PHYSICAL AND MICROSCOPIC EXAMINATIONS
A combination of microscopes (stereo, transmitted light and polarized light) at magnifications of 2× to 100× is used to examine the layers in a paint. Many layers will be visible without preparing the sample, but definitive paint layer identification often requires some sample preparation techniques. The paint layer structure can be seen by cutting through the sample with a scalpel blade at an angle; this technique increases the visible area of the sample. The structure of the layers and any irregularities and inhomogeneities are typically easier to see after this sectioning. Very thin sections of the paint can be accomplished with a steady hand and a fresh scalpel blade; a device called a microtome can also be used. A microtome is a mini vice that holds a sample in place while a heavy and very sharp glass- or diamond edged knife slices off sections of a few tens of microns thick, as shown in Figure 1.
FIGURE 1 A microtome is a mini-vice that holds a sample in place while a heavy and very sharp glass- or diamond-edged knife slices off sections of a few tens of microns thick. Micro tomes are useful for paint analysis by conserving sample consumption and preserving the samples for subsequent analyzes.
Cross-sections of the paint sample, either embedded in a material for support or thin-section preparations, provide information about the layers, their thicknesses, and colors, in addition to the size and distribution of pigments. Embedded preparations, as shown in Figure 2, work well because the sample is easily handled and can be subjected to many analytical techniques with a minimum of additional preparation. Microscopic comparisons of paint layers can reveal slight variations between samples in color, pigment appearance, flake size and distribution, surface details, inclusions, and layer defects. Any visual comparisons must be done with the samples side by side in the same field of view (or with a comparison microscope), typically at the same magnification. Visual memory is quirky and samples must be seen next to each other at the same time so that subtle details are not overlooked. Polarized light microscopy (PLM) is appropriate for the examination of layer structure as well as the comparison and/or identification of particles present in a paint film including, but not limited to, pigments, extenders, additives, and contaminants. The use of the PLM in the identification of pigments is detailed in a chapter titled “Application of Particle Study in Art and Archeology Conservation and Authentication,” writ ten by Dr Walter McCrone, in The Particle Atlas (1979). Other components of paint, including extenders, are large enough that they can be identified by PLM, although some pigment particles may be too small to be identified this way. The use of PLM for the identification of paint components requires a good deal of intensive training and experience; many training courses are available in PLM for a fee and similar analytical courses may be offered by materials science or geology departments at universities.
FIGURE 2 Embedding paint chips in an epoxy-resin support and preserve the sample, allow for thin sectioning, and make a fragile specimen safer to handle and store.
