explorer.

To browse Academia.edu and the wider internet faster and more securely, please take a few seconds to  upgrade your browser .

Enter the email address you signed up with and we'll email you a reset link.

• We're Hiring!
• Help Center

Analysis of Increasing the Productivity of Using Tinting Machines Based on the VSM Method in the Paint Industry

2018, IJRESM

Paint manufacturers experience the complexity of production planning due to the many color variations for a paint product that is supported by a lack of flexibility in batch production systems. The development of coloring technology with color tinting machines has allowed paint coloring to be done by the paint shop, not necessarily done directly by the paint manufacturer. By using the Value Stream Mapping (VSM) method, the paint production process will be mapped and analysis of improvements with the application of production in the paint manufacturer only white / base and staining done in the shop using Tinting machines. From VSM mapping and comparison before and after the improvement was carried out, the results showed that with improvements made by paint manufacturers will experience increased productivity through reducing production lead time by 41%, reducing NVA (Non Value Added) activities by 21%, increasing VA activity (Value Added) up to 17%, controlling the flow of raw material and simplifying inventory control for finish good paint.

Related Papers

Transstellar Journals

TJPRC Publication

Management and Production Engineering Review (MPER)

Nguyễn Đạt Minh

Lean is one of the systematic approach to achieve higher value for organizations through eliminate non-value-added activities. It is an integrated set of tools, techniques, and principles designed to optimize cost, quality and delivery while improving safety. In Vietnam, industry waste management and treatment has become serious issue. The aim of this research is to present the effective of Lean application for industrial wastes collecting and delivery improvement. Through a case study, this paper showed the way of Lean tools and principles applied for wastes management and treatment such as Value Stream Mapping, Pull system, Visual Control, and Andon to get benefit on both economic and environment. In addition, the results introduced a good experience for enterprises in Vietnam and other countries have similar conditions to Vietnam in cost saving and sustainable development in waste management.

Prof. Vikas Kumar

The International Journal of Advanced Manufacturing Technology

Uday Venkatadri

Wai M Cheung

In the last decades, environmental footprint of the product manufacture has emerged as an important public concern, causing manufacturers to re-assess their product’s environmental impacts. Responding to global outcry on global warming, world leaders have agreed to limit global temperature rise to less than 2°C above the temperature in pre-industrial times. As a result, governments and industrial leaders around the world have proposed a roadmap for 80% emissions reduction by 2050. The aim of this work develops a novel approach of linking Life Cycle Assessment (LCA) and Lean manufacturing to reduce the negative environmental impacts of a plastic injection moulded product. Products that use plastic as their primary source of raw materials are mainly produced by plastic injection processes. Although plastic injection moulding has many benefits such as ‘high production rates’, there is a lack of through investigation on the effects of this process has on the environment such as climate change, ozone depletion etc. This paper proposes a novel cross-functional mapping approach of linking lean-thinking and LCA. The implementation case study has been presented with the view to re-assess carbon footprint of an existing plastic product. The reported implementation work demonstrates that the adaptation of lean thinking and LCA could significantly minimise negative environmental impacts of a plastic injection product. A case study indicates that the overall environmental impact has been reduced by approximately 40% in climate change, human toxicity, photochemical oxidant formation, acidification and eco-toxicity.

The 2018 International Conference of Organizational Innovation

Tuwanku Aria Auliandri

Based on the National Statistics Bureau, the number of train passengers in Indonesia has steadily increased; hence, it has increased demands for train car bodies. Train production in Indonesia is conducted by PT. Train Industry (INKA), Madiun. A manufacturing enterprise that processes raw materials into finished goods through a complicated process before delivering the goods to consumers. During the process, wasteful activities frequently occur and do not add any value stream. Wasteful activities extend and delay the production process, which harms the company. This research was conducted at the Fabrication Division of the train manufacture company, PT. INKA, Madiun. It is aimed to identify and minimize wasteful activities during train car body production process conducted by the Fabrication Division using Lean Manufacturing approach. Analysis on wasteful activities is conducted using Value Stream Mapping with Process Activity Mapping (PAM) and Supply Chain Response Matrix (SCRM) and calculated with VALSAT. Based on the research findings, the most wasteful activity is waiting time. Waiting time is caused by the delay in the arrival of raw materials, which causes further delay in production time, unfit machinery capability and barely-skilled labors. Based on PAM, wasteful activities, which are mostly caused by delays in non-value adding activity (NVA), took up 271.5 hours or 16.80% of total production time. Based on SCRM, total lead time of car body production until it reached the Finishing Division amounted to 131 days. After analysis, suggestions for improvement are given to minimize wasteful activities. Based on the suggestions, a decrease in NVA was achieved at 88.67% rate and lead time decreased by 15.64%.

