Gesso is the Italian word for "board chalk", and is a powdered form of the mineral calcium carbonate used in art. This calcium sulphate compound was traditionally mixed with animal glue and used as an absorbent primer coat for panel painting with tempera paints. It provided a permanent and brilliant white surface when used on wood or masonite. As it was rather brittle and susceptible to cracking, it was not suitable for priming canvas.
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.
The word "gesso" originates from the Greek "gypsum"
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