Gum arabic is the dried sap that oozes from the trunks and branches of two acacia species. It is collected from wild trees, mostly in Sudan and across the Sahel from Senegal to Somalia.

The hardened gum dissolves readily in water to make a clear, sticky solution. That solubility is the whole trick of watercolor: dried paint softens again the moment a wet brush touches it.

Pigment powder is stirred into the gum solution along with glycerin or honey, which keep the dried binder flexible and quick to redissolve. Ox gall and preservatives may go in too, and the batch ends up as a thick paste.

The paste is fed between three horizontal rolls that turn in opposite directions, each faster than the last. The speed difference shears the paste in each narrow gap and breaks up pigment clumps, and a knife scrapes it off the last roll. Operators may narrow the gaps and repeat the cycle several times.

Most watercolor today is sold in small collapsible metal tubes, as a paste about as thick as toothpaste. The painter thins it with water on the palette.

For pan colors, paint is set into small open plastic wells and left to dry into a solid cake. Pans come in two standard sizes, full and half.