b) SODA, CLAYS AND FIRE (by P.E. Maddalena)

by

Gay Nichols

Gail Nichols is a potter whose technical research and artistic approach to soda firing has literally opened up new aesthetics in the world of ceramics.

In contrast with the general rule of using high contents of silica in the clay, she uses high alumina clay bodies which reacts to soda vapour to create a generous opaque glaze, dimpled and frosty matt, giving the impression of glacially flowing ice.

It bears little resemblance to traditional salt glaze, or to the bright glassier styles of soda glaze. Unlike slips or detailed surface marking of much salt and soda glaze work, this very special technique tries to focus toward another aesthetic in which the thickly formed glaze flows over, interacts with and softens the form.

Fairly slow cooling is required to create frosty mattness (two hours to 1100 °C and three more to 800 °C). It was also discovered that water vapour in the kiln can alter the glaze colour and increase the crystalline mattness. Compounds like calcia, magnesia, silica, iron or wood ash are to be avoided.

Firing cycle

After reaching 900°C start reducing and fire up with light to medium reduction.

Soda introductions begins at cone 8 with intervals of about 15 minutes with temperature raising up to cone 9 and 10 down.

At cone 10 maintain temperature for two hours (cone 11 down) then fire down to 1000 °C in two hours and again cool down normally to 800 °C in three hours.

Gay Nichols

Considerations

In Gail Nichols experiments, the soda “ice” glaze formed most successfully in oxidation. Its maturity and colour were enhanced by reducing during the first cooling stage. When reduced cooling below 1000 °C, the colours became muted, producing dull browns instead of reds and oranges.

Reduction during the high temperature glaze forming stages produced some strong colours with close resemblance to traditional salt glaze. Shino style slips and glazes also developed much stronger colour when exposed to this reduction.

It seems that for colour enhancement during cooling, water vapour is most effective between 1000 °C and 800 °C. Water during cooling can also be used to create matt glazes from otherwise glassy surfaces.

Copper and manganese pigments blend well with soda “ice” glaze.

Strong red colours are assisted by a reducing atmosphere and water introduction during glaze forming stages, as well as by reduction during the early stage of cooling.

The “ice” glaze owes its opacity and mattness to a complex mixture of surface distortion and crystallisation. Its icy character is caused by its high soda content, the complete absence of other fluxes, its low silica/alumina ratio, and the absence of crystallised iron. The best ice glaze seems to be obtained in oxidation.

Gay Nichols

Gail Nichols has written a book, “Soda, Clay and Fire” on her distinctive approach to soda vapour glazing published by The American Ceramic Society in 2006.

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2 Responses to “b) SODA, CLAYS AND FIRE (by P.E. Maddalena)”

  1. Mirka Golden-Hann Says:

    It is the ratio of alumina and silica in the clay body and in the slips that will govern the size of ‘orange peel’. Peter Meanley concludes that the level of orange peel could be governed by firing to cone 10 flat and soaking for further 15 minutes thus enlarging the spaces between ‘blob and blob’ . In my experience it is also the size of filler (grog, molochite or sand) which influences the size of the break-up in the glaze. Where microscopic particles of silica attract the glass the large particles often in the form of the filler will repulse it hence creating the more pronounced orange peel and in heavy salted examples even a pitted surface. However, the heavy orange peel effect can also be acheaved by using two slips with varying point of vitrification on top of each other. I can supply some recipes if you would like.
    Looking at the salt-glaze produced by the artists working for the Royal Doulton potteries (a source of inspiration for my colour in slat-glaze research) one could easily mistake the elaborately decorated vessels for non salt glaze. Their surface is often extremely smooth and flat without the orange peel effect. Alistair Young’s research into the Daulton practice led me to the understanding that the art pieces were never saggar fired as one might think from the surface quality, but were fired in the middle of the massive kiln stack shielded by sewerage pipes and other commercial wares produced by the factory. The recipe of the Famous Doulton Salt body (62.5 BBV ball clay, 18.75 China Clay, 18.75 Potash Feldspar) is highly siliceous and smooth. This perhaps explains the quality of glaze created in those spaces of the kiln penetrated only by minimal salt vapour. This would also indicate that the quality of the glaze was also governed by the size of the kiln. My experience is with salt-glaze, but I would like comments specullating the behaviour of soda which as I beleive does not travel around the kiln as freely as salt. Please let me know how does soda behave in the middle of the kiln stack?
    The above mentioned Famous Doulton clay recipe makes very attractive golden coloured clay body for salt firing with the addittion of 2.5% of terra cotta clay. In my experience I would not recomend adding straingt iron oxide into clay. Iron oxide beeing a flux promotes formation of crystobolite which inturn leads to serious dunting problem.

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