Egg yolk is never used by itself with pigment; it dries almost immediately and crackles when it is dry. Some agent is always added, in variable proportions. One recipe calls for vinegar (1:1 proportion to egg yolk by volume); other recipes suggest white wine (1 part yolk, 2 parts wine). Some schools of egg tempera use various mixtures of egg yolk and water.
The paint mixture has to be constantly adjusted to maintain a balance between a "greasy" and "watery" consistency by adjusting the amount of water and yolk. As tempera dries, the artist will add more water to preserve the consistency and to balance the thickening of the yolk on contact with air. Once prepared, the paint cannot be stored. Egg tempera is water resistant, but not water proof.
Different preparations use the egg white or the whole egg for different effect. Other additives such as oil and wax emulsions can modify the medium. Egg tempera is not a flexible paint and requires stiff boards; painting on canvas will cause crackle and pieces of paint to fall off.