1 INTRODUCTION

Identification of resins used for furniture finishes is important for art historical analysis of an artifact as well as for an initial material survey in restoration or conservation treatments. A detailed chemical analysis of surface finishes can also provide important information on previous surface treatment procedures and techniques.

The materials and techniques for furniture surface treatment have, for the most part, been developed empirically throughout the ages. The craft was industrialized in the second half of the 19th century, yet many craft procedures were retained until the beginning of the 20th century. Collections of old recipes provide some insight into the chemistry of varnishes used for furniture surface treatment (Brachert 1978; Mussey 1982; Penn 1984). However, the natural resins used to a great extent for these surface finishes are very difficult to analyze due to their complex composition, natural variability, and susceptibility to polymerization and oxidation. A review of the chemistry of natural resins important for research in art and archaeology was published by Mills and White (1977).

The analytical methods used for natural resin identification were reviewed several years ago by Masschelein-Kleiner (1986). While simple tests can provide some information on whether resins are present, there are primarily two instrumental techniques—gas chromatography and infrared spectroscopy—that can provide detailed characterizations of resins. Gas chromatography and gas chromatography mass spectrometry are widely used for the identification of resin types and quantities and are particularly useful for mixtures of components. Infrared analysis can identify a broad range of components that may exist in the sample without using any derivitization procedures.

The advantages of infrared spectroscopy for the analysis of varnishes and surface finishes was recognized relatively early (Feller 1954, 1958). Since then, infrared spectroscopy has found a widespread application in art conservation laboratories and laboratories that deal with analysis of natural resins (Thomson 1963; Bruun and Sjoholm 1964; Soteland and Elletsen 1964; von Jayme and Traser 1969; Low and Baer 1977, 1978). More recently, a paper was published on the capabilities and limitations of Fourier transform infrared (FTIR) spectroscopy for the identification of natural resin mixtures used in historic furniture finishes (Derrick 1989).

The standard infrared (IR) analysis method requires that the sample be scraped from the surface and mixed thoroughly with KBr. This procedure can result in a misrepresentation of any stratigraphy in the surface finish. In evaluating the role of infrared spectroscopy in identification of natural resins, Masschelein-Kleiner (1986) wrote: “The method [IR] needs previous purification of the sample to ensure as far as possible that only one layer [of varnish] has been taken.” It is difficult to remove just one layer because some varnish layers are very thin and some surface finishes represent complex multilayer structures. However, the recent advance of infrared microspectroscopy allows for analysis of small sample areas through the coupling of an infrared microscope to an FTIR spectrometer. A method is described in this paper for the analysis of multilayer structures of surface finishes using infrared microspectroscopy to identify individual varnish layers in a cross section. This technique has been applied to many other types of samples ranging from multilayer paintings (Derrick et al. 1990; Landry et al. 1990) to deteriorated parchment (Derrick 1990).