View Larger Image This fired vessel by Robin Hopper demonstrates the tree or leaf-like patterns that are possible with mocha diffusion.

April 2, 2008

Mocha Diffusion

by Robin Hopper | Read Comments (2)

Though it might sound like a fancy coffee drink, mocha diffusion is a little-known technique of ceramic surface decoration developed and used in the southwest of England. Through a reaction between acid and alkali, intricate veined patterns reminiscent of trees or ferns are created in this process. In the April 2008 issue of Ceramics Monthly (on newsstands now!), Canadian potter Robin Hopper, a mocha diffusion expert, provides an explanation of the technique in the Answers column. The following is excerpted from the article.
–Jennifer Harnetty, editor.


In this technique, thick slip is applied to wet or leather-hard clay (I prefer leather hard) that hasn’t started to change color in drying. Then an acid/color mixture (or “tea”) is applied. Mocha diffusion was traditionally done on both red and white earthenware, but may be done on almost any clay body at almost any temperature. From my experience, a smooth clay body with a high degree of ball clay or plastic kaolin, such as EPK, is the most ideal.

Various slip recipes are good, the most important ingredient being a high percentage of ball clay. A basic recipe, which will fit most bodies, and which can easily be colored with stains or various oxides, is shown at left. This slip is good on most clay bodies from Cone 04 to 12, in any atmosphere. The thickness should be like double cream, or room-temperature 10W30 motor oil.

Acid/Color Mixture
The mixture that is used to form the patterns is called “mocha tea.” It was originally made by boiling tobacco leaves and forming a thick sludge that was then thinned with water and mixed with colorant. However, nicotine solutions are only one form of mild acid; many others will work, such as citric acid, lemon juice, urine, coffee or vinegar, particularly natural apple-cider vinegar. One of these would be mixed with colorant. Most colorants work quite well, although carbonates and stains are usually better than oxides, since they are typically a physically lighter precipitate than oxides. Heavy materials such as black copper oxide, black cobalt oxide and black iron oxide do not work well, because the acid can’t adequately hold them in suspension. A ratio of about one heaping teaspoon of colorant to a quarter cup of mild acid is usually a good starting point. However, a good deal of individual testing has to be done to get the two liquids to work together to create significant dendritic formations or diffusions.

Method
The leather-hard pot is dipped, brushed or poured with slip. While the surface is still wet, and before it has begun to lose its shine, the acid/color mix is dripped or trailed into it. It is best done using a well-loaded brush held just touching the slip. If the viscosity of the slip, and the acid/color mix is right then the feathering pattern will take place quite naturally, as the acid eats a fern-like pathway through the slip pulling the colorant with it. Traditionally, the surface is coated with a thin coat of clear glaze, or transparent colored glaze, but this might cause the color to bleed out or become absorbed into the glaze, particularly at temperatures above Cone 4. I prefer to use the technique on high-fired ware that does not need to be glazed. This is a technique that usually takes a while to get used to, but can give interesting results when used sensitively.

For a more detailed explanation of this process and its history (including where it got its funny name), see the April 2008 Ceramics Monthly.