Component One: Chemical Analysis of Synthetic Paint Media, Synthetic Organic Pigments, and Paint Additives
The ability to establish the type of paint on a work of art is essential to an understanding of how the paint might alter in response to age, environmental conditions, or conservation treatments. It is also helpful in studying artists' techniques and in examining authenticity issues.

Prior to the establishment of this collaboration, the Conservation Department at Tate had worked for several years on developing appropriate analytical methods for the qualitative identification of the various synthetic binding media that are frequently used in modern and contemporary works of art. Consequently, it was already possible to detect all major types of modern paint binding media—such as oil, acrylic, alkyd, PVA, nitrocellulose, and polyurethane—from microscopic paint samples using a combination of pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and Fourier Transform infrared (FTIR) spectrometry. Work carried out at the FOM-AMOLF Institute in Amsterdam also showed direct temperature resolved mass spectrometry (DTMS) to be a very promising technique for analyzing organic pigments, as well as synthetic binding media.

However, as with traditional media, there is often a need for a more complete quantitation of the analysis, both within the individual classes of polymer/copolymer and with mixed media. A single, fully quantitative, analytical procedure for identification of the binding medium in all types of modern paints would be ideal, but it is simpler to develop a number of techniques that are more applicable to certain groups of media. For example, it may be necessary to analyze solvent-borne paints (oil, alkyd, nitrocellulose, acrylic solution, etc.) by a different method to that suited to water-borne media (such as emulsions and gouache).

Given GCI's experience and expertise with quantitative analysis of oil media, the starting point towards this goal has been with alkyd (which have a significant oil component), modern oil paints, and water-mixable oil paints, all of which are being examined by established and novel derivatization procedures for GC-MS analysis. Supplemental qualitative investigations using Py-GC-MS and FTIR are also in progress, in order to identify marker compounds from the additives in these media.

Identification of synthetic organic pigments is also an area in which little systematic analytical work has been done. There are hundreds of organic pigments, many of which are of intense tinting strength. They are therefore often present in small concentrations; this makes their detection problematic. Several analytical techniques are being assessed and compared for their ability to identify these products, such as pyrolysis-mass spectrometry (Py-MS), X-ray diffraction (XRD), FTIR, and Raman spectroscopy. Spectral libraries from these studies are shared among the partner institutions.

The vast array of additives—surfactants, dispersants, thickeners, coalescing agents, buffers, biocides, defoamers, and co-solvents—that are present in acrylic emulsion paints also pose significant analytical challenges. Within each category of additive, there are many classes of chemical compounds, making it difficult to devise a single analytical method to characterize them. Moreover, emulsion paints may contain ten or more additives in varying concentrations, which complicates chromatographic procedures for separation and identification of the individual additives. To address the many issues involved in identification of additives, analytical investigations are being conducted on the most commonly used additives in acrylic emulsion paints.

Work Completed

Alkyds and Modern Oils
A range of unpigmented alkyd resins with known composition and tube colors has been tested to assess how accurately a quantitative analysis is in determining the:

• proportion of oil to alkyd (the so-called oil length);
• oil identity (i.e., to assess whether this is affected by the presence of polyester);
• identity of polyol (and polyacid, although this is almost always phthalic anhydride);
• proportion of polyol and polyacid.

A novel GC method, initially developed by the American Society for Testing and Materials (ASTM) for quantitative analysis of polyols in alkyd resins, has been modified to accomodate small sample sizes typically removed from paintings. In a significant development, this modified GC-MS procedure is also capable of detecting polybasic acids (e.g., phthalic anhydride) and fatty acids from oil components at a semi-quantitative level. It is also encouraging that common modern pigments have not interfered with the analytical results.

Water-Mixable Oils
The project to characterize water-mixable artists oil products (WMO) has been carried out at the GCI by Casey Greet and Jesús Jiménez (graduate students in the chemistry program at California State Polytechnic University, Pomona) and Roberta Renz, a GCI graduate intern. They discovered that some patents of WMOs gave specific details about their composition and additives; this information proved useful in interpreting analytical test results. Preliminary studies of the major brands of WMO revealed that FTIR could not differentiate them from traditional artists' oil media. However, it was possible to extract and isolate some of the additives in WMOs by simple solvent-solvent partitioning. FTIR spectra of the solvent extracts closely matched those of polyoxyethylene-based surfactants (PEOs), which is consistent with the patent information. It was also found that the same set of additives could be detected in unpigmented WMOs with Py-GC-MS.

