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Firing Log

ancient kiln | 21st century logbook

November 6, 2009

Building and Firing a Steve Harrison Throat Kiln (Overview)

Filed under: Firing,Kiln,non-anagama,non-anagama,Pieces,sound & video — odin @ 9:44 am

For a while now, I have been thinking about building a smaller wood fired kiln — one which I could fire alone in a short time.  I love firing the anagama, but it requires a large amount of preparation and a large block of time to fire.  It isn’t the sort of kiln where I can think “Hmmm … I feel like firing this weekend”, and then just go for it.  Preparing to fire the anagama is huge undertaking, and I’m not only talking about wood or pottery.  There is a huge list of little things that must be done as a firing approaches and a person can really run ragged trying to do everything.

Aside from the work, firing the anagama is expensive.  Just the wood itself is a significant expense.  For example, I’ll usually burn 2.5 – 3 cords of firewood (@ $200/cord) and two cords of millends (@ $125/cord), plus use about $75 worth of propane.  Then there are lost minerals — every firing uses a decent amount of fireclay and grog in wadding (I’m guessing about $20 for wadding).  Other consumables include welding gloves (I go through three pair per firing: another $40), I easily lose five firebricks per firing at $2.50 each (and I have to Canada to get them): another $12.50.  It seems I buy a kiln shelf every other firing at about $35 each, so $17.50 per firing in shelves.  Without much effort, we’re talking about a cost per firing of approximately $1000, not including clay to make the pieces or food to eat during the firing if, as with me, you don’t live where the kiln sits.

So while I love firing the anagama, I’ve been wanting a kiln that would allow me to wake up on a Saturday, decide on a whim to fire, and be doing it that evening.  My interest in a throat kiln which would allow me to do such firings began with my interview with Steve Harrison some years ago, and even more so after I received a copy of his book “Laid Back Woodfiring“. 

Building (only a sketch, more details in later posts)

I plan to post a more detailed description of the construction in the coming weeks.  For now, I will say this: I basically built the kiln from interview with Steve Harrison’s plans in Laid Back Woodfiring, although in a nod to Furutani, I built a tiny sort of sutema between the kiln body and the chimney.  I didn’t build in any mouseholes, and instead of connecting  lower legs of the angle-iron braces on the firebox corners near the kiln body by running the brace under the kiln, I simply connected them at the top and buttressed them at the bottom.  I also set up the front air vents in a manner that I’m more familiar with.  I did none of these things for technical reasons, and I am not in the least suggesting that my kiln is better than Harrison’s — I just did them because it was either easier and I’m sort of lazy, or because I wanted to continue working with systems I’m already somewhat familiar with because of the anagama.

OK, by now, you’re probably wondering what the kiln looks like. Here are a few pics:

preheating throat kiln with propane preheating with propane (larger image)

firebox and kiln body firebox and kiln body (larger image)

glowing kiln glowing kiln (larger image)

firemouth firemouth (larger image)

chimney chimney (larger image)

I had the lid seen in some of the pictures above fabricated from steel and I attached refractory fiber to the inside of the lid (nicore wire strung through porcelain buttons I once-fired in my raku kiln). I cut out spaces in the fiber for the primary air inlets, and riveted a piece of angle iron to the top of the lid to act as a ledge for soft insulating firebrick splits which I intended to use to control the primary air inlets.  During heat testing, I realized that fire seeps up through air inlets, travels along the metal under the insulating firebricks, and then escapes.  Because some parts of the lid are hot and others are not, the surface takes on the shape of the ocean in a hurricane.  To remedy this, I riveted another piece of angle iron to the lid an tested again — same result although more constrained in the affected area.

Worse than the seeping fire though, was the smoke.  Prodigious amounts of smoke exited through the primary air inlets and made it practically impossible to stand next to the kiln.  Seriously, to fire the kiln with those air “inlets”, or smoke outlets as I think of them, I’d need a fireman’s respirator.  It would have been worth trying to see if carving the bricks so that they filled in the air inlets but also had a ledge so that they could not fall into the firebox, would have worked better, but there was a major problem with this idea — I would need six bricks and I had only one, so a trip to Seattle (three to four hours round trip) would have been necessary.  Besides — if I did open them to allow air “in”, what exactly prevents tons of smoke from seeping out the top of the boury box?  Either I don’t have a good understanding of how the boury box design works — I never hear about them belching smoke like a coal fired steam engine — or people don’t talk about that.  The only thing I can think of that would make it work would be a chimney twice as tall or taller, but I don’t want to build a 20 ft chimney let alone buy any more bricks.  Besides, smoke rises and a hole right over the fire is going to let smoke out unless the chimney has enough draft to power a rocket, and I don’t want the flame to move through the kiln that fast.  Anyway, I didn’t want to suffer so I took some scrap fiber and sealed up the holes, figuring I’d just use the front air inlets as I do with the anagama.  Then I weighted the lid to give it a good seal and moved on.

