Source Wikipedia - Tempera (also known as egg tempera, poster color or poster paint) is a type of artist's paint and associated art techniques that were known from the classical world, where it appears to have taken over from encaustic and was the main medium used for panel painting and illuminated manuscripts in the Byzantine world and the Middle Ages in Europe, until it was replaced by oil painting in Europe. It has remained the required medium for Orthodox icons. It is paint made by binding pigment in an egg medium. However, the term tempera in modern times is also used by some manufacturers to refer to ordinary poster paint, which is a form of gouache that has nothing to do with real egg tempera.,

One might observe simply by washing breakfast dishes that egg yolk dries quickly and adheres firmly. Tempera was traditionally created by hand-grinding dry powdered pigments into egg yolk (which was the primary binding agent or medium), sometimes along with other materials such as honey, water, milk (in the form of casein) and a variety of plant gums. Many of the Fayum mummy portraits use tempera, sometimes in combination with encaustic. Oil paint was invented in the north of Europe during the Middle Ages (Theophilus mentions oil media in the 12th Century) and was the principal medium used from the 15th century in Early Netherlandish painting in northern Europe. Italy, Greece, and Russia were the major centres of tempera painting. Around the year 1500, oil paint replaced tempera in Italy. Tempera continued and continues to be used in Greece and Russia. In the nineteenth and twentieth centuries there were intermittent revivals of tempera technique in Western art, among the Pre-Raphaelites, Social Realists, and others.

Tempera paint dries rapidly. The techniques of tempera painting can be more precise when used with traditional techniques that require the application of numerous small brush strokes applied in a cross-hatching technique. The colors, which are painted over each other, resemble a pastel when unvarnished, and are deeper colors when varnished.

Tempera is normally applied in thin, semi-opaque or transparent layers. When dry, it produces a smooth matte finish. Because it cannot be applied in thick layers as oil paints can, tempera paintings rarely have the deep color saturation that oil paintings can achieve. On the other hand, tempera colors do not change over time^[1], whereas oil paints darken, yellow, and become transparent with age.^[2]

True tempera paintings are quite permanent, and examples from the first centuries AD still exist, eg the Severan Tondo and some of the Fayum mummy portraits.