Oxgall is a wetting agent that will reduce the surface tension of water. This means that watercolour, with added oxgall, will be less likely to ‘pool’ into droplets and more likely to spread over a greater area. It can also be used to apply to a highly sized surface to reduce the surfaces tendency to repel water or watercolour.

As well as watercolour, it is used in marbling, lithography and engraving.