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Firing Log

ancient kiln | 21st century logbook

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February 3, 2010

Advice + Revisions = Success with Short Throat Kiln

Filed under: Firing,Kiln,Pieces,short throat kiln — odin @ 10:49 pm

I received advice from Steve Harrison (link 2, link 3) and Claudia MacPhee, see below for their comments, regarding the short throat kiln I built. Specifically, they gave me some suggestions regarding the smokey lid and honestly, after reading the suggestions I was sort of chagrined at how silly I had been. To sum it up, the large front air inlets I like rob power from the lid’s inlets and thus allow smoke to escape from the lid. Kind of like a hole in a straw makes it hard to drink a milkshake.

Secondly, my chimney was also a bit short, so I gritted my teeth and bought another taller ladder, and enough bricks to add ten rows to the chimney. The expensive part was the ladder — $90. I also dropped my grate one brick level so that the firewood rests closer to the coals.

The upshot is that the kiln fires to cone 10 without having to resort to careful stoking of kindling at the end and it doesn’t smoke while being used in boury box mode. In order to melt cone 11 flat, I still have to stoke small sticks by watching the chimney till the flame disappears, then stoke a stick, watch the chimney flame till it disappears, stoke a stick, etc. etc. I don’t need to go that high, but I like to know I can. I think the extra high temperatures sometimes cause interesting cracking patterns on the surfaces of the pieces.

I still like want this kiln to perform somewhat like an anagama — just smaller with a shorter firing period. So periodically I open up the front inlets, brave the smoke, and beat and stir the coals vigorously. It seems to be working. For example, the piece below was glazed only on the inside. Despite the fact that the outer surface was completely raw, the fire exposed side developed almost the same glaze color as the white liner glaze except with the added bonus of areas of ash based olive green (bottom right photo shows the front side). I think with a little practice, I may be able to avoid glazing completely and just rely on the fire. Click thumbnails for the big picture.

Steve Harrison‘s (link 2, link 3) Suggestions:

I think that the problem with the smoke coming out the lid is mostly to do with the short chimney, as you identified, but I also think that it is to do with the big air inlet/stoke holes at the bottom. Once I have enough ember to get the wood on the hobs to ignite, I close up the bottom hole completely and only have a smidgin of air going in under the embers through the mouse hole. This concentrates the draught in through the top holes and draws the smoke and flame down into the throat arch and chamber.

Claudia MacPhee‘s Suggestions:

Thinking about how my little kiln fires makes me really wish I could go out and run a load through it now! Alas, too cold….Just let me know what parts you want to see and I will try and get some photos up on my blog.

I made the bourry box exactly like the one in the diagram in the book. Counted the brick and didn’t use any ‘innovation’. I put in the brick hobs and cut my wood exactly 26″ long. A bit of a pain, but works well.

Usually I start a small campfire in the bottom of the box about 9 pm. Two big pieces with a bunch of paper and small stuff between them. I feed it from the bottom until it is going good with a few coals, then pile on more whole logs through the top. Don’t get too excited about a huge fire at this point. Mostly just keep it going until there are lots of coals then heap if full of wood, close the damper and put the three bricks in the front.Primary and secondary air vent are closed all this time. It is usually around midnight and I go into the house for a nap. Set my alarm for 4 am., get up and make a pot of tea. Fill up my thermos and get a cup full and go to check the kiln. There will be some coals left and I unbrick, pull out the damper and stoke it up, again on the bottom. Just keep filling the box with all the junk wood I have collected from around the yard and odds and ends from bucking the slabs exact. About every ten minutes I throw more stuff in. At this point I use aspen (tons of ash), pine/white spruce and hunks of willow. Willow is the hardest wood I can get. It grows very slowly up here with lots of cold and little rainfall so is tight and heavy. The willow I use has been dead for many years and is bone dry. It makes a really good, long lasting coal bed.

After about fours hours there are enough coals to go to the hobs. This is one of the coolest parts of the firing. I will take an armload of slabs, three to four inches thick, open the top lid and quickly place them on the hobs-it is like magic! They burst into flames and are pulled down into the kiln! After this point I keep the hobs full. The point here is to never let a bunch of cold air flow between the pieces of wood. So as it burns up put more pieces on the opening holes. Hard to describe. I try to lay the slabs in with the cut side up. There are always at least two layers. Also start using the primary air. The bottom three bricks are put in and clammed up.

I take a clock and a book out for the firing. Put new wood on every 10 to 15 minutes. Fill her up, go off and feed the chickens, fill her up, go inside to get some food.

My first firing I grossly under estimated how much wood I’d need. By 6 pm I was using my time between loads to buck up more slabs. Had enough time to do an entire wheel barrow full between stokes. That never happened again! Also I have never used the pyrometer to fire. Every body gets so neurotic with those things. I try to take the “Laid back” part seriously. I live in a beautiful and quiet place. I sit there and listen to the birds, read my book, hear a boat out on the lake or a plane going by.

After I go to the hobs I’ll begin the side stoking. Now this is the pyro dream, a technique from the Aussies known as ‘flamethrower’. You pull the brick, place a handful of small, long wood in and just hang on to it for a bit. It blows up and the flame goes in huge circles.

So the rhythm is-fill hobs, pick up sticks, pull brick, put in wood, hang in there for a couple minutes, drop them in, replace brick, sit down, drink tea, read book or stare into space for ten minutes. I have mouse holes but have never used them. Also have never needed secondary air. Usually keep the primaries about half open, just enough air to keep it from ‘chugging’. My first firing I had cone 11 (highest one I had) melted. Got some awesome hare’s fur on some bowls near the front. Didn’t side stoke that one so the pots near the back when in for round two next time.

I am dedicating my life to melting as much of the Coast Range as possible…lots of goodies to melt in this part of the world, plus a nifty bunch of volcanic ash, local copper and glacial silt that makes wonderful glazes. Chinese stoneware glazes, especially the iron ones really do it for me, I love them lots. I put in lots of glazed pots, but want flashing on them rather than a ton of ash. By this time I know where to put pots for what I want the kiln to do to them. Believe me, you can get all the ash you want at the front end. This is getting much too long. If there are any more questions, just ask. I have found this a really fun kiln to fire, and can do it by myself. I have children your age, so if an old lady can do it, how hard can it be??

Good luck!

Claudia

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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