Egg yolk (the yellow part of the egg) dries and sticks very firmly. It is used to mix and set the paints in a tempera painting. The colours are ground up into dry powders that must be mixed by the artist. In earlier times, some of the paints were very poisonous and the artist had to be careful not to breathe in the powdered paint. The paints are generally mixed with equal parts of egg yolk and water. If there is too much egg, the paint is gooey. If there is too much water, it is runny. Neither of these things are good. Some artists like to mix some of the egg white as well, for a different effect. Sometimes some oil is added. In ancient times other things were added which might include honey, milk and plant gums.