Note 240 — Chapter XIV · (XXVIII.)

For the various processes of preparing a panel for painting and for gilding reference must be made to Cennini’s Trattato, where many technical matters are elucidated that Vasari passes over almost without notice. It must be remembered that Cennini writes as a tempera painter, while in Vasari’s time these elaborate processes were falling out of use. In his chapters 115–119, Cennini gives recipes for what he calls ‘gesso grosso’ and ‘gesso sottile.’ They are made of the same materials, ‘volterrano,’ or plaster from Volterra, which is a sulphate of lime corresponding to our ‘plaster of Paris,’ and size made from parchment shreds; but the plaster for ‘gesso sottile’ is more finely prepared. The plaster, produced by calcining gypsum, is first thoroughly slaked by being drenched with water till it loses all tendency to ‘set,’ and is then as a powder or paste mixed with the heated size. The size makes the composition dry quite hard, and Cennini speaks of its having a surface like ivory.