What Is Gesso? The Layer Between the Panel and the Painting

Traditional gesso is an inert white powder, either gypsum or chalk depending on region, mixed with animal glue and applied warm in many thin layers over a panel that has usually been covered with cloth to bridge any joints. Each layer is allowed to dry and the whole is then scraped and polished to a hard smooth surface of almost ivory quality. It does several things at once. It seals the wood so that it does not absorb the binder from the paint. It provides a uniform white ground that reflects light back through translucent paint layers, which is what gives early panel painting its luminosity. It provides a surface smooth enough for fine detail and for the application of gold leaf, which shows every irregularity beneath it. And it can be carved, incised and moulded before painting.

The ground is not a neutral undercoat and its properties shape the finished work:

• •Absorbency, which controls how the paint sits and how much binder is drawn out of it
• •Whiteness, which is the light source for any translucent paint applied over it
• •Hardness, which allows burnishing gold to a mirror finish against it
• •Smoothness, which permits the fine detail that egg tempera technique depends on
• •Workability, since patterns could be incised, punched and modelled in relief before gilding
• •Rigidity, which is why it suits panels and cracks on anything that flexes

The move from wooden panels to stretched canvas from the fifteenth century onward made traditional gesso unusable, since a ground that hard cracks and falls off a flexible support. Oil grounds containing drying oil replaced it, being flexible enough to move with the canvas and less absorbent, which changed how paint behaved on the surface and contributed to the different look of oil painting on canvas. Coloured grounds became common, since an oil ground can be tinted and a mid-tone ground suits working in both directions. The white reflective ground that early panel painting depended on was therefore abandoned along with the panels, which is one reason the change in support produced a change in appearance that goes beyond the material. Modern acrylic primers sold under the same name are a different material entirely, being flexible and non-absorbent.

The connection between this ground and gold leaf is close enough to explain much of why the material was made so carefully. Water gilding applies gold leaf over a layer of fine clay laid on the polished ground, wetted so the leaf adheres, and the gold can then be burnished with a smooth stone to a mirror finish, which is possible only because the surface beneath is hard, perfectly smooth and slightly yielding. Any irregularity in the ground appears in the gold magnified by the reflection. Punched and incised decoration is worked into the gilded surface with tools, producing patterns that catch light, and tooling the ground before gilding produces raised ornament. The technique dominated devotional panel painting for centuries, and the appearance of those works, with figures against burnished gold, depends on the preparation as much as on the painting.

The layer is the source of most problems affecting panel paintings. Wood moves with humidity and the ground does not, so cycling humidity produces cracking, cupping and eventually separation of the paint and ground from the panel, which is the commonest serious damage to such works. Animal glue is sensitive to moisture and loses strength, and it is attractive to insects. Panels warp and have historically been forced flat by attaching rigid supports to the back, which restrains the wood and increases the stress on the ground, and that treatment has caused more damage than it prevented in a number of documented cases. Current practice favours allowing the panel to move within a flexible support. Stable environmental conditions matter more than any intervention, which is why climate control in galleries is a conservation measure rather than a comfort.

Chalk or gypsum in animal glue, built in thin layers and polished hard, seals the wood, reflects light back through translucent paint and takes gold leaf and incised detail. It cracks on anything flexible, so stretched canvas required oil grounds instead, which are less absorbent and frequently tinted. Humidity cycling separates the ground from the panel, which is the commonest serious damage to such paintings.