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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. 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 extremely thin in 10 layers or more. 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 only for rigid surfaces. For priming flexible canvas a emulsion of gesso and linseed oil, also called a "half-chalk ground", is used.
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 the "gesso" to remain flexible enough to use on canvas. High concentrations of calcium carbonate, or substandard latex components will cause the resulting film to dry to a brittle surface susceptible to cracking.
Acrylic gesso 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), will 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.
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 less resistant to cracking than a solid acrylic gesso.
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