Derived from the bile of domestic cows or other bovines, ox gall is added to paint as a surfactant or wetting agent to allow paint to flow more freely.<br><br>A waterless, oil- and water-soluble, translucent, nearly transparent brown liquid, ox gall is the bile taken from the gall bladder of a cow. It is used with watercolors, in engraving, in marbling, and in lithography as a wetting agent ? reducing what's called the "surface tension" of liquids, improving water's ability to penetrate and be absorbed. It is also used in the marbling of paper to more smoothly disperse oil color on the size. Ox gall has a long history of use, but often replacing them today are modern synthetic wetting agents available from art supply dealers (photographic and general chemical supply dealers too) in bottle, dropper-bottle, and aerosol forms. Ox gall and other wetting agents are also employed to eliminate pin-holes in gesso surfaces, by mixing it into the gesso before the gesso is applied. Also see absorption, binder, marbling, and medium.