Last weekend I had the great pleasure of helping Joe Morgan with the third firing of his new wood/soda kiln. I helped a little with its second firing at the end of last year.
Joe, Joseph Travis and Hilary Firth packed the kiln on the Saturday, then we fired on the Sunday. I arrived at 6.30am and the boys had already got going. I’m a novice at wood firing but I’ll try to explain a little of the process involved…
The kiln is based on the ‘long’ kilns that Sandra Lockwood and Steve Harrison use in Australia. The fire box is of bourry box design, the chamber long and low. To start, a small fire is lit on the firebox floor using small pieces of kindling. It is imperative to slowly increase the temperature until the kiln has passed all the ‘danger’ points. Pour water at 100˚C, chemically combined water at around 350˚C and lastly quartz inversion at 573˚C. Over the next few hours the fire is built up increasing the embers in the ash pit until around 600˚C is reached on the pyrometer. At this point we switch from adding small pieces of wood through the front to much larger/longer pieces onto the upper part of the bourry box. These rest on hobs over the ash pit and allow the kiln to draw the heat and flames down through the kiln. The rate of draw and atmosphere can be adjusted by adjusting the primary and secondary air holes at the front of the kiln and dampers in the chimney.
It is continual work to keep the fire fed… sawing, chopping and splitting wood to the right size; feeding and maintaining the fire; monitoring temperature and so on. It is intoxicating though – such a medieval process, involving proper graft, but massively exciting and hopefully rewarding. It is particularly enjoyable when with others… the chat and company spurs everyone on.
The temperature in the kiln’s chambers is checked firstly by using a pyrometer. In addition to this draw rings made from terracotta and stoneware are removed at various points to monitor the atmosphere in the kiln. Pyrometric cones are also observed through spy holes to check the progress in each of the ware chambers.
Once cone 10 goes down (1300˚C) in the first chamber (closest to the firebox) we introduce soda via bark ‘canoes’. These are carefully added to the firebox – the sizzle of the soda vaporising is a very welcome sound. The sodium vapour is then carried through the kiln with the flames and combines with silica in the clay. This will hopefully create the characteristic dimpled ‘orange peel’ glaze associated with soda firing. The last of the wood is added to the firebox before closing down the front of the kiln. Then side stoking begins through four small holes on either side of the kiln. These are located between the three ware chambers. Long, thin pieces of wood are carefully introduced to continue feeding the fire, all the while ensuring that they don’t touch the pots on either side. Finally more soda, wrapped in paper parcels are introduced through the four side stokes before the firing is completed.
By this time it was 9pm and we were exhilarated, but shattered. The firing was better than last time, but we won’t be sure of the results until we open up the kiln next weekend.
As we were tidying up in the darkness, we witnessed the perfect finale for our day’s labour… the most fantastic display of the Northern Lights. Breathtaking.
With many thanks to Joe Morgan, for help with the technical details.