Sizing is the first layer of protection you put on raw canvas. It serves to protect the fibers from the oil that will eventually be used in the painting. Without sizing, the oil will cause the fibers to rot, and eventually tear. Sizing is basically an animal glue that is mixed with water. Used in combination with a water mix gesso, it allows you to avoid both the turpentine-based oil gessos and the acrylic gessos with plastizier additives. Glue sizing combined with marble gesso is the historic approach to preparing a canvas. It has the added benefit of allowing the artist to select a specific raw canvas to use, based on expense and the style of the painting. See also GESSO GROUND.