Saltglaze

The porcelain pots are biscuit fired prior to slip and glaze application. Any stoneware pots, such as casseroles, are slipped and fired raw.I will often throw the porecelain pots for saltglazing a little thicker than for glaze firing to reduce warping and to give them more strength. The extra weight isn't noticable in the fired pot as they do not have the thickness of glaze on them. The slips are applied very thin to the porcelain and glazes are applied at a milk thin consistency. Pots are glazed before slipping as the slip tends to peel off the pot if done the other way around. In the kiln the pots are wadded using a mixture of alumina hydrate, china clay and paper pulp - applied at frequent intervals around footrings to reduce severe warping. The wadding is predominantly alumina and the paper pulp is included to increase the maleability of the mixture and, as it burns out during the firing, it makes it easier to crumble and grind the wadding off the pots.

If all pots are biscuit fired the saltglaze firing progresses quite quickly. Up until cone 06 it is pretty similar to a glaze firing (about 5 -6 hours). There follows a moderate/heavy reduction to 1100°C over an hour. Then the atmosphere is altered to light reduction for an hour until 1200°C. The rest of the firing is in oxidation. To cone 9/10 in another hour when about 4 Kg of fine salt is blown into the kiln via tubes above the burners. This technique has the advantage of reducing the amount of salt needed in the firing, preventing excessive wear of the fireboxes and getting an even salting. Salting last 1 - 1.25 hours. Then there is a 20 minute soak until cone 11 is down. At this point I reckon the temperature inside the kiln is around cone 9 (cone 11 bending early due to the fluxing action of the salt). When working in Australia I never crash cooled but clammed up the kiln immediately the firing was over. Here I've been fast cooling to 1200°C only. I don't soak for that long at top temperature due to the silca content and the warping of porcelain, but I reckon that the longer cool brings out warmer colours and gives some interesting crystal growth in some of the glazes.

The salt kiln is a catenery arch structure built with 42% alumina dense bricks. The lower half and door is insulated with diatomaceous bricks, the arch with 3" ceramic fibre. It can be fired with opposing LPG burners or wood (using the internal fireboxes). So far, due to problems finding a suitable wood supply, it has only been fired with gas.

Click here for slip recipes.