"Tempera" means any medium used to temper dry pigments so that they may be applied and bound to a surface. Usually tempera signifies only media containing egg or glue. Other types are classified as oil or water color. The use of tempera goes back many centuries. The basic medium is composed of egg-yolk, to which an equal amount of water has been added. On a glass slab, the pigment is ground with this medium by means of a spatula. It is most often used at the consistency of heavy cream. Because of its oil content, egg yolk may be regarded as closely related to th£ oil medium. As it drys the water content evaporates, causing the paint to set quickly. In time, the albuminous substances become insoluble, and the oil dries quite slowly, leaving a strong flexible film.
The oil content of the egg medium may be increased by the addition of a drying oil or oil varnish, the result being known as an "emulsion medium." (See the chapter on painting recipes and equipment.) In a very similar manner a water solution of gum arabic may be added, making an egg and gum medium. The American tempera painters prefer these media to the pure yolk of egg. Glue and size are similar substances, softening easily in cold water and soluble in hot water. Glue and size contain no oil, being composed of nitrogenous matter ; hence the film formed by them is always soluble unless covered with varnish. Preservatives and plasticizers are added to glue and size media to prevent purification and cracking.
The support for tempera painting may be wood, canvas, or some of the modern wall boards. In many cases cardboard and paper are used, especially in commercial art where temperas are largely employed. A priming of gesso on the support is used for tempera painting. Its smooth luminous ground is particularly adapted to this technique, since it increases the brilliance of the picture. The gesso is sometimes coated with a weak solution of size to render it less absorbent. Illustrators' board and tinted drawing papers usually need no priming when used with glue tempera. The medium does not contain oil, and the paint remains opaque with little tendency toward translucence.
Pigments used in tempera are the same as those used in oil or water-color work. The medium is regarded as more permeable to moisture and gases than oil, and the fugitive, changeable pigments are avoided. Pigments containing lead and copper are also excluded ; they may be affected by sulphur in the albumen of the egg yolk.
In the Renaissance, tempera was applied with a very fine brush and the forms built up by a series of hatched lines. Today it is used more flexibly and has occasionally been employed as an under-painting on which oil glazes are superimposed. Tempera has its own distinctive quality, however, and is quite different from oil in the soft, flat brilliance of its tones.