home   info   gallery    articles    diary   links   contact    trivia

Firing the New Kiln                                                               by Owen Rye

First published in The Logbook Issue 13, 2003, pages 16 to 20.

My new anagama/tunnel kiln has two chambers; the first a tunnel some 5 metres long with a ‘paved’ brick floor and the second an offset catenary with a setting space around 25-30 cu ft ( 0.7 – 0.85cu.m). The kiln construction was described in the March 2001 issue of Pottery in Australia. It has now been fired twice and the basic design works well. Given that I am not getting any younger, everything around the kiln has been designed to minimise effort. Stokehole closing bricks are suspended on chains. The wood is all prepared ready to stoke, and stacked along the sides of the kiln shed so there is minimal movement in stoking. There are slopes rather than steps, with a soft gravel surface underfoot.

Firing the tunnel is straightforward. The overall plan is to heat to the required maximum cone in 24 to 36 hours; then over stoke for very heavy reduction and much ash; then lighter reduction and heat again to melt ash. Keep repeating this cycle on an 8 to 12 hour basis. Use heavy reduction in the first stage of cooling. The details of firing are worked out as it goes along, within this general plan.

The second chamber is something new to me and is still a bit of a mystery. I am familiar with the usual noborigama-firing concept where the chambers are fired in sequence. The first chamber is heated to full temperature and then left, concentrating on the second chamber and so on until all have reached temperature. I do not want to fire that way, mainly because it would cause the tunnel to oxidise while cooling, whereas I need heavy reduction. So I have to find a way to fire the second chamber at the same time as the first rather than in sequence. If any readers have suggestions based on experience I would appreciate hearing from them.

A simple answer to this problem is to use glazes in the second chamber that need a low maturing temperature, so the second chamber is heated sufficiently from the residual heat from the first or tunnel chamber. It appears that if I get cone 11 midway along the tunnel then the temperature through the second chamber is around cone 9 so I’ll work on that basis next firing. I realise that by contemporary woodfire standards these temperatures are low, but my interest in strong colour development means the lower the temperature the better the results.

In the interests of colour development the kiln has a water supply through pipes under the floor. In the first firing water/steam did not ‘flow’ through the brick floor, so for the second firing I made a trench filled with quartz chips along the pipe. The result was still not very effective so now I will probably remove the quartz chips and have a straight brick-lined trench for the water to flow into, as in Richard Bresnahan’s kiln.

To me the most important stage of the firing is stacking the kiln, and this gets slower each time. For the last firing I took three days for the back chamber and six days for the tunnel. Setting the tunnel is something of a challenge; areas under ember are about 18 inches(460mm) wide and about a foot (300mm) deep. The side stokeholes are in pairs, staggered for maximum width under ember. In the past I would have set most pots lying on their side, but now most are wadded apart standing up. In between each under-ember area there is space for one row of shelves 18 inches (460mm) deep by about 4 feet (1.22m) width.

The second most consideration is the firing crew. Big kilns cannot be fired solo over a period of days; it is always a team effort. My crew (three pairs of two, each pair doing 8-hour shifts) is selected on the basis of experience, compatibility and reliability. My set-up is different to a school or workshop situation where the aim is education, and inexperienced people are accommodated. I aim for the best possible results, and that means forming a team from the most experienced people around. Every firing is different because it contains different work, resulting in a different setting, and every firing incorporates experimentation in some form, and application of new knowledge from many sources. So the team must be capable of informed innovation based on good powers of observation and concentration. My personal preference is working with people who have read and travelled widely and are good conversationalists, because that makes work more interesting.

From the beginning the heat rise is rapid (all the work is bisqued). The draft is so strong it is difficult to build up a good ember bed even with the air inlets fully closed. Overstoking with pine helps, and the reduction is satisfactory, but I must shorten the chimney before the next firing, or block up the flues slightly.

The radiata pine fuel has about reached its temperature limit 36 hours into the firing. This is the time to change to Californian Cypress (macrocarpa), the ‘warp drive’ of local timbers. It burns fast and hot with big easily burned embers, like pine on steroids. Its ash is fluid and potent so in this kiln, which was designed for as much work as possible under the embers, it needs to be used with moderation at high temperatures. I have a choice of many eucalypts but these tend to make finer, more closely packed embers that need a strong air flow for combustion, and thus tend the firing towards oxidation. So I prefer to use the softwoods that produce large embers, easy to burn. I sidestoke specific materials in some locations, aiming at known exotic surfaces.

The overall approach mid firing is to get the temperature up quickly with cypress and then sidestoke heavily with either pine or cypress to get heavy ash and reduction, dropping the temperature again. Overfiring in the front is controlled by sidestoking, decreasing the temperature from the main firebox. These cycles of heavy stoking to lay down ash, followed by heating to melt it, continue through the heating stages of the firing.

During the early part of the heating cycle I use a stoking method which was first worked out by Graham King, involving the oxyprobe. Stoking leads to heavy reduction; you then watch the probe until it falls to a certain number, then stoke again. The number varies in different firings but is generally in the light reduction range, so stoking prevents the atmosphere moving over into neutral and then oxidation. This technique produces extremely effective heating when all other methods of stoking fail.

Three days into the firing, the tunnel was okay but the second chamber used for work with applied glazes would not get hot enough. The dampers changed the heat distribution; more draft heats the top more, less draft heats the bottom more. So it was possible to get an even temperature, but it was still two cones short of maximum. We tried opening air inlets to the second chamber grate, and stoking heavily with cypress. The temperature increased half a cone in 45 minutes but then stalled. Rob Barron arrived for a look at the firing and suggested light sidestoking in the tunnel at the point that would place the tip of the flame in the second chamber. We tried that for five hours using cypress. I was concerned that the stoking was too light and that the second chamber was being oxidised; the temperature increased slightly. Choung then decided to stoke much more heavily in the same location, and that got cone 11 moving. It never did go down. Tony Stewart and I had a long discussion about the heating merits of ‘the tip of the flame’, favoured by some woodfirers, versus ‘volume of flame’ which seems more appropriate to my needs for very heavy reduction.

After cooling for five days, opening the second chamber was a depressing experience. Most of the work was bad; underfired, too dark in hue, and one of the transparent glazes was obviously mixed wrongly because it looked like dark chocolate. Everything in the tunnel also had a very dark cast. Where there should be transparent green ash glaze there is dark brown instead. Later I realised that the end of the firing was handled very badly. In a state of tired stupidity I had filled the firebox to create a reduced cooling condition, but neglected to completely shut off the draft first. So the draft had carried fine dark ash and carbon through the kiln as it cooled, depositing a black layer widely. It’s hard to believe I could be so stupid about something I’ve done better many times before.

The compensation came, as it usually does, some days later when I realised that most of the pots fired under ember in the tunnel were good; and that some of the others that I had dismissed at first glance were exhibitable. It will take many months to analyse the results, and make notes about what to do next time.