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The Alchemy of Watercolors On Porcelain - Part I

There is something enigmatic about the 'Watercolors on Porcelain' technique. The elusive colors, that may or may not be as expected; the transmutation in the fire -to get that most precious, beautiful surface. The parallels with alchemy of the Middle Ages certainly are there: European porcelain was 'discovered' by the German Alchemist Johann Friedrich Boettger around 1709, after numerous vain attempts to copy Chinese Porcelain in the 15th, 16th and 17th centuries. There is the use of precious metals -gold, silver, rhodium, palladium; the transformation in the high temperature firing; the knowledge of the underlying chemical processes; the chance discoveries... While it is not gold that we are trying to discover (although precious metals including gold may be used), the 'Watercolors on Porcelain' technique is still used by relatively few. This has its reasons -some of the raw materials are prohibitively expensive, they are highly toxic and necessitate extreme care, the results are not always promising and more research needs to be undertaken, despite knowledge already pioneered by others.

Porcelain Bowl Just what is WOP? (I shall use this acronym from now on). As most readers will know, the majority of traditional ceramic colors come in the form of oxides or carbonates that are used in underglazes, onglazes and as slip and glaze additives. From these oxides and carbonates come the broad palette of decorative effects that we have known for centuries, even millennia. In the late 1980's, Professor Arne Åse investigated the use of watersoluble metal salts, derivatives of the oxides and carbonates, for color effects and possible applications in studio ceramics. He chose to test these substances on porcelain, as the whiteness of this particular clay should give the most untainted color response. Thus the WOP technique was born.

Which chemicals are we actually talking about? As already mentioned, they are watersoluble metal salts: chlorides, sulphates and nitrates of some common and other not so common metals, eg. Gold Chloride, Nickel Sulphate or Iron Nitrate. In most cases there will be a chloride, nitrate and sulphate version of a given metal available, eg. Copper Chloride, Copper Nitrate and Copper Sulphate. There are also some useful colorants that aren't actually metallic salts, but nonetheless watersoluble, eg. Potassium Dichromate. Being watersoluble, these chemicals will react in a different way when applied to a clay surface. Instead of sitting on the surface they will stain and penetrate the clay, sometimes going right through to the other side, so some unexpected results can be reckoned with!

Porcelain Bowl While a fair amount of research has been done with the chlorides and nitrates, I think the sulphates have been overlooked somewhat. The fact that Iron Sulphate, Cobalt Sulphate and Copper Sulphate were readily available and relatively inexpensive led me to investigate these chemicals. I'd also like to talk about Gold Chloride, Potassium Dichromate and to a degree Uranyl Nitrate, plus the effects of resist materials and the use of Phosphoric Acid. These materials can sometimes be bought from potters suppliers, some will need to obtained from a supplier to laboratories. For our purpose, the grade or purity need not be high. Generally they will be supplied in a powder or crystalline form from which a solution must be made. When dealing with these chemicals it is advisable to wear thick rubber gloves, a mask with gas attachments and also goggles to protect the eyes. Eating or drinking anywhere near these chemicals is of course out of the question (and something one shouldn't do near ceramic materials anyway). A few other things will be necessary: some glass jars that will not leak (food jars aren't good enough!); a chemists measuring tumbler, with measurements from 1 to 50 ml; an accurate set of scales, with 1 gm gradations. All decoration is carried out on bisqued ware and then fired to approximately 1270 degrees celsius in a reduction atmosphere.

Cobalt Sulphate

A useful concentration for Cobalt Sulphate is about 10%. To make this, we weigh 10 gms of Cobalt Sulphate powder and add it to 100 ml of water. I have previously used distilled water, but this isn't really necessary unless your tap water is really impure. Carefully add the powder to the water without spilling any. This should be done wearing a good mask with gas filter and good rubber gloves that are impervious to water and definitely not inside any living quarters. Brushes must be washed well in running water and not used for any other purpose. Always ensure there are a few layers of newspaper underneath the work to avoid soiling the working area. If anything is ever spilt, clean it up immediately and thoroughly. Never reuse cleaning utensils such as sponges.

The solution is now ready to be applied. This can be done with a brush, freehand or on the wheel. Protect any bench surfaces or wheel heads with newspaper. Just how many layers of solution should be brushed on is a matter of finding out what works for you, as the result also depends on the type of brush you are using and on the density of the clay. Spraying is not recommended as it can be messy, even with a spraying booth. One of the greatest difficulties in applying these chemicals is the fact that they are very hard to see once painted on. One way to get around this would be to add ink or food colorants to the liquids.

After applying the Cobalt Sulphate, I sometimes apply a layer of Uranyl Nitrate over the top, also with a brush, in a concentration of 7.5%. The greatest care needs to be taken with this chemical, it is highly toxic! After a short drying time, Phosphoric Acid (85% concentration) can be applied to achieve the 'halo' effect. Of course great care also needs to be taken when handling the acid - goggles should be worn! The effect of the Phosphoric Acid is to displace the colors and concentrate them on the edge. This will often reveal the white color of the underlying porcelain.

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