(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987 revised 1998)
In addition to varying drying times among pigments, there are also varying film characteristics, which can range from tough and flexible to hard and brittle. It is the brittle film characteristics that are of greatest concern, particularly when used in underpainting. If a brittle paint is used underneath, it may cause the overpainting to crack.
Zinc-based paints, such as zinc white, are the most brittle and are not recommended for use in underpainting. Cobalt-based colors, barium yellow, strontium yellow, and red oxides, although not as brittle as zinc paints, are also not recommended for use in underpainting. Lead-based paints, raw umber, and burnt umber are among the most flexible paints and are perfect not only for underpainting, but also for mixing with more brittle paints to stabilize them.