Gesso
A white coating of chalk or gypsum and glue, used to prepare a surface for painting or gilding.
Also: chalk ground
Gesso is the Italian word for gypsum. Traditional gesso is a mixture of animal glue and fine white powder, either gypsum or chalk, brushed warm onto a wooden panel in many thin coats and sanded smooth as ivory. Medieval panel paintings and gilded frames were made on it. It is brittle, so it suits rigid supports. The product sold as gesso in art shops today is usually an acrylic primer with white pigment, flexible enough for canvas and ready to use from the tub.