Egg tempera painting was the primary panel painting medium for nearly every painter in the European Medieval and Early renaissance period up to 1500. For example, every surviving panel painting by Michelangelo is made with egg tempera.
One of the oldest permanent painting media, artists have used egg tempera to create delicate, detailed works of art since antiquity. The technique evolved from “secco fresco” which is the application of pigment on to dry plaster using egg as a binder. The fact that egg tempera paint has a short shelf life would not have been a concern to mural painters because they were accustom to using up their materials within a work day.
However many artists today are accustom to storing art materials for a long time. Unlike artists' oil paints and many brands of acrylics, manufactured brands of egg tempera have to be laced with strong chemicals and preservatives to keep the paints stable. Think of egg tempera like organic food which also cannot be stored for more than a few days. Painters who enjoy working in this luminous media should continue the tradition and make their own paints.
Here is a reliable recipe:
1. Add enough water to wet pigment, grind together. Continue adding water and grinding pigments until you make a paste. This is difficult and takes time! Place paste in covered jar.
2. Separate egg yolk from egg white. Discard egg white. Gently roll yolk back and forth from palm to palm until yolk feels dry. While holding egg yolk between index finger and thumb, pierce the egg yolk sack so that yolk drains into a cup. Add an equal part of water (doubling the volume) and shake until a pale emulsion is formed.
3. As you paint, add equal amounts of pigment paste to egg/water emulsion.
You can grind pigments directly into egg/water emulsion if you plan to use all the paint you make in one painting session. While it may be easier to grind pigment into egg/water emulsion, again the mixture does not store well.