For centuries, artists and craftspeople have used natural organic materials in the creation of works of art and artifacts. Proteins, oils, waxes, resins, and plant gums have a long history of use as coatings, varnishes, adhesives, and paint-binding media. Identification of these materials aids in the planning of appropriate conservation treatments, provides information on an artist's methods, and sheds light on the processes of deterioration.
One of the most powerful analytical tools for studying organic substances is gas chromatography-mass spectrometry (GC-MS), a technique designed to identify the components of complex mixtures. Since the early 1990s, a team of GCI scientists led by Dusan Stulik and Michael Schilling has been committed to advancing the state of the art in GC-MS analysis of organic materials. The team's work, which incorporates new technological developments, extensive use of quantitative analysis, and improved data-interpretation techniques, has led to a number of exciting findings that are described in more than 10 publications.
For example, a new procedure developed to analyze amino acids in proteinaceous binding media (such as animal glues and egg tempera) permits the identification of protein mixtures in paint. Luiz A. C. Souza, a former Scientic Program intern who worked on the team, used the procedure for studying painted altarpieces from Ouro Preto, located in Minas Gerais, Brazil. Dr. Souza found that the gesso layers of the altarpieces were prepared with animal glue, and that some of the colors were done in egg tempera.
Recently, the team reconsidered a procedure developed 30 years ago for the analysis of fatty acids and glycerol in food oils, and modified it for use in analyzing paint. The updated procedure is remarkably effective for assessing the extent of degradation of oil-paint media, as demonstrated in a collaborative study with Susan Lake of the Hirschhorn Museum of paintings by Willem de Kooning. The tests confirmed anecdotal accounts that de Kooning employed slow-drying oils in his paints in order to prolong their workability, a practice that has led to increased hydrolytic breakdown of his paints.
An especially promising procedure currently being evaluated by the team allows the analysis and identification of proteins, oils, waxes, and resins that are present in a single paint sample. The procedure, dubbed "POWR" (for protein, oil, wax, and resin), accomplishes in one method what previously required four separate tests, thus eliminating the need for multiple sampling from a work of art. Tests of Dead Christ Supported by Mourning Angels, a late-15th-century tüchlein painting from the Bonnefanten Museum in Maastricht, The Netherlands, confirmed the presence of plant gum in the medium instead of the more commonly encountered animal glue. GC-MS team member Herant Khanjian and Hélène Dubois, a former intern in the J. Paul Getty Museum paintings conservation department, presented a poster that described this work (which happened to be the first recorded occurrence of the use of this medium in an easel painting) at the 1995 Leiden Conference on historic painting techniques.
Other recent applications of GC-MS include: (1) the detection of blood as a binding medium in pigment cakes from the Chumash Indians of central California; (2) the identification of meat as a ritual meal from a 2,000-year-old Chinese Zhou dynasty lacquer earcup; (3) the discovery that plant gums contain characteristic levels of amino acids that can be used for their identification, and the use of that discovery to confirm that the paint used in the wall paintings in the tomb of Nefertari in Luxor, Egypt was applied with a gum-arabic medium; and (4) detection of free fatty acids in "ghost images" (the white, hazy deposits on the glass inside picture frames).
At present, the team is developing a procedure for identifying plant gums which will complement the POWR procedure results. Traditional gum-analysis procedures, pyrolysis-GC, and direct insertion probe ms techniques are all under consideration by Mr. Schilling and David Carson, the newest team member.
Broad dissemination of the research results is a high priority for the team. Although the GC-MS publications reach a wide audience, hands-on training in the new procedures provides unique opportunities for collaborating with other institutions and for studying unique collections of art objects. The first example of the team's efforts to reach other museum professionals was the "Analytical Techniques in Conservation" course held at the Winterthur Museum, Delaware, in 1996. The team is now negotiating with potential partners from other major institutions to study significant collections using the new procedures.