IAEME Publication

Lean manufacturing is a procedure to eliminate the wastes consistently by continuous progress in manufacturing industries. The main objective of lean manufacturing is to reduce the overall production cost, to increase the efficiency and to minimize the overall lead time. In this research an attempt was made to enhance the overall production rate in a medium scale industry. The aim is to make the production line to accomplish the target of producing twelve homogenizer machines per month from the actual production of ten homogenizer machines. These machines are used for the homogenization of milk, where the milk fat globules are reduced in size and dispersed uniformly through the rest of the milk. To enhance the productivity, an approach of lean tools like 5S, value stream mapping, line balancing in assembly, Gemba walk and kaizen was used. The main reason to increase the productivity is to meet the increasing customer demand at right time and also to minimize the overall production lead time. From the results it was found that, the company achieves the customer demand of twelve homogenizer machines and the same was validated using flexsim simulation software. Also it was observed that the overall lead time was reduced about 776 minutes and efficiency of liquid end assembly line was increased about 28%.

IOSR Journals publish within 3 days

Bus body manufacturing play a key role in automotive manufacturing, as vehicle manufacturers often sub contract such enterprise to complete bus manufacturing process. In a fiercely competitive global environment, the bus body manufacturing enterprises are constantly looking for ways of meet their customer requirements of delivery on time, cost and quality in order for them to survive. However, bus body manufacturing enterprises barely meet the customer demands in terms of delivering on time due to long cycle times on the production floor due to low labour utilization, material wastage and unorganized work flows. In this research, we critically looked at improving productivity such enterprises by considering a case study of one of the leading bus body manufacturing enterprises in Kenya. Value Stream Mapping was used as the main productivity improvement tool supported by line balancing techniques. The results from the improved value stream map indicated an increase in efficiency of 13.1% and a reduction the cycle time by 7 days, demonstrating the potential of these tools for improving productivity in bus body manufacturing.

Purpose – The purpose of this paper is to develop an effective methodology for implementing lean manufacturing strategies and a leanness evaluation metric using continuous performance measurement (CPM). Design/methodology/approach – Based on five lean principles, a systematic lean implementation methodology for manufacturing organizations has been proposed. A simplified leanness evaluation metric consisting of both efficiency and effectiveness attributes of manufacturing performance has been developed for continuous evaluation of lean implementation. A case study to validate the proposed methodology has been conducted and proposed CPM metric has been used to assess the manufacturing leanness. Findings – Proposed methodology is able to systematically identify manufacturing wastes, select appropriate lean tools, identify relevant performance indicators, achieve significant performance improvement and establish lean culture in the organization. Continuous performance measurement matrices in terms of efficiency and effectiveness are proved to be appropriate methods for continuous evaluation of lean performance. Research limitations/implications – Effectiveness of the method developed has been demonstrated by applying it in a real life assembly process. However, more tests/applications will be necessary to generalize the findings. Practical implications – Results show that applying the methods developed, managers can successfully identify and remove manufacturing wastes from their production processes. By improving process efficiency, they can optimize their resource allocations. Manufacturers now have a validated step by step methodology for successfully implementing lean strategies. Originality/value – According to the authors’ best knowledge, this is the first known study that proposed a systematic lean implementation methodology based on lean principles and continuous improvement techniques. Evaluation of performance improvement by lean strategies is a critical issue. This study develops a simplified leanness evaluation metric considering both efficiency and effectiveness attributes and integrates it with the lean implementation methodology.

Tonny Rosado

Implementing Lean Manufacturing in not only in big production lines but also in small workshops

Loading Preview

Sorry, preview is currently unavailable. You can download the paper by clicking the button above.

