Soda Firing Workshop October 2004

This workshop was run by Paul, Vivienne and Bippin. There were four ‘students’, including Richard, a new member from Derby. The weather was autumnal, but at Northfields, being closer to the Arctic, it felt distinctly nippy. Numerous layers of clothing were the order of the day, and as we gathered on the Saturday, the group looked a motley crew.

Despite the weather, the workshop began in high style in Dorley and Murray’s conservatory, with tea and jam tarts fresh from the oven. Then we set to, unpacking our biscuit pots and selecting pieces for slipping and glazing. There were ash, celadon and shino glazes, and blue and orange slips to choose from. Bippin cleaned out the kiln, and the shelves were batt washed. Wadding was made up and rolled into small balls, and each pot was ‘wadded up’ with three balls stuck to the bottom, to prevent the soda sticking them to the shelves. All the props were ground flat. The kiln was then ready for packing.

Meanwhile, the slipping and glazing was going on. When it was finished, we stopped for lunch. Whereupon Dorley appeared bearing tea, and Murray produced a bottle of wine. Our picnic was turning into a feast!

Fortified, we got back to work. Bippin climbed into the kiln to pack the back stack, a back breaking job. The rest of us put the wads on our pots, and also the props, as they were handed into the kiln. This is a fiddly business, as the wads have a tendency to drop off. Eventually the middle stack, and at last, the front stack, were done. Cones had been placed at the front and to the rear of the stack. The packing had taken about five hours. It was now getting cold, and the gloom of the late afternoon had turned to night. Torches were brought out. The door was carefully bricked up and bricks were re-cut to accommodate the spy holes for the cones. Mortar was made up from crushed fire bricks and fire clay, and then plastered all over the joints. The last task was to set up the burners, and check the gas connections to ensure that there were no leaks. It was all done by 8pm – everything takes a lot longer than you think.

The next day, Dorley nobly volunteered to get up at 5.30am to light the two burners. She then had to turn up the gas at 6.30, 7.30 and 8.30am. (I am ashamed to admit it, but some of us were still in bed then.) Dorley’s heroic efforts helped enormously, as the kiln temperature was about 8oo C by 9am, and 1000 C by 11.30am. The group reassembled gradually around the kiln, and, as the burners roared, the day began to appear most congenial.

By 11.30am the temperature was 1105 C, the gas was turned up to 15psi, and reduction was commenced by closing the damper a little, and also the venturi on both burners. (The venturi are the adjusters on the burners that control the air intake.) This continued until 1220 C, when the damper and venturi were opened up. It was now time for action. Time to add the soda!

The soda is put into the kiln in a hot concentrated solution. This unpleasant concoction was prepared on a camping stove, sieved, and poured into a plastic pressurised spray gun so that it could be squirted through an open port and into the flame. This is, of course, terribly exciting, and everybody has to have a go. You should wear shades so that you can have a good peer into the kiln without damaging your eyes. It takes about four minutes to empty the gun and then it must be washed out. As blockages often occur, two guns are used. When the soda hits the flame, it explodes into a vapour, using some of the heat as it does so. The temperature tends to drop during this process. So that it can recover, there is a short gap, of about five minutes, between the applications of soda. Then the operation is carried out from the other side, so that it alternates from front to back, about 12 times in all. After each shot, the damper was pushed in and out very quickly. This creates a pressure wave that helps the soda vapour circulate around the kiln. The soda was finished by 2.30pm. A total of 1½ Kg of sodium bicarbonate had been used.

The firing had to continue until 1300 C was reached, which is always a slow process, in this case requiring 1 1/2 hours. So, we spread out a picnic under a damson tree behind the kiln. Vivienne served a hearty vegetable soup that she had made, and there were a number of relishes to accompany our sandwiches, including a delicious runner bean chutney. (Does anyone have a recipe for this? It was rather good!). We finished off with damson crumble made from fruit that we had picked the day before. Even the sun came out. To cap it all, Murray appeared with a bottle of whisky and cigars!

By 3.45pm all the cones were flat, indicating that 1300 C had been reached. It was time to crash cool the kiln. The gas was turned off and all the bungs opened. It took about half an hour for the contents of the kiln to cool from a glowing gold to a dull red, and for the pyrometer to show 980 C. We set about clamming up the kiln. All the bungs were replaced and the damper was closed. All the cracks in the wicket (door) and around the bungs were sealed with mortar.

It had been a lovely day and a cracking good firing. We looked forward to the results, and when we opened the kiln, a week later, we were not disappointed. Well, mostly. Most of the pots had taken the soda well, with a good lustrous surface and lots of rich orange and red tans, bleached golds, and subtle orange peel effects. You may have seen some of these in our exhibition at Open Day.

In a few cases the magic of soda had not worked. With one batch, it seemed that the clay colour predominated, and the slip colours had been lost. These pots had had a pinkish hue at the biscuit stage. After the soda firing they were dark brown and shiny. Another group of pots had been covered with some other slip, and had fired quite mat. Conclusions may be drawn that results depend on many factors, including:

1. Clay. Use a fairly white, grogged clay, or mix of clays. (Heavy iron content can work fine, especially if well grogged, and you like brown pots.) The grog appears to aid the formation of the orange peel effect.
2. Slips. The orange tan slips work well and are pretty reliable. Colours vary from reddish orange to gold, depending on how much soda the pots get. Blue slips, on the other hand, are much more elusive, and may give blue, black, brown or a pink beige.
3. Position in the kiln. Some areas get much more soda than others, e.g. the back wall usually gets lots. However there is little consistency, as the stack is never the same twice.
4. The firing varies according to the weather, (wind direction and speed, air temperature, humidity, etc), the state of the gas tanks (whether full or half empty), and the operator.

All this goes to make every soda firing like a journey into the unknown. The results may be stunning or disappointing, but always quite fascinating.

A big thank you to the team who led the workshop, and especially to Dorley and Murray for their kind hospitality, and for getting up so early.