"Gesso" ['dʒɛsːo] is the Italian word for "Board chalk" (akin to the Greek word "gypsum"), and is a powdered form of the mineral calcium carbonate used in art. Gesso was traditionally mixed with animal glue, usually rabbit-skin glue, to use as an absorbent primer coat for panel painting with tempera paints. It is a permanent and brilliant white substrate, as long as it is used on wood or masonite. This mixture is rather brittle and susceptible to cracking, thus making it unsuitable for priming canvas. In Geology, Italian "Gesso" corresponds to the English "Gypsum", as it is a calcium sulphate compound (CaSO4·2H2O).
Modern acrylic "gesso" is actually a combination of calcium carbonate with an acrylic polymer medium latex, a pigment and other chemicals that ensure flexibility, and ensure long archival life. It is sold premixed for both sizing and priming a canvas for painting. While it does contain calcium carbonate to increase the absorbency of the primer coat, Titanium dioxide or titanium white is often added as the whitening agent. This allows the "gesso" to remain flexible enough to use on canvas. High concentrations of calcium carbonate, or substandard latex components will cause the resulting film to dry to a brittle surface susceptible to cracking. Typically, a canvas should be sized prior to being gesso'd as a sizing coat will sink into the substrate to support it as opposed to a gesso coat which is just put on top of the substrate.
Acrylic gesso can be colored, either commercially by replacing the titanium white with another pigment, such as carbon black, or by the artist directly, with the addition of an acrylic paint. Acrylic gesso can be odorous, due to the presence of ammonia and/or formaldehyde which are added in small amounts as preservatives against spoilage. Pre-gessoed canvases can be obtained commercially.
Acrylic gesso is a modern art material, and is used as a primer for oil painting and acrylics. Many of the solvents used in oil painting, such as turpentine or odorless mineral spirits (OMS), will leach some oil through a thin acrylic primer coat and damage the canvas underneath just like traditional hide glue sizing. However, sufficient coverage and penetration of an absorbent support is archivally acceptable.
Although it is generally believed that it is acceptable to paint in oils over acrylic gesso, it has been stated in several painting textbooks such as "The Painter's Handbook" that it is unwise to paint in oils over acrylic gesso because--unlike time-tested alternatives such as rabbit skin glue--the oil paint will eventually delaminate from the acrylic gesso surface. This effect may not make itself manifest for several decades and then mostly affecting thick impasto. The cause for this problem is the inability of oil paint to establish both physical and chemical bonds with the acrylic base. In a canvas that has been primed with rabbit-skin glue, oil paint is able to penetrate the ground (which is porous, unlike acrylic gesso) and establish a permanent bond, both chemical and physical.
Gesso is also used by sculptors, to prepare the shape of the final sculpture (fused bronze) or directly as a material for sculpting. Gesso can also be used as a layer between sculptured wood and gold leaf. In this case a layer of red shellac called "assiette" is used to cover the Gesso before applying the gold. A collection of gesso sculptures is properly called a gypsotheque.