This habit of manipulating the precious metals gave neatness and precision of touch to the painter, and in the art of illuminating manuscripts taught the artist to use his gold so as to produce the richest and most decorative effect. _The gold mordant._ _The mordant._ We now come to the most difficult part of the illuminator's art, that of producing a ground for his gold leaf of the highest hardness and smoothness of surface. It is a subject dealt with at much length by all the chief writers on the technique of the illuminator, from Theophilus in the eleventh century, down to Cennino Cennini at the beginning of the fourteenth[259]. Though differing in details, the general principle of the process is much the same in all; the finest possible sort of _gesso_, plaster, gypsum or whitening, was very finely ground to an impalpable powder, and then worked up with albumen or size to the consistency of cream, so that it could be applied with a brush. After the first coat was dry, a second and a third coat were added to bring up the mordant to the requisite thickness of body, so that it stood out in visible relief upon the surface of the vellum. In order that the illuminator might see clearly where his brush was going, and keep his mordant accurately within the required outline, it was usual to add some colouring matter, such as bole Armeniac (red ochre), to the white _gesso_, which otherwise would not have shown out very clearly on the cream-white vellum. In many cases, however, this colouring matter is omitted. _Application of leaf._ When the last coat of the _gesso-mordant_ was dry and hard, its surface was carefully polished with the burnisher and it was then ready to receive the gold leaf; several days' waiting would often be required before the whole body of the mordant had set perfectly hard. White of egg was then lightly brushed over the whole of the raised mordant, and while the albumen was still moist and sticky, the illuminator gently slid on to it the piece of gold leaf, which he had previously cut approximately to the required shape. He then softly dabbed the gold leaf with a pad or bunch of wool, till it had completely adhered to the sticky mordant, working it with special care so as completely to cover the rounded edges. After the albumen was quite dry, and the gold leaf firmly fixed in its place, the artist brushed away with a stiff brush all the superfluous gold leaf; all the leaf, that is, under which there was no mordant-ground to hold it fast. _Burnishing process._