Egg yolk, (provided by Sue's 'girls' above) is mixed with distilled water to make a wonderful translucent medium into which ground pigments are added. There are numerous recipes for egg tempera and as egg yolk is a natural emulsion it can also be mixed with drying oils such as linseed oil or poppy oil.
Egg tempera dries very quickly and so is gradually built up by applying delicate brushstrokes in numerous layers and glazes, the white gesso ground beneath giving luminosity. The paint surface dries to a velvet matt finish which cures over months to become stable and even waterproof. Modern materials are unable to replicate the beauty and durability of this ancient technique.