Tempera (or egg tempera) is the primary type of artist's paint and associated art techniques that were prevalent in Europe's Middle Ages. 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.

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. After the invention of oil paint in the Late Middle Ages, tempera continued to be used for awhile as the underpainting (base layer) with translucent or transparent oil glazes on top. This transitional, mixed technique was followed by a sole oil painting techniques, which for the most part replaced tempera in the 16th century.

Tempera paint dries rapidly. The techniques of tempera painting can be exacting 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, or the 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.

True tempera paintings are quite permanent.

Making tempera

1. Place a small amount of the pigment paste onto a palette, dish or bowl.
2. Add about an equal volume of the egg medium and mix well making sure there are no lumps of pigment. Some pigments require slightly more egg medium, some require less.
3. Add distilled water (usually less than a teaspoon per egg yolk), trial and error will dictate just how much water is required.

Only the contents of the yolk are used. The white of the egg and the membrane of the yolk are discarded. After isolating the yolk and drying the membrane slightly by rolling it on a paper towel, pick up the yolk gently by the membrane, dangle it over a receptacle and puncture the membrane with [for instance] a toothpick to drain off the liquid inside.

If the paint contains too much yolk, the paint will look greasy and clumpy; too much water makes it run. So makers of paint have to finely adjust the amount of water and yolk to achieve a consistent paint. 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.

Many of the pigment used by medieval painters, such as Vermilion (made from mercury ore), are highly toxic. Most artist nowadays use artificial pigments, such as the quinocronones are less toxic but have similar color properties to the older pigments.

Source: WikiPedia

Some artist known for using egg tempera include Reginald Marsh, Thomas Hart Benton, George Tooker, Paul Cadmus, Robert Vickrey, Andrew Wyeth, Jared French and many others.