The fine art of paints and coatings.

Understanding the technology of paints and coatings used in the production of fine artwork makes it possible to properly protect, conserve, and, at times, restore paintings, sculptures, and other pieces treasured for their beauty. Modern analytical tools are helping conservators expand their knowledge of the materials and methods used by artists throughout the centuries. With this greater understanding has come an enhanced ability to preserve valuable works of art.

Analytical evaluation of paints is done for two main reasons, according to Dr. Paul Whitmore, director of the Art Conservation Research Center at Carnegie Mellon University. "The first mission focuses on determining whether or not the painting or other object is what it is purported to be." For paintings, the process involves identification of the different types of paints used by the artists and the components of those paints. This information can be used to confirm the authenticity of the painting or uncover any previous restoration work.

In many cases, ingredients were introduced at a specific period in time and were used during certain historical periods, so this information helps identify the age of a painting. Synthetic paints, for example, were introduced in the 20th Century. Depending on the level of detail obtained about the ingredients, it is also possible to confirm that a painting was completed by a specific artist. A particular painter may have been the only one to use a fish-based glue. Other painters may have modified the binder material in distinctive ways. According to Richard Wolbers, associate professor in the Art Conservation Department at the University of Delaware and a conservator for the Winterthur Museum, the famous painter Rembrandt emulsified a unique combination of materials into his paints to give them more body. "Evaluation of the different styles of manipulation of binders helps us learn more about the different techniques employed by various artists," he comments.

Often the methods of application of the paint can be visualized as well. Some artists sketched with charcoal first, then applied successive layers of paint, while others worked on the final image from the inception of the project. "With the information gathered from these evaluations, we can often get an understanding of the conditions under which an artist was operating," Whitmore notes.

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Insight into trade and cultural exchange in historical times can also be gained through evaluation of art pieces. "Through analysis of different artworks over a period of time, we can track the appearance and location of the use of a given pigment, for example, and watch the spread of technology across the world," explains Wolbers. He also notes that throughout history there has often been a blending of different technologies as people shifted responsibilities. These shifts are reflected in the blending of different types of paints in artwork. In Colonial times, for example, it was common for sign makers to become portrait painters. In many of the portraits from this period, analysis reveals the use of paints designed for both of these applications, providing information about the painter's history.

The second key reason for analyzing artwork is to gain an understanding of the materials used to create a piece so that it can be properly conserved and preserved. "Knowing what materials were used to produce a piece of art provides us with information about how it might respond to temperature and humidity changes, its sensitivity to light, and the solubility of the paint itself," Whitmore says.

When restoring a painting, the goal of the conservator is to match the colors and textures of the original painting while making the restored area easily distinguishable from the original upon close inspection. The restoration also needs to be reversible. "It is possible that we could learn more about the original intent of the artist, or new technologies might be developed that would enable a better approach to the restoration. Therefore, it is very important to design the restoration so that it can be easily removed without affecting the original painting," he adds.

Usually, restored areas are isolated from the original painting with a layer of varnish or easily soluble polymer that is first applied before the restoration is initiated. For older, oil-based paintings, this concept works well. Newer acrylic-based paints have very different properties than oil paints and are ideal for restoration of older works. Newer paintings, however, are often completed with acrylic paints. Conservation of these more modern works is a much greater challenge. "In this area, conservators continue to learn about new technologies and find out how new paints can be used," comments Whitmore. "We don't have broad experience with many of these new paints and new technologies, so it is a work in progress."

The art world is indeed embracing technology to achieve its conservation goals. Polarizing light microscopy (PLM), x-ray fluoresence (XRF), x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), liquid and gas chromatography (LC and GC), mass spectrometry (MS), scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS), thermal analysis (TGA, DSC), gel electrophoresis, laser technology, staining techniques, radiological tools, and DNA analysis are all utilized to gain insight into the composition of paint and methods of application in historical pieces. Nondestructive testing methods are, of course, preferred. Advances in technology, though, are making it possible to conduct these analyses on smaller and smaller sample sizes.

PLM, XRF, XRD, and SEM-EDS are used to analyze inorganic materials. XRF and XRD are use to detect and identify metallic elements in inorganic pigments, for example. Organic materials are more difficult to identify because they are not distinguishable by different elements, according to Whitmore. FTIR and LC/GC analysis coupled with MS is often used. Advances in mass spectroscopy are making it more feasible to use this analytical technique without prior separation.

