The Practice of Tempera Painting. Yale University Press. Fourth Printing, 1946. Some of the most useful and agreeable of these techniques depend upon the use of artificial emulsions.
Egg yolk is a natural emulsion, a mixture of watery and oily parts.
Artificial emulsions may be made in innumerable combinations, many of which have strikingly individual working characters as painting media. Some are good, some bad, technically considered. Only the most cursory reference to these media can be given here. A Basic Formula Certain substances act as emulsifying media. Oily materials can be added to them, and then watery materials, and the oil and water will not separate, but remain for some time in intimate mixture.