Oil Painting - Rule

There is another important reason for this rule. As previously explained, linseed oil oxidizes as it dries. It unites with oxygen from the atmosphere, becoming heavier in the process. Furthermore, it moves as it dries, expanding and contracting its bulk considerably. Since the film dries from the top (where the air is) toward the bottom, it may be dry or tacky on the surface while it is still oxidizing and swelling below the surface. If a film of "leaner" paint containing less oil is placed over such a half-dry underpainting, the lean film may become thoroughly solid and dry before the fat film has completely gone through its drying process. In such a case the movements of the lower film may cause the dry upper film to crack and fracture, in much the same way that heaving ground may cause a concrete sidewalk to crack.

Because of these facts, a painter who develops pictures in several layers of paint should use a painting medium in the upper layers that contains a higher percentage of oily ingredient than does the medium in the lower layers of the underpainting. (Varnish is not considered an oily ingredient since it does not expand on drying.) Therefore the medium for underpainting should contain somewhat more varnish and turpentine, while the medium for overpainting should contain more oil. If possible, the underpainting should be applied as a denser, drier paste (thinned with less medium), whereas the overpainting should be richer, oilier, and slightly more fluid. Fast-drying colors, such as lead white or Naples yellow, are to be preferred in the underpainting, and the slower-drying pigments, such as zinc white or cadmium yellow, should be reserved for overpainting whenever possible. [p. 83]