Gesso is an art supply used as surface preparation, or primer, for painting, gilding and sculpting. Its origins are uncertain, but it is believed to have been developed in Italy, because the word is Italian for "chalk." Preparation varies according to intended use but usually consists of mixing glue with plaster, chalk or gypsum.
Created for Use in Painting
This substance resembles paint but is thinner and dries hard. It is applied with a brush and must dry before the surface can be painted. Gesso was first created for use in painting, to give the surface the right properties to receive paint. In Gothic and Renaissance panel painting, it 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.
Traditional gesso was equal volumes of filler or chalk dust; white pigment, either from the powdered chalk or another mineral, such as zinc; and animal-skin glue. The mixture was then heated and stirred. This substance was brittle when dry and therefore susceptible to cracking.
In 1955, an acrylic paint company, Liquitex, developed the first water-based acrylic gesso. Modern gesso is a mixture of calcium carbonate, a pigment and an acrylic polymer medium. The pigment usually is titanium dioxide or titanium white.
Modern gesso retains the absorbent qualities of the older version but is more flexible and can therefore be used on canvas. It also can be colored during the manufacturing process by replacing the titanium white with another pigment. The artist also can color it using watercolor, acrylic paint or another coloring agent to tint the surface to be painted. Canvases with gesso already applied are available commercially.
Use in Sculpting
Gesso also is used in sculpting. During the 18th century, it was commonly used as a base for decorative gilding or otherwise embellishing carved woodwork, such as picture frames or furniture. Gesso is not always attached to or painted over another surface. Sometimes, it is used directly to form the actual artwork.
This material can be cast in a mold or used to make the mold itself, or it can be modeled or carved. It is useful for molding or building up into relief designs. Gesso also is used in manuscript illumination because it forms a raised area on the page that can then be gilded and burnished.
Some artists question whether modern gesso should be used under oil paint on canvas. Certain materials that are used in oil painting, such as mineral spirits, can leak oil through and damage the underlying canvas. The archival properties of the acrylic type is unknown as well.