Mocha Diffusion is a technique that originated in England in the 18th century and was popular in the 19th C. It consists of dribbling an acidic solution with coloring oxides onto a wet slip of a contrasting color. The solution spreads in the slip with a tree fern-like pattern. Legend has it that in a pottery in Stoke-on-Trent around 1780 a drop of tobacco juice dripped on some slip and the distinct mocha fern magically appeared. (Maybe a potter was chewing some tobacco and spit on the pot!) The name is supposedly derived from the Saudi Arabian town and religious center of Mecca, where the semi-precious Agate stone was traded, which has a similar fern-like vein structure on its surface.
The Mocha Diffusion technique consists of making a concoction of tobacco juice and coloring oxides, called Mocha Tea. Other forms of Mocha Tea use lemon juice, vinegar or even urin – anything that is slightly acidic. A typical recipe for Mocha Tea would be
25 gr. tobacco
1 pint water
30 gr. iron or manganese.
The method is fairly simple – a leather hard pot is dipped in slip and the Mocha Tea is dribbled onto the wet slip surface. The pattern can be influenced with the help of gravity by turning the pot (or tile for that matter) on an angle, so that the Mocha Tea solution runs in a particular direction. The fern-like pattern will appear as the acidic solution spreads through the more alkaline slip.