Firing Andrew´s kiln
Carol looked at the photos of Andrew´s kiln and wanted to know how it was fired, so I emailed some questions to him, using the photo at right for reference. His reply referred to a further diagram which is reproduced below, along with the questions and his answers.
I have attached a cross-section of my kiln which may be of some use
to look at in conjunction with your photo. I have since altered the
floor of the throat to make the steps two bricks long instead of one.
It was too hard to pack with all those little steps. The illustrated
pack is denser than anything I can actually achieve. I now use only
one bung of shelves in the first bay. More space was needed there for
flame expansion and the top front was always a cold spot. The diagram
shows the sidestoking ports, corresponding to 3, 5 and 6 on your
photo, but does not show the mouseholes. There are three, one at the
front of the firebox and one on each side of it, at the level of the
floor. There's another one on each side of behind (in front of?) the
bagwall at position 7.
Do you keep the main firebox going throughout?
Yes, I do, even if it is only ticking over. I think of it as an old V8 burbling away with lots of deep down reserve power. The sidestoking ports can give a quick burst but have insufficient "grate area" or regulated air supply (see below) to keep the kiln going on their own. They may do so in a no-bulli-gama (1) but not when there is a whole chamber tacked on the end.
When do you start sidestoking?
No hard and fast rule for this. There are two considerations: moving thefire along the kiln to narrow the temperature difference between front andback; and getting enough reduction at the end of the throat and in thechamber. The first bay of the throat can be at 1100 or even 1200 before cone06 starts moving in the second bay, while the chamber may only be 500 or 600. I would probably begin by stoking at 3 at this stage to try to drop 06 in the second bay in a reducing atmosphere. I might stoke a bit at 5 aswell, depending on how I feel about the overall state of the kiln.I used to think it was better to start sidestoking early, before the first bay got too hot in case I ended up overfiring it. I thought there would be some virtue in trying to get the kiln even, before top temperature, and then bring it all up together. I now think that's probably impossible and even though cone 12 is flat at the front it never gets any hotter and [the clays I use] can withstand long times at high temperatures, the longer the better actually.
Once started do you sidestoke all the throat ports all the time ?
More or less, although I usually concentrate on one set at a time according to where I think the heat is needed. This also gives the other one a chance to burn down since they can get quite full.
Do you stoke the main chamber directly?
Yes, eventually I always end up stoking at 6 because I don't think the chamber is ever going to make it. There's a big difference in the effect of stoking right behind the bagwall at 6 compared to 5. Sometimes I may stoke in a wave: 3 then 5 then 6 and back to 3 again to pull the fire through. It's a long way for the flame to travel from the firebox to the other end of the kiln without some help along the way. Yet it's surprising how just a tickle of a sidestoke can result in a very quick rise and an impressive flame at the top of the chimney. I would like to try a firing with no sidestoking to see if it could be done and how long it would take. I suspect it would be a struggle and the results might be a bit bland but it would be much less work (splitting as well as stoking) even if the firing took longer.
The bricks near the numbers [on the photo]seem to be removable. What are they all for?
Of the numbers I have not already identified, 1 and 4 are spyhole bricks and 2 is where a thermocouple goes in.
Are there any ports on the other side of the kiln? Do you have any air intakes and grates below the sidestoke ports as does Sandra Lockwood?
There are stoking ports in exactly the same positions on the other side of the kiln as well as a couple more spyholes. Grates and air intakes would probably improve their performance markedly. They are pretty inefficient as they are and not all that easy to use.
What do you do with the damper, and why? It seems to me that if you can get this kiln to fire reasonably evenly then we should have been able to get our old salt kiln to fire more evenly than we ever did. We never knew what to do with the damper.
I have to admit that often I am puzzled too. There is a school of thought that says you should be able to fire with the damper wide open, using only the air controls at the front of the kiln but I haven't been able to do it. This kiln can pull a lot of air. The chimney is one and a half bricks square internally and there is all that horizontal draught as well as vertical. I don't think the firebox is big enough to keep up with the draught and it's all too easy to pull excess cold air through and lose temperature. Observing the behaviour of the fire at start of the firing (fire on the firebox floor) is instructive. If the damper is open the fire is erratic, flaring up and dying back quickly with consequent cycles of soaring and plummeting temperature, impossible to stabilise. This occurs even if the stokehole is virtually closed. It is much more manageable with the damper set at a bit less than half from the start and it will rarely exceed this at any stage of the firing. In fact it seems to need closing to 20% or 30% for reduction or even less to get decent back pressure (2) from the spyholes in the chamber.
It's easy to succumb to worry that you're not getting anywhere and to keep fiddling with the damper with an eye on the digital pyrometer. These quick reacting pyrometers have developed in tandem with lightweight kilns. There's a lot of inertia in a brick kiln and you just have to be patient and watch the flame. A nice fat, lazy flame takes time, not big draughts of cold air. If I'm honest I must say that I don't really know what I'm doing a lot of the time. There are a lot of variables, not least the fuel. I'm still learning the kiln. I've only fired it ten times and with different people, including Bob Connery and Colin Drake, who have all had their ideas too. There are lots of ways to fire it. Once I fired to cone 12 in the first bay, cone 10 in the second and a load of planters to between cones 03 and 2 in the chamber with no attempt (and no necessity) to even out the temperature.
How even is the temperature in the main chamber, at best, when you finally decide to go to bed?
Looking back over the logs I see that the cones have been pretty even over the last four or five firings, at least cone 10 consistently on the bottom with cone 12 moving higher up. Sometimes cone 12 is nearly half down on the bottom too. As for the pots themselves, they can be a little cool down near the exit flues and perhaps all the way down the back wall. But if there's a good soak it's not too bad.
(1) The term no-bull-i-gama was used by Steve Harrison as the name for a kiln of his which is essentially the same as Andrew´s kiln with the chamber removed and with a horizontal throat.
(2) Back pressure refers to the situation where flames tend to come out of the spyhole when the spyhole brick is removed.
SideStoke home | Andrew home | Kiln home