There is a glazing technique that gives rise to complex dendritic patterns on pottery. Sometimes called Mocha diffusion, the resulting pieces are called Mochaware. The name Mocha comes from the Red sea port of Mocha, now in Yemen, a city associated in England with the export of dendritic, or moss agates (Mocha stone). The technique dates from the 1780s, and was invented in Staffordshire in the UK. The earliest written reference to it comes from the Lakin & Poole factory in Staffordshire, and mentions "mocoe beakers" in 1792-1796. A dated mug from 1799 is in the Christchurch Mansion Museum, Ipswich, England. The term "mochaware" has unfortunately expanded to include many types of slip decoration which do not involve the dendritic patterns at all.

The original recipe involves a "tea" made by boiling tobacco, which is then colored with e.g. Iron oxide. The piece is first coated with a wet "slip" (very runny clay/water mixture). Then the tea mixture is touched onto the wet surface. The acidic tea reacts with the alkaline slip and the dendrites grow quickly from the point of contact.

The dendritic pattern is clearly the result of a dynamic process in which the contact line between the two liquids, tea and slip, becomes unstable. The surface tension of the tea is less than that of the slip. The instability is probably driven by a combination of capillary and Marangoni (surface tension gradient) stresses, coupled somehow to the acid/base chemical reaction. Similar looking instabilities are known in surfactant driven flows.

Thanks to Jonathan Rickard for many details.

Dendritic patterns

on mochaware  pottery


Mochaware bowl (1820). Image courtesy of Jonathan Rickard.

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A sample about 2 cm across made with Iron oxide and tobacco tea.

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References and links

  1. Mocha diffusions article by Robin Hopper in Ceramics Today.

  2. Mocha and Related Dipped Wares 1770-1939, a book by Jonathan Rickard, The University Press of New England. [Amazon link].

  3. Mocha, Banded, Cat's Eye, and Other Factory-Made Slipware, by Lynne Sussman, No. 1, Studies in Northeast Historical Archaeology, 1997.

  4. S. M. Troian, X. L. Wu, and S. A. Safran, Physical Review Letters, v. 62, p. 1496 (1989). [online version].

  5. S. M. Troian, E. Herbolzheimer, and S. A. Safran, Physical Review Letters, v. 65, p. 333 (1990).  [online version].

  6. Streamlets and Branching Dynamics in Surfactant Driven Flow from The Gallery of Fluid Motion, by Benjamin J. Fischer, Anton A. Darhuber and Sandra M. Troian.

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