Old tempera

Natural tempera was known in ancient times. It enjoyed its greatest popularity in the Middle Ages. Egg, as a natural emulsion, was used in both easel and wall painting.

Composition of tempera

Cennino Cennini says this about egg tempera:

"Two types of tempera are used - one better than the other:
The first consists of mixing the white and the yolk, adding fig tree juice to them and mixing them thoroughly. Then about half of the diluted wine is added to this mixture, and on top of this the dyes are rubbed off.
If you put the tempera in a thick layer, the paint will quickly crack and pull away from the wall.
The second type of tempera consists of egg yolk alone.
Tempera is usually used in wall technique and also on wood and metal."
Tempera based on yolk alone also contained fig tree sap, according to Vasari's treatise.
Cennino Cennini describes tempera during its heyday and greatest development.
He describes interesting details of the technique, such as, for example, the ability to use the color of the yolk.
Light yolk (urban hens) was to be used to paint women's faces. The dark yolk (of rural hens) to paint drab faces.

Composition of the egg

A hen's egg weighs approximately 40 to 60 grams, with the yolk weighing 20 grams and the white weighing 40 grams.
The largest weight of eggs is in spring, and the smallest in winter.

EGG COMPOSITION	Yolk	Protein
Water	51,5%	84,8%
Albumin and vitellin	15%	12%
Egg oil	22%	0,2%
Lecithin	9%	traces
Mineral bodies	1%	0,7%
Other components	1,5%	2,3%

Protein, like yolk, belongs to nitrogenous substances that contain sulfur. These substances easily decompose and rot.
Protein is actually (judging by its components) an aqueous solution of albumin. It consists of cells with very thin membranes and lacks liquidity to some extent.
The protein has an alkaline reaction (reaction). It congeals at increased temperature. Heavily diluted with water, it does not shear.
Shear of the protein, or albumin, also occurs when exposed to spirit, phenol, creosote, strong acids and alkalis, sublimate and tannin. When mixed with turpentine, it also shears off.
With lime, the protein gives a permanent, water-insoluble bond.
In the dry state, the protein itself becomes insoluble in water over time. The shearing of the protein occurs when the water is completely separated from it.
Egg white, after applying a thin layer of it to glass, when it dries forms a transparent film, very brittle and water-soluble. Its brittleness does not allow to use for tempera the white alone, but together with the yolk.
The protein is strained several times through a thick cloth - then it becomes liquid.
The yolk is bounded by a thin membrane and more or less colored. It consists of a stable emulsion.
The emulsifiers are albumin and vitellin (vitellin is stronger).
The yolk contains oil, but is still capable of emulsifying as much oil as it weighs by itself.
The coloration of the yolk is impermanent and quickly perishes when exposed to light. Zinc white rubbed with the yolk whitens quickly (lead whitens more slowly).
Egg oil belongs to animal fats. In its pure state in the air, it dries very slowly (as does fish fat).
Yolk used in painting in larger proportions than it should be, causes the paint layer to crack.
As the yolk dries, it still loses water and further short-circuits the paint layer, which can also lead to cracks.
For the conservation of the egg, fig juice was introduced in Italy. In Germany, beer (a certain percentage of spirit) was introduced for this purpose, in Russia - bread acid (in proportions equal to the protein).
According to Cennini, dye was added to the egg binder in equal volumes, and then the paint was diluted with water (to a liquid state).
With the above tempera it was possible to paint in very thin layers. As it dried, it lightened slightly and became duller.
Yolk-based tempera was painted mainly on boards. It was also used for wall painting.
Both yolk and white were used to paint walls. Such tempera was also used for retouching frescoes.
Since the egg white added brittleness to the binder, fig juice and wine were present as plasticizers in this type of fresco, in a volume equal to the egg.
The egg itself was never used, but diluents were always added - always in proportion to the volume of the egg.
In later tempera recipes, instead of the above-mentioned diluents, there is skimmed milk (for two eggs - 1 liter) or solutions of vegetable adhesives.
According to Theophan, for some dyes, protein itself was sometimes used as a binder. Especially in miniature painting for lead white and minia.
Such a binder was an intermediate link between the glue technique and watercolor.