Glossary of Terms Casein tempera Casein, usually referred to as a glue, is an organic protein compound generally made from the curds of milk. In casein tempera, the medium is made from skim milk or lime or from powdered casein dissolved with ammonia. Pigments are mixed with this medium and applied to a variety of supports including paper, canvas, plaster, and wood panels. It resembles gum tempera in application of paint to support and in final appearance; however, the distinguishing difference is that casein tempera dries to a very hard finish not soluable in water. Egg tempera Egg yolk is mixed with pigments ground in water, and the mixture is applied to the smooth gesso surface in thin layers. The water evaporates quickly, and the oil in the egg hardens slowly, producing a hard, strong paint film. Gesso Gesso is a mixture of chalk or gypsum and glue and is applied in several thin coats over a sized wood panel or board to produce a panel that is smooth and white. Glue tempera and gum tempera Almost identical in appearance with and commonly called "gouache." Gouache (also called distemper) Actually a watercolor (or gum tempera) and a term synonymous with "opaque watercolor".… Ordinarily it is applied on paper support but with thicker layers than in a watercolor technique and is further distinguished by the use of mixed tints of white for the light colors instead of transparencies of color. Ground The preliminary coating given to the support to make a more desirable surface for rendering the expression, e.g., gesso, sizing, lead white. Provenance The background and history of ownership. Support The paper, canvas, hardboard, or other material that acts as the base upon which the painting or drawing is executed. Watercolor Used to describe a standard preparation of pigments ground in water-soluable gums. The typical transparent watercolor painting is executed with paint applied thinly and with a degree of transparency on a support, usually paper.