Firing

Firing the kiln went very well.  I candled the kiln overnight through the front air inlets (secondary inlets in a proper boury box, now primary in mine with the lid vents sealed) with the same raku burner I use to warm up the anagama.  Over the course of three hours, I brought the burner to full power and added in a weed burner also gradually brought to full power.  At 11:30 am, I started throwing in charcoal while the gas burners ran, and in about an hour, emptied a 20 pound bag of charcoal into the firebox.  This gave me a nice coal bed, at which point I began stoking small sticks and adjusting the gas to keep my temperature rise steady with same method I use for the anagama as demonstrated here:

Once I hit about 20 mV in the front and 17 mV in the back (I could see the edges of bricks inside the kiln glow in the mid-teens), I attacked, stoking aggressively with wood chopped thinly enough to stoke through the front air inlets.  I would periodically hold my breath, and drop larger logs on the hobs through the top lid, letting those burn till they were coals and would crumble on the next stoke.  Two to three logs seemed like a good amount to stoke.  Prior to stoking the top, I’d crush down the coals on the floor into small pieces and push them into the kiln, hoping to get some koge effects on the front pieces.  After stoking the top, the temperature would rise, and when it began falling, I’d go back to stoking the front inlets.

This process worked very well, and with a little side stoking, the front and back temperatures were pretty even.  Eventually, cone 9 laid down and when cone 10 was bending, I called the firing.  This occurred around 50 – 52 mV (new thermocouples — they will get less sensitive as they get used more).

NOTE: pyrometers are voltmeters and pyrometers that will output to a computer via a serial port are expensive while voltmeters that will do the same are cheap. I am mostly interested in whether the temperature is rising or falling rather than a temperature readout, so I use voltmeters to monitor temperature. This is why all my references to temperature are in millivolts rather than degrees.

I capped the chimney (I made a slot in the chimney for a damper but have not cut a shelf to fit that yet, so I fired damperless like Furutani suggests for anagama kilns) sealed up the kiln with a good amount of coals on the floor and wood still in the burning phase on the hobs.  My thought was that the kiln would cool quickly and closing the kiln with remaining wood would slow that down.  My worry was that ashes would blow through and make dry gray sandpapery pottery.

The firing took about 11 hours.  I burned about a third of a cord of wood (mostly firewood, but a good armload of thin millend sticks I have) and used approximately five or six gallons of propane (I use two 40 pound tanks on the raku burner and one 30 on the weed burner — they were still very heavy after I was done).  I used a couple pounds each of grog and fireclay, and 1/4 package of coconut husk as combustible material in the wadding.  I could stoke the kiln with regular leather/canvas work gloves that cost $2/pair and ruined only one pair.  I doubt I spent more than $85 firing the kiln.

Results

I was fairly happy by the results.  The pieces I put in the kiln were a random assortment of rejects that have been sitting about the studio for a while made of various stonewares, porcelains, and recycle clays.  I received a bit of haikaburi glaze on the front guard piece that received the brunt of the coals I’d push into the kiln. I’m quite pleased with the warm blush on the stonewares but the porcelain is a bit too pasty-dumpling-white.  I didn’t quite get high enough in the back, or wasn’t high enough long enough there.  Feldspar granules in the recycle clay peaked out, but did not melt.  Also, I wouldn’t mind a bit more wood ash on the fronts of the pieces.  All that said, for a first firing in a new kiln, I feel it was a complete success.  For my next firing, I will sidestoke a bit more aggressively and add 50% to the amount of time spent firing with wood (measured from when the charcoal first hits firebox).  I think that will ensure I fire the back of the kiln to temperature, will deposit a little more ash, and might be enough time to let the feldspar melt.  I might also let the kiln get a little hotter — my silica sand didn’t melt at all and I’ve been having fun sprinkling that on pieces recently.

Here are some pieces fresh from the kiln. I’ll devote a post to pieces later.

cones 9, 10 & 11 cones 9, 10 & 11 (larger image)

fire blushed bowl fire blushed bowl (larger image)

thin liner glaze thin liner glaze (larger image)

light ash glaze on front light ash glaze on front (larger image)

coals make haikaburi coals make haikaburi (larger image)

back underfired back underfired (larger image)

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