Mocha decoration relies on a chemical reaction between acids and alkalis. The mocha decorator dripped a dark pigment made by mixing acidic tobacco juice, manganese and stale urine onto a pot that had been dipped in alkaline clay slip. Contact with the alkaline surface made the pigment spread out into frond-like forms, a reaction that could to some extent be controlled by the potter. The technique was introduced in the 1790s, having possibly been discovered by accident, and was widely used as an inexpensive method of decorating utilitarian beer mugs and jugs. It is named after the patterns found in mocha stones (moss agates).