RH- PORCELAIN DISC FORM - IRON AND MANGANESE MOCHA DIFFUSIONS WITH RUTILE AND WHITE SLIPS - UNGLAZED EXTERIOR - GLAZED INTERIOR
GAS FIRED REDUCTION TO CONE 10
BLACK COLORED PORCELAIN WITH RUTILE AND WHITE SLIPS
MANGANESE DIFFUSIONS - FIRED IN OXIDATION AT CONE 8
MOCHA DIFFUSIONS is a little known technique of surface decoration developed and used in the Southwest of England, and subsequently brought to parts of Eastern North America, particularly Canada by itinerant, immigrant, European potters. 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 Muslium 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 colored with manganese or iron, have permeated between layers of alkaline sedimentary rock. Compression and geothermal heating has hardened the stone into a gem quality. In nature these patterns may take hundreds of years to develop.
In ceramics, the process is done in seconds. 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 wet to leather hard clay – I prefer leather hard - that hasn’t started to change color in drying. 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/color mix will not move, too thin and it will run excessively and become blurred. With experience it is quite easy to control.
MOCHA DIFFUSIONS was traditionally done on both red and white earthenware, but may be done on almost any clay body at almost any temperature. Clays that have a high degree of sand, grog or lignite in them are sometimes prone to cracking. From my experience, a smooth clay body with a high degree of Ball Clay or Plastic Kaolin such as Edgar Plastic Kaolin is the most ideal. All clays and slips are primarily alkaline, a basic necessity for the reaction to take place with the acidic Mocha “tea” or vinegar mix.
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 would be; BALL CLAY 75, KAOLIN 10, SILICA 10, FELDSPAR 5. I have taught this process all over the world and you can use any ball clay, any feldspar, any kaolin and any 200 mesh silica, flint or quartz. They will all work. 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 10 W 30 motor oil. A liquified porcelain clay slip will not usually work well since a porcelain body usually contains a maximum of 50% plastic clay material, the remainder being non-plastic fluxes and fillers such as feldspars and silica.
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 color to apply. It probably originated by pottery decorators chewing tobacco while they worked, and spitting in the paint pot, creating a murky brew. 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, coffee, or vinegar, particularly natural apple-cider vinegar, which is what I always use. The mix is made by making a solution of acid, mixed with colorant. Most colorants work quite well, although carbonates or stains are usually better than oxides, since they are usually a physically lighter precipitate than oxides. Heavy materials such as black copper oxide, black cobalt oxide and red iron oxide do not work well, since the acid can’t adequately hold the color in suspension. If you want to use iron, either use black Iron oxide or yellow ocher. If the colorant is gritty the grit will sink to the bottom of the mix and impede movement. A ratio of about 1 heaping teaspoon of color 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, and to make things to work correctly.
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 clear, 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 wares that do not need to be glazed, and have been doing it that way for over 50 years. It is a technique that usually takes a while to get used to, but can give interesting results when used sensitively. It can also be done quite easily on once-fired glazes, providing that they have enough ball clay in them.