A tiny sample taken from a paintings and mounted in a resin, which cures to become a solid block that may then be ground so that a cross section of the paint layer is produced. The cross section represents the layer structure of the painting in the area sampled and may include the ground layer, preparation layer(s), underdrawing materials, paint layer(s), and surface coating(s).

Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 26.