Ox-gall is a liquid which is obtained from the gall bladder of a cow. This liquid is water soluble and is transparent. It is usually mixed with alcohol to obtain a greenish-brown translucent fluid to use as wetting agent in lithography, engraving and marbling. This reduces the surface tension of liquids improves the absorption ability of water. It can also be used to remove pin-holes in gesso surfaces. Now synthetic wetting agents are available in the market instead of ox gall.