Glaze (painting technique)
Often, because a paint is too opaque, painters will add a medium like linseed oil or alkyd to the paint to make them more transparent and pliable for the purposes of glazing. While these media are usually liquids, there are solid and semi-solid media used in the making of paints as well. For example, many classical oil painters have also been known to use ground glass and semi-solid resins to increase the translucency of their paint.
In oil painting, the simplest form of a glaze is a thin, oily, transparent layer of paint spread over the top of an opaque passage that has been given some time to dry. Light travels through the glaze and is reflected back off of the opaque layer below. This can cause a glowing effect similar to looking at a brightly lit white wall behind a film of colored cellophane. The thin oily layers of a glaze can facilitate the rendering of details that would be more difficult with opaque paints—e.g. the complexities of skin tones.
When multiple layers of glazes are used, the colors in all visible layers can appear combined. However, the pigments are not physically mixed, since the paint is left to dry before each successive glaze is applied. The artist may apply several layers of paint with increasing amounts of oil added to each successive layer. This process of applying the fat layers (more oil in the painter’s medium) over the lean layers (less oil) can minimize cracking; this is the "fat over lean" principle.
Scumble is a technique similar to glazing, except that the coating is opaque, and is just painted on very thinly to allow bits of the paint below to shine through. Scumbling works by a principle similar to that used by pointillists, mixing colors optically. While most painters glaze with dark colors, scumbling is more popularly used for lighter colors; especially atmospheric effects when rendering fog or clouds.