Gesso
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"Gesso" is the Italian word for "chalk" (akin to the English 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. This mixture is rather brittle and susceptible to cracking, thus making it unsuitable for priming canvas.
Acrylic gesso
Modern acrylic "gesso" is actually a combination of calcium carbonate with an acrylic polymer medium and a pigment. 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 flexable enough to use on canvas. High concentrations of calcium carbonate will cause the resulting film to dry to a brittle surface susceptible to cracking.
Acrylic gesso can also be colored, either commercially by replacing the titanium white with another pigment, such as carbon black, or by the artist them self, with the addition of an acrylic paint. Acrylic gesso can be odoriferous, 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 has an unproven record as a primer for oil paintings. 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. And, while a pure acrylic polymer is more flexible than traditional animal size, the addition of calcium carbonate increases the brittleness of the primer coat. Thus, the archival nature of acrylic gesso on canvas is uncertain.
Gesso and sculpture
Gesso is also used by sculptors, to prepare the shape of the final sculpture (fused bronze) or directly as a material for sculpting. A collection of gesso sculptures is properly called a gypsotheque.