Fiel de bœuf

Natural wetting agent for degreasing substrates before painting with gouache or watercolors and (used sparingly) as a flow agent for all watercolors. In the rice varnish technique, the ox gall ensures that the rice varnish forms a bond with the base varnish and does not roll off. Since ox gall promotes emulsification, it is also used as an emulsifier for making emulsions from oils and water. The painter applies it to "greasy" substrates that repel water to prevent the applied water-based paints from "running together", or he mixes it with paint powders to make them more easily wettable with aqueous binders.