RELATED PAPERS

bagus kurniawan

Green Energy and Technology

Al-IQTISHAD Jurnal Ilmu Ekonomi Syariah , Irfan Syauqi Beik , Aslam Widigdo

Transstellar Journal

gumewa udiantoro

AK'Aphisak Kraart

IJRAME Journal

International Journal of Advanced Manufacturing Technology

shanae cooper

Javier Santos , Martín Tanco

International Journal of Industrial and Systems Engineering

Rapinder Sawhney , Soundararajan Kannan

International Journal of Lean Enterprise Research

Fazleena Badurdeen , William Templeton

Kassu Sileyew

Khadija Al Madani

Journal of Optimization in Industrial Engineering

Ripon Kumar Chakrabortty

kassu sileyew

Kassu Jilcha , Kassu Sileyew

Hoang Nguyen

Ramsi Kalia

Journal of Intelligent Manufacturing

Fazleena Badurdeen

Production Planning and Control

Dr Abhijeet Ghadge

GRD JOURNALS , p Vikraman

abbas Sharif

Dr. MD. Mamunur Rashid

Ashma Sorna

International Journal of Production Economics

Satish Tyagi

Ylenia Sette

RELATED TOPICS

•   We're Hiring!
•   Help Center
• Find new research papers in:
• Health Sciences
• Earth Sciences
• Cognitive Science
• Mathematics
• Computer Science
• Academia ©2024

China scientists’ low-cost solid-state battery could be EV industry breakthrough

• New approach by USTC research team breaks cost barrier to next-generation rechargeable lithium batteries, according to paper

While there have been significant advances in traditional battery packs, their bulk, weight – and potential fire risk – are out of step with the requirements of lightweight, reliable EVs.

Replacing the liquid electrolytes of conventional lithium batteries with a solid-state alternative could promise faster charging times, better performance and improved safety standards.

But large-scale use of solid-state alternatives to conventional lithium batteries has been hampered by high material and manufacturing costs, sparking a global race for a commercially viable solution.

The researchers, from the University of Science and Technology of China (USTC) in the eastern province of Anhui, have developed a type of solid electrolyte that could hold great promise for commercial applications.

According to a paper published on June 30 by the German Chemical Society’s Angewandte Chemie, one of the world’s leading chemistry journals, the team took a new approach to a problem that has plagued researchers for more than a decade.

Major industry players, including Japan’s Toyota and South Korea’s Samsung, have invested substantially in research and development for a suitable solid-state electrolyte, with three main contenders – oxide, sulphide and chloride types.

Sulphide is generally considered the most promising candidate for the eventual practical application of all-solid-state batteries because of its excellent performance, but its high cost remains prohibitive.

In an interview with state-run newspaper Science and Technology Daily last week, USTC researcher Ma Cheng explained the root of the problem in terms of simple economics.

A commercially viable solid-state battery electrolyte must cost less than US$50 per kilogram to produce, but sulphide electrolytes are generally priced at more than US$195 per kilogram.

“Although researchers around the world are striving to reduce the cost through various methods, long-term exploration has shown that it is quite difficult to achieve this goal,” Ma said.

To overcome this challenge, Ma and his team set out to develop a new sulphide solid electrolyte, which they have called LPSO, that does not require lithium sulphide as a raw material.

LPSO is synthesised from two low-cost compounds, at an ingredient cost of just US$14.42 per kilogram – less than 8 per cent of the raw material costs of other sulphide solid electrolytes.

According to the researchers, the price reduction comes with no loss of the unique advantages offered by the best performing current forms of sulphide solid electrolytes, including acceptable anode compatibility, which determines performance stability.

LPSO matches well with high energy density anodes such as lithium metal and silicon, with a battery made from a combination of the new material and lithium metal maintaining more than 4,200 hours of stable cycling at room temperature, the paper said.

But Ma told Science and Technology Daily that LPSO “is still expected to achieve further improvements in performance, and we are working towards that”.

‘Overtaking on a bend’: how China’s EV industry charged ahead to dominate the global market

Companies from Japan, South Korea, Europe and the United States – which have lagged behind China in traditional lithium batteries – are pinning their hopes on next-generation battery technology, including solid-state batteries, to take the lead.

Both Toyota and Samsung are reported to be targeting 2027 for the commercial release of their all-solid-state batteries.

As other nations accelerate their efforts to achieve a commercially viable version of the technology, China’s battery and car makers have joined forces as part of a government-led drive to build a solid-state battery supply chain by 2030.

In January, Beijing launched the China All-Solid-State Battery Collaborative Innovation Platform, a consortium that brings together government, academia and industry, including EV battery giants CATL and BYD.

In May, Chen Liquan – “father of the Chinese lithium battery” – said the strategy aims to keep China in the race to develop what he described as “the future of rechargeable battery technology”.

“Only by seizing the first opportunity can China remain in a proactive position” in the development of solid-state lithium batteries, said Chen, 84, from the Chinese Academy of Sciences’ Institute of Physics, in an interview with China Science Daily.