The application of gold or silver to a surface. Gold could be applied as an ink, in an expensive powdered form, for use in detailed work and in chrysography, but it was more frequently applied in medieval illumination in the form of gold leaf. The gold leaf could simply be laid down on an area to which a binding medium such as glair or gum (perhaps mixed with honey to prevent it from cracking) had been applied, as was the case during the early Middle Ages; it could also be laid on a raised ground of gesso. In order to enrich the tonality of the gold and to make the areas to which the ground had been applied more visible, a colorant such as bole (a pink earth colour) was often added to the base. Gesso grounds enabled the gilded surface to be tooled. However it was applied, the gold could be burnished or left in its slightly duller state. Gilding formed the first stage in the painting processes of illumination, since it was a messy activity, the gilded area often requiring trimming with a knife. The gilding of a manuscript illustration was carried out by the artist or by a specialist.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).