Gesso consists of an adhesive with an inert filler. Traditionally, animal glue is used as the adhesive but gesso can be based on casein, acrylic or other materials. The term gesso originally referred to a gypsum- (calcium sulphate)
based gilding ground such as those commonly used in southern Europe. In the general context of painted, japanned or gilded furniture, the term is extended to include grounds based on calcium carbonate. This may not be the case in other conservation disciplines, where the use of the term 'gesso' may be specific to calcium sulphate grounds and where other grounds, such as those based on calcium carbonate, may be simply described as 'white ground'. Cennini's fifteenth century commentary on artists' materials recommends that the best size for gessoing panels is made from parchment prepared from the necks of goats and sheep by trimming, washing, soaking and then boiling it (Thompson, 1960). Other historic treatises such as those by Theophilus and Watin also advise on the preferred animal glue for making gesso, although interestingly, none mentions rabbit-skin glue (Souza and Derrick, 1995).
The filler is typically calcium carbonate or calcium sulphate but other materials can be used. Historically, calcium sulphate has been used in the Mediterranean basin and calcium carbonate in Northern Europe, where chalk was plentiful, but this is not invariable. Kaolin (A12O3.2SiO 2.2H2O) and sometimes dolomite (CaMg(CO3)2) were also used (Cession, 1990).
Large deposits of gypsum are found near Bologna and Volterra and also occur in varying grades throughout the world. Chemically, gypsum is calcium sulphate dihydrate formed by the deposition of salts in inland lakes. The raw, unburned gypsum has no function in the making of gesso and is only used once it is calcined and ground to make plaster of Paris (CaSO4.>2H2O). The powder, known as gesso in Italian, is used in two ways. It is soaked in excess water (so that it does not set) for four or five weeks with frequent stirring and changes of water. The residue, after the water is poured off, is made into small loaves and dried. This very fine powder, known as gesso di Bologna, can then be crushed, sieved and mixed with size to make gesso sottile. To make gesso grosso, unadulterated plaster of Paris is sifted and mixed into a paste with either parchment or rabbit-skin size. Gesso di Bologna is slightly greyer than chalk whiting (see below), it feels cooler and softer, and compacts more easily. It is lumpy and therefore it is always necessary to sieve it before mixing it into the size.
In Northern European regions, particularly England and France, where the chalk deposits are plentiful and of a high quality, a very fine variety of calcium carbonate (chalk whiting -CaCO3) known as gilders' whiting is used to make gesso. Natural chalk is a soft, white rock which is largely composed of the remains of minute sea organisms. It is extracted by open-surface mining of deposits with minimal impurities and a high degree of whiteness. Once the material is extracted it is dried to lower its water content from 20% to 0.2%. For gilders' whiting the rocks are ground to a particle size of 3.3 or 4 pm. The fine, soft, white powder is added to glue size, made from parchment cuttings or rabbit skins, to make gesso.
Southern European grounds prepared with gypsum (natural gypsum is calcium sulphate -CaSO4.2H2O), usually consist of a coarse preliminary preparation followed by a smoother one. Cennini describes the two stages, gesso grosso (coarse) and gesso sottile (fine). Typically, gesso grosso refers to anhydrite (CaSO4) or plaster of Paris (CaSO4.MH2O), which is then coated with a gesso sottile made of calcium sulphate (CaSO4.2H2O), the difference being the degree of hydration and the consequent chemical link. Anhydrite and plaster of Paris set by hydration, regaining the water lost during heating, but when mixed with glue, they become much harder.
Von Endt and Baker (1991) explain that in a filled animal glue system, the hydrogen bonds of the glue are pulled apart to allow the filler into the matrix, which reduces the flexibility of the chains. This makes the matrix stiffer, but also weaker as less force is required to pull it apart. Fillers such as calcium carbonate and sulphate do not interact chemically with the matrix and consequently form a system that is weaker than those in which there is chemical linking between the filler and the polymer. Although filled glue system are less permeable to water than unfilled ones, gesso can be permanently deformed by the introduction of humidity because the molecules do not always go back to their original positions and less stress is required to provoke subsequent deformation.
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