General binder analysis can provide information on what type of binder was used--plant resin, oil, carbohydrate, protein, synthetic polymer, etc. More detailed analysis can provide a more precise evaluation including information about which type of protein or oil was used. According to Wolbers, particle size distribution analysis can provide information about the methods an artist used to prepare his or her paints.

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Researchers are using infrared rays to penetrate through the layers of a painting to reveal the carbon-based materials, such as charcoal, used to sketch out the initial image. Being able to see these original drawings can help confirm the identity of the artist. At Stanford University, the Cantor Center for the Visual Arts has used CT scans to produce 3-D animations that show the inner structure of the wood backing on some paintings. The Stanford Synchrotron Radiation Laboratory has also analyzed microscopic samples of paint from artworks with intense, precise x-ray beams that can be finely tuned to spot trace elements in very small samples. This information can help determine the geographic origin of minerals used in the paint.

Advanced technology is also being used to aid in the cleaning of art pieces, an often necessary step in the restoration process. Wolbers has employed the use of fluorescent molecules to tag particular components such as protein and oils within the paint of an artwork. This staining technique enables the identification of undesirable materials that can then be removed with enzymes. The polymerase chain reaction (PCR) has also been used to identify bacteria and fungi that harm paintings; DNA samples from the infected paintings are amplified using PCR, then the DNA is sequenced and matched to a database of known microorganisms. Once the infectious agent is identified, an appropriate treatment can be prepared.

Both infrared (IR) and ultraviolet (UV) lasers have been used to selectively remove layers of paint and grime from sculptures and paintings. IR lasers work by destroying the uppermost paint layers with heat and are appropriate only for certain pieces. UV lasers work by breaking down the molecular structure of paints, which is better for paintings where only specific layers need to be removed. The conservator controls how much material is removed by continuously analyzing the material that comes off the surface. In this way they can detect when the laser has moved from one layer of material to another.

"As technology advances, we are gaining the ability to make finer and finer distinctions," says Whitmore. This information is helping conservators develop increasingly enhanced preservation and restoration processes. Unfortunately, only larger museums and universities are able to afford to establish a well-equipped analytical laboratory. There are about half a dozen sites in the U.S. Other institutions do have access to the technology, though; samples are frequently sent to the major research sites for evaluation.

Several research groups and institutions are investigating new coatings technology designed to improve the protection of fine artwork. According to Whitmore, some researchers are developing solvent-based paints made with low molecular weight resins to impart improved application properties and also modify the optical properties so that they are more similar to the original oil-based and resin-based paints. These new materials have a high refractive index as compared to traditional acrylic paints and thus offer an appearance closer to that of the components in historical artistic paint formulations.

Dr. Tara J. Shedlosky (currently with DuPont Imaging Technologies), as a graduate student with Professor Gordon Bierwagen at North Dakota State University, developed a polyester urethane coating for protecting outdoor sculptures. "Protective coatings for sculpture need to be weather resistant, but they also need to be removable. Developing a coating with these juxtaposing properties has been quite a challenge," she says. Her solution was to include a weak link in the coating that could be easily broken by treatment with a base. The university has applied for a patent and is looking to license the technology. Shedlosky tested the coating in UV and salt fog chambers with positive results. The coating can be removed by applying a gel containing ammonium hydroxide and letting it sit for just 15 minutes.

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UCLA professor Eric Bescher and retired UCLA professor John Mackenzie developed a multi-layer coating composed largely of a solgel and covered with a removable top layer made of traditional polymer material. The ceramic nature of the sol-gel allows it to resist the effects of weather and corrosion better than traditional coatings, while the liquid form makes it easy to apply. The research was sponsored by the Getty Conservation Institute in conjunction with the Office of the President in the Czech Republic. This new technology was used to restore "The Last Judgment," a medieval mosaic by St. Vitus that is located in Prague Castle. According to one of the principals who recently reviewed the mosaic, "the conservation treatment is performing perfectly."

Whether technology is used to analyze the components of paint in an historic piece of art, or to look for hidden sketches beneath the surface of a painting to identify the artist, or to carefully remove dirt buildup as the first part of a restoration project, it has become an important instrument in the conservator's toolbox. Recent advances in both analytical techniques and coatings chemistry are making it possible to continuously improve our ability to protect and preserve our cultural heritage as represented by so many beautiful works of art.