Cross sections yield a wealth of information as they reveal the buildup of layers in a painting including the ground layers, paint layers, glazes, and any varnish coatings. Using a scalpel or similar tool, a very small amount of material, usually no bigger than the head of a pin, is carefully removed from a painting. In some circumstances multiple samples are taken from different areas of a painting. The sample is imbedded in a resin, such as polyester, that hardens overnight. The cross section is subsequently ground and polished to reveal the stratigraphy of the sample. A microscope is then used to examine the exposed paint layers under high magnification from which digital images can be captured. Reflected light or, if the sample has been thinned, transmitted light is used to observe the cross section.
Examination will show the thicknesses and interior composition of the layer. More specifically, a cross section can display the pigments, their colours, the proportion of each in a mixture, and the surrounding binding medium. From the examination of this extremely tiny sample, we can gain an understanding of the process used by the artist to build up the various layers that result in the final image. An indistinct boundary between paint layers may indicate that the first paint layer was still wet when the second layer was applied (wet-into-wet method). Cross sections can also yield evidence of another painting hidden beneath, or a previous design element covered over by subsequent restorations. Comparing a cross section with those from other paintings by the same artist will put the information into context. Cross sections can even show the causes of degradation, for example, when too many paint layers prevent formation of a stable structure, when the ground layer is cracking, or when the pigment and oil have reacted chemically and degraded to form metal soaps. Thus, a cross section can be a powerful analytical tool to answer questions of technique, condition, and authenticity.
A cross section can also be examined with other analytical techniques including fluorescence microscopy and scanning electron microscopy. Indeed, some cross sections have been stored for decades and can now be examined with new techniques. Recent work on femtosecond pump-probe microscopy indicates that virtual cross sections, where no samples are taken, may be a possibility in coming years. The hope for the future is to be able to examine cross sections without taking a sample.