The past three days have been spent at HDK’s kiln site at Nääs where we fired both the anagama (Mamagama) and Elinor (the smaller wood kiln).
Since Elin and I were scheduled for the first shift on Monday morning, we arrived on site around 6am and started up the anagama. We pulled out the bricks from the central air hole at the base and built a small brick box in which we set fire to some newspaper and placed dry kindling on top. We began by taking the temperature up at a slow pace, 25C per hour until reaching 100C, with the fire still mainly in the box outside the kiln. As a temperature gauge we used a pyrometer stuck through a crack in the door but later decided to place it in a hole in the kiln’s roof to get a more accurate reading of the inside temperature. We sealed the door with the daub we made last week, blocking out the bumble bee who was desperate to get inside despite the rising heat!
After four hours we let another pair take over and we worked this way in shifts of four hours per team for the entire firing. Around 10am Elinor was started and taken up to temperature at a much faster rate. Reduction in Elinor took place on Monday afternoon when the kiln had reached 1000C. Creating a reduction atmosphere before this temperature means the clay can get reduced instead of the glazes which can cause it to trap carbon and turn a very dark colour which might be undesirable.
Sometimes when too many logs were fed in too quickly, black smoke started spewing out the chimney – a sign reduction was taking place, and we had to be more patient. It’s difficult to get the balance between allowing in enough oxygen for the flames and allowing in too much which starts the reduction process because the cold air blocks the flow.
When feeding logs into the kilns the temperature would fluctuate up and down a lot, sometimes peaking three times before finally decreasing for good. We were told to focus on the fire and not rely on the pyrometer though. It’s possible to hear when the kiln is ‘hungry’ again because the fire goes quiet and the crackling stops. We also used pyrometric cones placed at the front, middle and back of the kilns to check temperature. When the cones were bending unevenly e.g. if on the left side there were four standing and on the right five, we put in less wood and slowed down the temperature gain. Another way to check if the glazes are melting is to poke a stick in through one of the anagama’s peep holes and see if the pot’s surface is shiny enough to reflect off it, a technique I’ve used before when firing raku.
It’s important to make sure that the last logs have turned into embers before more have been added. Unless you do this you find yourself in a situation like we did early on Wednesday morning when the anagama would refuse to climb above 1220C. Looking into the airholes we realised that the embers were so high that they were blocking the oxygen flow into the kiln so the fire couldn’t grow. By pulling out logs and moving around the embers inside we fixed the problem, but the temperature dropped dramatically so we worried that we would be behind schedule. This could be fixed though by filling the door with long thin sticks sticking into the flames which raised the temperature.
In between shifts we took turns breaking down the logs with a hydraulic wood splitter and cutting some down even smaller with an axe. Smaller pieces of wood raise the temperature because they burn quicker but it’s best to use a mixture of thick and thin, long and short logs to get an even rise. When stoking the kiln, sometimes we would place two small logs crossed in the doorway to conserve heat.
Our fifth and final shift started at 6pm on Wednesday night. We kept the temperature around 1250C until 7pm when we topped at 1300C before bringing it back down to 1270C. We topped another 5 or so times before filling the kiln with as many long sticks as possible and sealing as many holes as possible in turns. This was probably the most stressful part of the firing because it needs to be done fast and the kiln is at its hottest. It was impossible to feed in logs for very long because your legs feel like they’re burning! It was necessary to wear welding goggles, scarves over our hair and mouths, long sleeves to cover arms and legs, sturdy boots and flameproof gloves. After sealing the gaps in the kiln with daub, cold water was poured all around the kiln to make sure none of the logs piled around it would catch fire once we left.