Gesso (Italian:ˈdʒɛsːo "chalk," from the Latin gypsum, from the Greek γύψος) is a white paint mixture consisting of a binder mixed with chalk, gypsum, pigment, or any combination of these. It is used in artwork as a preparation for any number of substrates such as wood panels, canvas and sculpture as a base for paint and other materials that are applied over it.
 Traditional gesso
"Gesso", also known "glue gesso" or "Italian gesso" is a traditional mix of an animal glue binder (usually rabbit-skin glue), chalk, and white pigment, used to coat rigid surfaces such as wooden painting panels as an absorbent primer coat substrate for painting. The colour of gesso was usually white or off-white. Its absorbency makes it work with all painting media, including water-based media, different types of tempera, and oil paint. It is also used as a base on three-dimensional surfaces for the application of paint or gold leaf. Mixing and applying it is an art form in itself since it is usually applied in 10 or more extremely thin layers. It is a permanent and brilliant white substrate used on wood, masonite and other surfaces. The standard hide glue mixture is rather brittle and susceptible to cracking, thus making it suitable for rigid surfaces only. For priming flexible canvas, an emulsion of gesso and linseed oil, also called "half-chalk ground", is used. In geology, the Italian "gesso" corresponds to the English "gypsum", as it is a calcium sulfate mineral (CaSO4·2H2O).
 Acrylic gesso
Modern "acrylic gesso" is technically not gesso at all. It is 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 (CaCO3) to increase the absorbency of the primer coat, Titanium dioxide or titanium white is often added as the whitening agent. This allows gesso to remain flexible enough to use on canvas. High concentrations of calcium carbonate, or substandard latex components make the resulting film dry to a surface that is brittle and susceptible to cracking.
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. Art supply manufacturers market canvases pre-primed with gesso.
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), leach some oil through a thin acrylic primer coat and damage the canvas underneath just as traditional hide glue sizing did. However, sufficient coverage and penetration of an absorbent support is archivally acceptable.
Although it is generally believed acceptable, several painting texts such as The Painter's Handbook state that it is unwise to paint in oils over acrylic gesso because, unlike with time-tested alternatives such as rabbit skin glue, oil paint eventually delaminates from the acrylic gesso surface. This effect may not manifest for several decades. The cause for this problem is the inability of oil paint to establish physical and chemical bonds with the acrylic base. Applied to 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. Manufacturers of commercially sold, pre-gessoed canvases deny that delamination takes place. However, curators in the Smithsonian Museum are not permitted to use acrylic gesso under oil paint, precisely because of the delamination problem.
 Soy-based gesso
Soy-based gesso is a low emitting bio-based gesso made from recycled soy content. Soy gesso is made with new bio-based dispersion technology that uses a soy ester with a modified soy-vegetable oil acrylic. The surface is similar to acrylic gesso, but is not a solid acrylic. Soy gesso is made using a thin film of a modified acrylic and the soy ester. The penetration and adhesion of the soy ester to the substrate and the thin film of modified acrylic may have advantages in creating a surface that allows a physical bond between the gesso and the oil paint. In addition, the thinner modified acrylic film is more resistant to cracking than a solid acrylic gesso.
 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. 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.
- The materials and techniques of medieval painting By Daniel Varney Thompson. page 32
- The painter's handbook: a complete reference By Mark David Gottsegen. page 321
- Lettering & Calligraphy Workbook By Diagram Group, page 148
- The painter's handbook: a complete reference By Mark David Gottsegen, page 64
- The painter's handbook: a complete reference, by Mark David Gottsegen. Page 321
- Maor, Yonah (2008), Delamination of oil paints from acrylic grounds, Kingston, ON: Queen's University, http://qspace.library.queensu.ca/handle/1974/1487, retrieved 2011-07-10