SOME IMPROVEMENTS IN THE STUDY OF CROSS SECTIONS
JIA-SUN TSANG, & ROLAND H. CUNNINGHAM
THE STUDY of cross sections can provide a large amount of precise information about the composition of a very small area from a work of art. It complements other techniques such as x-radiography and examination under ultraviolet and infrared light, which give rather more general information over a large area. Pioneering work on painting cross sections was conducted by Raehlmann, who published his results in 1910. Another important earlier paper was published by Plesters (1956) on cross sections and the chemical analysis of paint samples. It is still widely referred to today. Although the technique has become widespread in conservation laboratories in museums and in conservation training schools, the basic approach of microscopic examination and chemical staining and testing has remained relatively unchanged for many years.
During this time, however, techniques of instrumental analysis have changed tremendously. While instrumental analyses have been used to solve other problems in conservation, they have not always been suited to the study of the individual layers in cross sections either because of the limited sample size, or the fact that most instrumental analyses are destructive, or that sample preparation methods were inadequate for the proposed techniques. Consequently, there is a lack of correlation between the technical information gathered from the microscopic study of cross sections and that from instrumental analyses. The use of a microscope for identification of the pigments and media in a cross section is a technique that is accessible and much used by most conservators in most conservation laboratories. Unfortunately, techniques such as gas-liquid chromatography (GLC) and high-performance liquid chromatography (HPLC) are accessible to only a few conservators and conservation scientists in a few conservation laboratories. The paper published by Erhardt et al. (1988) suggested a systematic approach to the instrumental analysis of natural finishes and binding media. That paper is useful as a guide and reference for those who do have access to an analytical laboratory. In that analytical scheme, the suggested first step is to obtain an infrared spectrum of the sample. The use of these spectra makes it possible to narrow the range of materials in a sample by excluding various potential components and to classify in a general way those major components that are present.
We describe below our method for providing cross sections thin enough for a transmitted light Fourier transform infrared (FTIR) microscope to obtain infrared spectra of the individual layers in a cross section. This is the first step in linking the study of cross sections directly to other instrumental analyses. In addition, by thinly slicing cross sections, we increase the number of samples that can be used both for microscopic and other analytical procedures. It is hoped that by doing this we can narrow the gap between simple chemical and physical tests and modern instrumental analyses.