The Alchemy of Watercolors On Porcelain

Iron Sulphate

o get a deep color from Iron Sulphate one needs a highly concentrated solution. To make this, we take 100mls of water and add 100gms of Iron Sulphate. This solution can be applied to bisqued ware, with or without resist decoration. In Fig.2 I have applied the resist material shellac in the form of the stripes and brushed several layers of saturated Iron Sulphate on top. The effect of the resist is to preserve the white porcelain body from the colorant. This technique can be used with any of the metal salts. Sometimes there will be a concentration of color along the edge, where liquid has collected. After decorating, it is necessary to carefully remove any residue of the salts from the shellac (with a sponge or moist tissue), as this will stain the surface underneath in the firing. It may become necessary to let the piece dry in between applications. Iron Sulphate can also be combined with other colorants, eg. Cobalt Sulphate.

Copper Sulphate

I have been lucky, that in my research with Copper Sulphate I have been able to achieve deep reds akin in depth of color to traditional copper red glazes. To achieve this type of color a couple of things are necessary. First of all a strong solution of Copper Sulphate is needed. To get this we dissolve 25-30gms of Copper Sulphate in 100 mls of water. This solution is applied in several layers onto a bisqued piece. After an hour or two, as the piece dries, blue crystals should be forming on top of the surface. They may literally grow in front of your eyes. If there seems too little color on the piece, more Copper Sulphate may be added. These blue crystals give an indication of what the piece will look like after firing, only they will have metamorphosed into a deep red! (In contrast, Cobalt Sulphate crystals are pink and turn blue in the firing.)

The firing cycle is very important. If the piece is oxidised, an insipid yellow color will result and the chemical also tends to attack the clay surface. Work decorated with Copper Sulphate needs to be fired in reduction, as with copper red glazes. The reduction should be medium to heavy, and most importantly, the kiln should be fired down in reduction for the first few hundred degrees, ie. from 1300°C (or whatever the maturing temperature of your porcelain) to about 1000°C, to avoid reoxidation of the copper. When fired in this manner, a deep red should result, with possible grey areas, where the solution was applied more thinly, also the surface stays smooth. In some cases there may even be areas with a silver appearance. Of course a gas kiln will be most suited to this type of firing.

Gold Chloride

Gold Chloride can be purchased from laboratory suppliers. I must admit, though, that I have never done this. Instead I use gold lustre which is readily available from potter's suppliers and contains gold chloride. I get quite a good color response from it, although this method isn't exact regarding quantities. I thin the lustre with conventional lustre thinner (4) . The color can be applied with a brush or nib and is clearly visible in contrast to the near colorless water solutions. Gold Chloride is a very reliable colorant in oxidation and reduction and will result in light to deep pinks, depending on concentration.

Potassium Dichromate

This chemical is a bright orange crystal in the raw state and is fairly toxic. Its saturation point is about 12%, after this, it just won't dissolve any more. 12 gms are added to 100mls of water. Because a large amount of this chemical is necessary to get a good color response, the 12% solution needs to be applied in several layers. As with Copper Sulphate, crystals -in this case orange- should grow on the surface whilst drying. If there don't appear to be enough, more solution can be applied. Potassium Dichromate will fire to an olive green in reduction. I find the color can be improved on by a small addition of Cobalt Sulphate, which can be applied separately or added to the solution itself. Potassium Dichromate also lends itself well to the use of resist materials.

Uranyl Nitrate

This is the most toxic and one of the most expensive of the metal salts. Extreme care needs to be taken when handling this chemical. The remarkable thing about this substance is its capacity to form an edge of concentrated color in combination with Phosphoric Acid, which displaces the color and reveals the whiteness underneath.

I do strongly recommend to anyone wanting to experiment with Uranyl Nitrate and any of the other chemicals written about in this article to buy a copy of Arne Ase's book, entitled 'Water color on Porcelain', Norwegian University Press, 1989, also published in the USA. It has all the information on how to use the metal salts and also what protective measures should be taken.

WOP and Glazing

Normally I don't glaze my work at all. As it is vitrified it will hold liquids without any problem. But the main reason is that the smoothly sanded surface of the unglazed porcelain offers a tactile experience unlike any glazed pieces. Nonetheless, in the case of functional ware that is to be used on a daily basis, it is necessary to glaze the work. In some cases I may glaze only the inside, in other cases the whole piece. Because any application of moisture on the bisqued ware will affect the applied metal salts, a different approach is needed for glazing.

One solution is to fire the decorated work to the maturing temperature and then glaze at a lower temperature in a subsequent firing. This requires the work to be carefully heated with a burner and the glaze to be meticulously sprayed on with a spray gun. Of course it will be essential to experiment to find a glaze that will fit the porcelain body and the application will need a bit of practice. I use a wide firing high gloss glaze and fire to about 1100°C.

But even this technique will work with only a couple of the metal salts without changing their appearance. This is the case with Cobalt Chloride and Uranyl Nitrate. There may be other colorants this glazing technique will work with, eg. Bismuth Nitrate or Vanadium Sulphate, but this needs to be looked into. In any case glazing in this manner will not work well with Copper Sulphate, Iron Sulphate or Potassium Dichromate without blurring the crystaline structure of the surface, and if the glazing is done in oxidation (which I normally do), copper will tint the glaze green.

Calligraphy & WOP

Because the WOP are chemicals dissolved in water and are much like ink in consistency, it is possible to use them not only with brushes, but also in conjunction with nibs and writing pens. Some of the metal salts will give very reliable results used in this way. Notably those colorants that need only one application can be used for writing on clay. Best of all are Gold Chloride, Cobalt Chloride and Uranyl Nitrate. To stop the colors from migrating, it helps to add some gum arabicum to the water solutions -this thickens the liquid slightly and stops them from blurring. Needless to say that the same precautions must be taken here as with any other decorating technique using metal salts, and extra care needs to be taken not to smudge already decorated areas. These problems aside, writing adds another dimension to the possibilities of the WOP technique.

Conclusion

The technique of using watersoluble metal salts is still fairly new and offers plenty of scope for experimentation. While no iron will be magically turned into gold, the WOP technique can yield some really fantastic results, but it can also be disappointing. colors will not always turn out as expected. Metal salts can be fairly unpredictable, because there are so many variables that will influence the outcome (eg. kiln atmosphere, amount of color, degree of vitrification, etc.). But this very same aspect of unpredictability makes it an exciting and sometimes very rewarding technique to work with.

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