Gesso is the white ground, the first layer, that painters spread over a wooden panel before any color goes on. It mixes a mineral powder, gypsum or chalk, with animal glue.
Brushed on in many coats, then scraped perfectly smooth, it gave paint and gold leaf a hard white surface to sit on for centuries.
The name is the Italian word for gypsum, the soft white mineral that plaster of Paris is made from. English borrowed it at the end of the sixteenth century from the workshops of Italy.
There, painters prepared their panels with gypsum and glue. North of the Alps, workshops generally used chalk instead, and both were called gesso.
The Wilton Diptych shows the northern way. This small folding altarpiece, a painted panel meant to stand on an altar, was made for King Richard II of England to pray before. Its oak was first covered with fibers that seem to come from parchment, then with natural chalk mixed into glue.
In the strict Italian sense, gesso means gypsum, so the Wilton Diptych has no gesso at all. The name stuck anyway, and today it covers three recipes: the Italian one, the northern one and the one sold in art shops.
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Wooden panels and their white skin
Before canvas became common, pictures not painted on walls were mostly painted on panels of wood, from the holy icons of the Byzantine Empire to the altarpieces of Italian churches. Wood, though, has a flaw, and every one of those painters had to hide it before picking up a brush.
What You'll Discover
How a painter made gesso around 1400.
How gold stood up off the panel.
Why canvas needed something else.
What the tub in the shop contains.