FURNITURE FINISH LAYER IDENTIFICATION BY INFRARED LINEAR MAPPING MICROSPECTROSCOPY
MICHELE R. DERRICK, DUSAN C. STULIK, JAMES M. LANDRY, & STEVEN P. BOUFFARD
Resins commonly used in the historic varnish recipes listed by (Brachert 1978, 1979) were selected for this study. They included an insect resin (shellac) as well as diterpenoid (copal, sandarac, and rosin) and triterpenoid (mastic) tree resins. Resin varnishes on furniture were typically either applied as spirit varnishes (resin-solvent mixture) or oil varnishes (resin-oil-solvent mixture). Only spirit varnishes will be discussed in this paper.
Solutions of the resins in ethanol (40 % w/v) were prepared to represent typical spirit varnish formulations (McGiffin 1983; Allen 1984). These solutions were used to make facsimile samples by brushing the varnish on mahogany veneer, Teflon, or polyethylene surfaces. Multiple layers, each containing a single resin component, were applied to each sample. Each layer was allowed to dry for several days before a new layer was added.
For the first set of samples, portions of the veneer-resin composite (e.g., 2 × 5 mm) were removed from the plywood support using a razor blade. The resultant sample cross section was of sufficient strength and size to allow for direct mounting and slicing on a Reichert-Jung Model 2040 Autocut microtome with a stainless steel blade. The samples were cut to produce 15–25 μm thin-section slices of the layered cross section.
The second method of cross-section preparation involved removing smaller portions (e.g., 1 × 3 mm) of a multilayer resin coat from either the Teflon or the polyethylene support. This sample was then embedded in a polyester resin (sold under the trade names of Bioplastic or Caroplastic). Once the mold was cured (≈ 24 hours), most of the excess plastic around the sample was trimmed away to minimize the contact area for slicing. Using this procedure, samples were routinely microtomed to give 5–15 μm thin-section slices. All samples of finishes obtained from museum objects (e.g., 0.5 × 1 mm) were embedded in polyester resin and then microtomed in this manner.
Once microtomed, the thin-section slices were placed on a BaF2 window and transferred to the sample stage of the infrared microscope for analysis. The infrared microscope used in this study was the Spectra-Tech IR-Plan II research infrared microscope positioned in the sample beam of a Perkin-Elmer Model 1760 FTIR spectrometer. The microscope was equipped with a narrow-band, cryogenically cooled mercury cadmium telluride (MCT) detector. The spectrometer was interfaced to a Perkin-Elmer Model 7700 Data Station. Spectra were collected from 4000 to 700 cm−1 using 120 scans with a resolution of 4 cm−1. The microscope contains adjustable knife edge apertures above and below the sample stage to isolate a rectangular window on the sample for analysis. A Spectra-Tech motorized micropositioning stage was used to control the stepwise movement for the linear mapping experiment. The X–Y stage uses two stepping motors with a minimum step size of 1 μm. While the use of the motorized stage simplifies the analysis for the operator, this same procedure may be done with manual stage movement.