Gum Arabic

Used in painting to increases flow, transparency, and the gloss of watercolour paints. Also reduces spread of wet on wet applications. Dries completely to a flexible film. A binder made from acacia trees - mature acacia trees exude the gum from incisions made in their bark. This dries into hard, transluscent balls, which are collected and ground into a fine powder. The lighter the colour of the gum arabic, the better the quality. To use it, gum arabic is dissolved in water, using two parts water to one part gum arabic.

http://en.wikipedia.org/wiki/Gum_arabic
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