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.
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