EMBEDDING PAINT CROSS-SECTION SAMPLES IN POLYESTER RESINS: PROBLEMS AND SOLUTIONS
MICHELE DERRICK, LUIZ SOUZA, TANYA KIESLICH, HENRY FLORSHEIM, & DUSAN STULIK
Polyester resins were introduced in the 1940s. Because of their transparency and ease of preparation, they were soon used for the encapsulation of natural history objects (Purves and Martin 1950). Although it was later discovered that the long-term life of the polyester was not appropriate for museum objects (Meurgues 1982), polyester resins are still commonly used by schools, scientists, and hobbyists for embedding and casting many objects. Polyester resin is also routinely used as a medium for embedding multilayer paint cross sections from art objects (Plesters 1956; Wolbers and Landrey 1987; Tsang and Cunningham 1991). In a recent detailed comparison of 14 types of embedding media, polyester resins were found to be the most satisfactory for embedding cross sections (Waentig 1993).
The preparation of cross-section samples has become standard practice for the examination of painted surfaces in art conservation and forensic analysis. Large, sturdy cross sections can often be polished and microtomed without embedding, that is, surrounding the sample with another material. However, tiny or fragile cross-section samples need to be embedded in a supporting medium to hold them together and in the correct orientation for examination. The mounted cross section is a valuable source of information for a painting's layered structure, pigments, and technique. Relating the structure to its materials can also provide indications of relining, overpainting, or other restoration procedures.
In the analysis of mounted paint cross sections for their pigment components using light microscopy and scanning electron microscopy with energy dispersive x-ray spectrometry (SEM/EDS), no apparent problems have been cited with polyester embedding resins. However, the determination of the types of binder in a sample can require other techniques, such as staining (fluorescent and nonfluorescent) and infrared microspectroscopy. With these techniques, it has become apparent that polyester resin and other polymeric embedding media can interact with some specific types of samples during the embedding process. This interaction may produce interferences in the analysis of the organic portion (binder and coatings) of the sample, and caution must be used when any results are interpreted.
Some samples may contain components that are soluble in the liquid prepolymer. Godla (1990) pointed out that polyester resin can dissolve some waxes in furniture finish samples, often making it difficult to examine wax finishes after embedding. (Bischoff 1994) has observed that some organic dyes on inorganic carriers in modern (post-1850) pigments are solubilized by polyester resin. In addition, polyester resin was found to dissolve fresh (less than one-year-old) natural resin samples of dammar, mastic, and copal (Derrick et al. 1992). Aged or oxidized resin layers on furniture finish cross sections did not seem to exhibit the same solubility problem.
Polyester resin has also been found to soak into or infiltrate porous, low-binder samples, as observed by (Baker et al. 1989) for embedded paper cross sections. In many cases, this impregnation is beneficial because it consolidates the sample, making it less fragile and easier to polish or microtome. However, embedding media infiltration also has unwanted characteristics. It may change the appearance of the sample and hinder infrared spectral analysis of the sample components. Penetration of the resin into the sample also interferes with the use of stains (fluorescent and nonfluorescent) for binder identification in cross sections by coating the particles in the sample and thereby inhibiting interaction of the stain with the binder. Thus, it is important to recognize that infiltration can occur with some embedded samples and to be able to prevent infiltration whenever it is undesirable. This paper will illustrate these problems and discuss alternate embedding materials and procedures.