Svend fired the new kiln last week end (29th August to 1st September 2013). The kiln was very damp so 24 hours at 100C were needed to get rid of the moisture before the firing even started. Svend then started with gas overnight to slowly heat up and draw the moisture out of the glazes; the next day, the actual firing started very slowly with gas and hard wood, mainly beech, until the alpha-beta conversion was reached (roughly 573C). The reason for using hard wood is that hard wood is better for keeping a constant temperature than soft wood and the alpha-beta conversion stage being critical, it is important not to let the temperature fluctuate too dramatically. The actual firing lasted 72 hours.
After the alpha-beta conversion, soft wood ( birch and Douglas fir) was used and what is called “heat work” started and lasted until the end of the firing. The crew was stoking three sticks at a time and for three days and nights worked four hour shifts each. The aim during all firings is to get the temperature to rise slowly with constant reduction and to get ash deposits on the surface of the pots. During heat-work, Svend also does some side stoking to encourage fly-ash and embers to deposit on the pots placed nearer the sides, and also to level out the temperature throughout the kiln.
At the beginning of the firing, Svend draws a firing chart that each crew member will follow on their own shift. They record the temperature throughout the firing, the time when reduction was reached and any unusual event that may occur for future reference.
On the last day of the firing, the three members of the crew work together to reach cone 12. One is stoking the front of the kiln and the other two fire the first stoke holes until cone 12 is flat. They then go on to the next stoke hole until cone 12 is reached there and carry on so until the last stoke hole has been fired. During side stoking, two people have the job of stoking whilst the third person checks the chimney for smoke; when all the smoke has disappeared off the chimney, it is time to stoke again. When all cones are down, the firing is finished and the kiln is clammed up with a mixture of one part clay and two part sand.
Svend explains in his own words and in more details what happened in this firing in the section below.
Over to Svend…
THE NEW KILN.
Some years ago David Frith asked me to build a kiln at one of his events. The kiln I built was a small, simplified version of a Sawankhalok kiln I had built several times before. The kiln fired very easily and the three day firing produced some beautiful, heavily ashed pots. When I returned home I pulled down my existing kiln and built a slightly larger version of the kiln I had built for David. The kiln worked well for many five day firings until suddenly it began to oxidise. It took five more firings before I discovered the cause. The soft bricks had cracked around the door, leaving a half inch gap all around it, which translated into the equivalent of pulling two bricks out. Once repaired, the kiln worked well again but the soft bricks began to deteriorate and crumble and so I decided to rebuild it with some minor alterations, using hard bricks. This was the first firing of that kiln.
I started this firing on Thursday 29th of August at midday. I began with small household beech logs and a gas burner to dry the kiln out. At 10 pm I stopped the wood and left the gas burner in until I got up at 6 am. I turned the gas up and went for my swim. At 8 am I switched over to wood only, using birch logs.The temperature rose steadily throughout Friday and passed 900C in the early hours of Saturday. At that point the kiln was already reducing naturally as the ember bed was higher than the stoke holes in the fire box and so I switched to side stoking and for the next four hours, simply riddling the fire box embers and side stoking the kiln for embers and temperature. By Sunday morning the temperature had crept up to 1200C and by late afternoon it was 1280C on the pyrometer but cone 12 was flat all over the front of the kiln and so I began side stoking to finish. We got cone 11 over at the first two spy holes and then noticed that cone 12 was falling at the very back of the kiln. In order to avoid overfiring the back, I stopped the firing. The firing, with the help of Bee and Charlie, doing 4 hours on and 8 hours off , was very easy and the kiln very responsive. It did everything I asked it to do and in spite of being built of dense bricks, used no more wood than the previous kiln.
On Friday 6th September I unpacked the kiln with Brigitte and Bjorn. Although there were some very good pots at the front it very quickly became apparent that I had made some basic mistakes.
The biggest mistake was starting the finishing side stoking too soon. If I had side stoked into the firebox for longer I could have got cones 11 and 12 down at the sides before the back got too hot. The result was that the middle of the kiln was under fired and the back too hot . The clue was in the fact that the pyrometer was only reading 1280C. Had I gone on at the front until 1300C+, things might have been different. The next big mistake was using a celadon at the front which is good at cone 10 but watery and dripping at cone 12. The shame of it is that I have a perfectly good celadon for the front which is both stable and reacts beautifully to heavy ashing. But it was by no means all bad. Some of the pots are beautiful. Time to pick myself up, dust myself down and get right back in there again.