Gesso resembles paint in appearance, but is thinner and it dries hard. It is applied with a brush and must dry before the surface can be painted on. Gesso was first created for use in painting, to give the surface the right properties to receive paint. In Gothic and Renaissance panel painting, gesso was applied over a panel of wood to give the paint something to which is could adhere. It created a slightly rough surface and prevented the paint from seeping into the wood. Gesso can be quite a brittle surface and if mixed wrong can crack.
Egg tempera is a permanent drying medium consisting of coloured pigments and egg yolk. Egg tempera painting was the primary method until the development of oil paints in the 1500s. Tempera also refers to the paintings in this medium. Tempera paintings are very long lasting and examples from the 1st centuries AD still exist, for example Egyptian sarcophagus painting and early panel painting by Michelangelo.
Egg tempera is traditionally created by mixing small amounts of pigment with egg yolk and distilled water. The egg is separated to remove the white and then the egg yolk is pierced to remove the outer membrane. The mixing and amounts cannot be measured, but must be judged by eye. The consistency should lie somewhere between watery and greasy. Some pigments are unstable and therefore require more binder, i.e. yolk. Depending on how thick you wanted your tempera to be, you would add more or less water. Despite being a water-based paint, it is not water resistant and therefore the paint will not keep more than a few hours.
If you wanted a quality of paint more akin to oil paint, you would mix a ratio of 1:1 of yolk and linseed oil to your pigment.
Tempera paint dries very quickly and is best applied thinly in semi-opaque layers. You will build up the colour in layers and it allows for experimentation with different coloured layers. The finished appearance is matte and smooth. The advantage over oils is that oil colours will yellow in time, but egg tempera colours do not alter over time.
Tempera adheres best to an absorbent ground with a lower oil quantity than itself, which is why oils are not suitable. Tempera and gesso requires a hard surface like board, and not a flexible one like canvas as it would crack otherwise.Tempera painting became out of favour in the Late Renaissance but it was rediscovered by artists such as William Blake, and the Pre-Raphaelites.