Binders: Gum Arabic and Egg

Gum arabic, egg tempera and oil compared: what each is, how it dries, and which paint it makes.

Gum arabic is the hardened sap of Acacia senegal trees and is the standard binder of watercolour paint; it has been traded from the Sahel for centuries. It dries by water evaporation, and a gum-arabic film can be re-wetted, which is what lets watercolour be worked again after it has dried.

Egg tempera binds pigment with egg yolk; it was the main panel-painting medium of European art before oil paint spread in the fifteenth century. The yolk sets as it dries and does not re-wet, so tempera layers are built up in thin strokes rather than washed back.

Oil paint spread across European panel painting in the fifteenth century, taking over from tempera. It cures slowly through the film rather than drying by evaporation, which gives long working time but a long wait before the next layer.

The choice of binder decides the behaviour of the finished paint more than the pigment does: the same ochre ground fine will behave as a re-wettable watercolour with gum arabic, a fast-setting panel paint with egg yolk, or a slow-curing film with oil.

For a first attempt, gum arabic is the most forgiving route: it is the standard watercolour binder, it re-wets if the mix is wrong, and it does not set irreversibly the way egg yolk does.

• Gum arabic — hardened Acacia senegal sap — dries by evaporation, re-wettable — watercolour
• Egg yolk — tempera binder — sets irreversibly — panel painting before the fifteenth century
• Oil — cures slowly through the film — long working time — spread from the fifteenth century

Further reading

• Which Plant Gives Which Colour
• Making Walnut Ink