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Woodfired Clay Colours
Owen Rye reviews a woodfire/salt glaze body

Flashing colours are typical of woodfiring, usually where a reduced atmosphere has been used in the  higher temperature ranges, followed by an oxidised cooling. The flashing colours are very dependent  on the nature of the clays used, and generally low iron clay bodies fluxed by felspars produce an  orange coloured flash.

I have for many years been working at improving colour response in woodfiring, both in glazes and on the clay itself. Rather than dull browns and tans I want reds, pinks, oranges and apricot colours. Experimenting for many years with clay body composition led to RSF, a woodfire/salt body sold commercially in Australia by Clayworks. RSF is one of the few specially formulated commercially available woodfire clay bodies in the world (the others being in Japan, e.g. Shigaraki clay). It has proved over the years to be very durable in the most testing woodfire conditions such as anagama firing, and even refiring multiple times, a practice that destroys most stoneware bodies. It has been successfully fired up to Cone 12-13 in the anagama but is best around Cone 10 for colour   development. Under the right firing conditions, RSF could produce strong pink/apricot/orange colours underneath wads, or where the clay was sheltered from direct flame.

Recently, because one of the materials in RSF became unavailable, the opportunity arose to experiment with further improvements in RSF. The new composition allows an even richer flashing quality.

In common with the earlier version of RSF, the new clay is also suitable for saltglazing. It has good throwing characteristics and is especially suitable for large pots, which I make by the coil and throw method. It is also used as a handbuilding body especially for large-scale work. I use it sometimes as paperclay, mixing shredded newspaper and some paper pulp into the commercial body in a clay mixer. This does not alter its firing characteristics in any way but allows making difficult shapes, and larger forms are much lighter than when made from the standard RSF.

Along with clay body experiments I have for many years investigated details of the firing process, and it is clear to me that the best clays in the world will not respond with good color if fired inappropriately. An oxidising fire, for example, produces no flashing (except an occasional slight coloration where works are touching during firing). My early experiments showed that RSF develops color when oxidation and reduction are alternated during firing, as can happen quite naturally during woodfiring if stoking is delayed until only brightly glowing ember is left. This helps temperature rise.

Better results later came from consistently reducing up to (and maintaining) top temperature, followed by reduction during cooling down to around 1100 Centigrade (1832 Fahrenheit). This procedure gives orange/apricot flashing colours. The strongest colors have developed since I started introducing water into the kiln during firing and cooling. Water in the kiln during cooling seems to give the strongest colors, which move towards pinks and reds. There is no final word on these experiments, which continue with each firing. And just in case anyone thinks that color development is only possible in long duration firings, I have seen strong flashing on RSF fired over six hours in a fast-fire kiln, followed by appropriate cooling techniques.

The next stage in experimentation will be to develop a higher-iron woodfire clay. The aim here will be to develop rich red colors as seen in some Bizen ware in Japan, notably developed under wads in work by Kei Fujiwara. We are planning that the new clay body will be available later this year.