A type of clay also called bole is used to facilitate the burnishing of water gilding. The name bole or bolus is derived from the Greek, meaning 'clod of earth', as the clay that was first used for this purpose came from deposits in the ground. Armenian bole was considered to be the best. It is composed of pipe clay (hydrated silicate of aluminium and organic sediments) and iron oxides, which give it its distinctive blood-red or orange colour. It may now be difficult to obtain.
The colours used depended upon the clay deposits that were locally available so that, for instance, the red clay on seventeenth century Spanish work is a different colour to French red clay of the same period. In later centuries, when individual clay deposits were no longer exclusively relied upon, fashion played a role in determining the clay colour. In nineteenth-century English gilding, for example, a dark blue/black clay was often used under burnished water gilding whilst in the last quarter of the eighteenth century a pinkish brown clay was common. The clays were made using a number of different ingredients, ranging from bullock's blood to soap. These additives gave varying characteristics but the base constituents remained pipe clay and pigments.
Today, bole comes in many colours and is most often sold in the wet state, but it can also be purchased in a more limited range of colours in the dry cone form. Modern commercially produced boles are manufactured from pipe clay combined with a colouring agent and water. A few contain an oil or lead to further facilitate the burnish but in most cases it is left up to the gilder to add a little oil if required.
Bole is made by mixing clay with warmed parchment size, rabbit-skin size, fish glue or glair and applying with a soft brush onto the prepared gesso surface on which the leaf is to be laid. Glair is derived from egg white (see section 14.2.13). Two distinct layers of bole may often be found on the same piece. A yellow ochre bole was applied directly on the gesso, to hide any breaks or 'faults' in the gilding by virtue of its colour. Over the yellow bole, there may then be a second bole made of clay of a deeper colour to which beeswax, tallow, oil, suet or graphite may have been added to enhance its burnishing capabilities. Souza and Derrick (1995) has shown cross-sections of Brazilian polychrome sculptures in which graphite particles can be seen lying par allel to the surface of the bole. This second layer of bole is applied to tone the colour of the gilding, and to serve as a soft 'seat' (assiette in French) which deforms under an agate burnisher to take a high polish. Bole is discussed further in Chapter 14.
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