Substance that facilitates the fixing of colours or other materials to a support, as in oil-gilding, where it is utilized as a mordant in one of the possible methods for the preparation of a gold ground. Cennino Cennini (end of the 14th century) provides indications for making a good mordant for various surfaces: "You take your oil, cooked on the fire or in the sun...and work up with this oil a little white lead and vertigris; and when you have got it worked up like water, put a little varnish into it, and let it all boil together for a while."