Gesso

Gesso (Italian pronunciation: ; 'chalk', from the Latin: gypsum, from 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. Gesso (Italian pronunciation: ; 'chalk', from the Latin: gypsum, from 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. '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 is 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 as it is usually applied in 10 or more extremely thin layers. It is a permanent 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', a calcium sulfate mineral (CaSO4·2H2O). Gesso is 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 sculpted wood and gold leaf. In this case, a layer of refined and coloured clay, called ‘bole’ is used to cover the gesso before applying the gold. This is usually red in colour. Modern 'acrylic gesso' is a widely used ground that is a combination of calcium carbonate with an acrylic polymer, medium latex, a pigment and other chemicals that ensure flexibility, and increase archival life. It is technically not gesso at all. Its non-absorbent acrylic polymer base makes it incompatible with media that require traditional gesso such as egg tempera. It is sold premixed for both sizing and priming panels and flexible 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. 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. The Painter's Handbook notes a problem with using oil paints over an acrylic gesso ground instead of a traditional oil ground, citing a mismatch in flexibility over time that could cause the oil paint to delaminate.