Synthetic Organic Pigments
A selection of two hundred synthetic organic pigments has been amassed and studied using several analytical techniques in order to develop databases of standard spectra; to date, the two most promising techniques have been FTIR and Py-MS.

The Py-MS library was developed by Suzanne Quillen Lomax (organic chemist at the NGA), who spent two months working at the GCI as a visiting scientist. The majority of the organic pigments displayed extremely characteristic Py-MS spectra, with the exception of pigments with anionic structures; for this group, the overall spectral intensities were quite weak.

All synthetic organic pigments were found to have extremely characteristic FTIR spectra, but since this semi-quantitative technique also detects other components in a paint film (e.g. the binder and extenders), the low concentration of organic pigment in a typical paint often means that it is very difficult to obtain a good match with the library of standard spectra. Julia Jönsson, Conservation Science Fellow at Tate, successfully developed some simple extraction procedures that significantly improve the detection limits of the FTIR technique for organic pigments, when present in a paint fragment from a work of art.

Additives
One important aspect of this research has been the acquisition and cataloguing of a range of commercial paint additives; because these products come from a wide range of chemical classes, they can be difficult to analyze with a single analytical technique. Gregory Smith, Samuel L. Golden Fellow at the NGA, tested numerous additives using size-exclusion chromatography, Py-GC-MS, and FTIR. Ultimately, he found HPLC to be one of the best analytical techniques for characterizing complex mixures of additives in acrylic emulsion paints, and developed a standard protocol capable of separating the most commonly used products. The protocol has been effectively utilized in studying which additives are potentially removed during cleaning treatments (as discussed in Component 3). A smaller set of commercial additives was also analyzed at the GCI using thermogravimetry (to obtain information about the oxidative and thermal stabilities), liquid chromatography-mass spectrometry, and FTIR.

Work in Progress

A wider range of oil-based products (alkyds, WMOs, and modern oils) are being assessed by the GC-MS technique to include oils such as castor, coconut, cotton, dehydrated castor oil (DCO), oiticica, perilla, soya, safflower, sunflower, tall, and tung. The sample set includes fresh tube colors and dried paint films, some of which have been placed in accelerated aging equipment to expose them to intense light and heat. As the project develops, it will be important to consider the effects of aging when critically evaluating the test protocols and the suitability of marker compounds. In the alkyd research, resins that contain typical modifiers such as styrene, vinyl toluene, acrylic, urethane, and polyamide are being tested by the GC-MS procedure in order to determine if these modifiers can be detected, and if they interfere with the test results for aged alkyd paints. In the paint additive research, the current focus is the testing of additives in WMOs by Py-GC-MS in order to ascertain whether PEOs can be used as reliable marker compounds, and which chemical derivatization procedure is most effective.

Future Work

One of the final steps in the alkyd and modern oil research will be application of the GC-MS protocol in technical studies of a number of modern paintings in order to study the paint media but also to assess the applicability of the newly-developed analytical methods.

Looking ahead, the next challenges for quantitative analytical methods would include:

• characterization of nitrocellulose (NC) paints, typically consisting of NC, alkyd, and plasticizer (separation techniques, i.e. GC-MS or Py-GC-MS, are to be assessed, as NC's diagnostic peaks in FTIR can often be masked by other components);
• quantitation of acrylic copolymers with different side chains, e.g. poly (ethyl acrylate/methyl methacrylate) and poly (n-butyl acrylate/methyl methacrylate) copolymers.

In the synthetic organic pigment research, new samples that have been added to the Tates pigment collection will be tested by Py-MS and FTIR, and their spectra will be compiled into the database. Additionally, the pigments and tube colors will be studied by a post-doctoral researcher at the GCI using various mass spectrometric techniques, including fast-atom bombardment, Py-MS with derivatization, and with negative ion monitoring. The entire collection of pigments will also be analyzed by Raman spectroscopy, which has proved to be a very promising additional technique for these pigments, from initial work carried out by David Wise at the University of Canberra, Australia.

Analytical work using Py-GC-MS and DTMS to characterize the additives will continue at the GCI. This work is expected to overlap somewhat with the search for stable marker compounds in the WMOs.