by Robin Hopper
ocha Diffusions is a little known technique of surface decoration developed and used in the Southwest of England, and subsequently copied in parts of North America, particularly Canada. It was only done on wares of a simple functional nature, and on forms that were simple in shape, such as mugs, bowl, jugs, chamber pots, etc. The name has nothing to do with coffee, but is derived from the word MECCA, the centre of the Moslem world, in Saudi Arabia. It is from here that the finest Moss Agate Gemstones come. These gemstones show the veined patterns reminiscent of trees or ferns. The slow evolution by which these are formed in nature is called Dendritic Formation, where acidic solutions, usually manganese have permeated between layers of alkaline rock. Compression has hardened the stone into a gem quality.
In ceramics, the process is done in seconds, which in nature may take hundreds of years. It is quite a simple process but demands exact timing and viscosity control. As with the natural occurrences it depends on a reaction between acid and alkali. It has to be done on a leather hard pot that has not started to change colour. The timing refers to the state of dryness that exists. If the pot is too dry, it might well crack or split; too wet, and it might sag or slump. The viscosity refers to the thickness of the slip coating which is used – too thick and the acid/colour mix will not move, too thin and it will run excessively.
Mocha Diffusions was traditionally done on both red and white earthenware, but may be done on any clay body. Clays that have a high degree of sand or grog in them are sometimes prone to cracking. A clay with a high degree of Ball Clay 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 coloured with stains or various oxides would be; BALL CLAY 75, KAOLIN 10, FLINT 10, FELDSPAR 5. This slip is good on most clay bodies from cone 04 to 12, in any atmosphere, the thickness should be like double cream, or 10 W 30 motor oil.
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 It was originally made by boiling tobacco leaves and forming a thick sludge which was then thinned with water, and mixed with colour to apply. It probably originated by pottery decorators chewing tobacco while they worked, and spitting in the paint pot. However, nicotine solutions are only a form of mild acid, and any form of mild acid will work, such as citric acid, lemon juice, urine, or vinegar, particularly natural apple cider vinegar. The mix is made by making a solution of acid, mixed with colourant. Most colourants work quite well, although carbonates are usually better than oxides, since they are usually a lighter precipitate than oxides. Heavy oxides such as copper do not work well, since the acid cannot hold the colour in suspension. A ratio of about 1 heaping teaspoon of colour to a quarter cup of acid is usually a good starting point, but a good deal of individual testing has to be done to get things to work right.
The leather hard pot is dipped or poured with slip. While the surface is still wet, and before it has begun to lose its shine, the acid/colour 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/colour mix is right then the feathering pattern will take place quite naturally, as the acid eats a fern-like pathway through the slip. Traditionally, the surface is coated with a clear glaze, but this might cause the colour to bleed out. I prefer to use the technique on high-fired wares that do not need to be glazed. It is a technique that usually takes a while to get used to, but can give interesting results when used sensitively. In theory it should be able to be done quite easily on once- fired glazes, providing that they have enough ball clay in them, but at the present time I have not explored this avenue.