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 with propane (larger image)

firebox and kiln body (larger image)

glowing kiln (larger image)

firemouth (larger image)

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 (larger image)

fire blushed bowl (larger image)

thin liner glaze (larger image)

light ash glaze on front (larger image)

coals make haikaburi (larger image)

back underfired (larger image)

Last Friday afternoon, I stopped by my friend’s office and picked up a chunk of his property — neatly contained in a ziplock bag. Then on Saturday, I pounded up the block by wrapping it in a heavy plastic bag and sieving out the powder.

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Once I had a bowl of dust, I took it into the studio and made four test clays (all measures volumetric, not weight based):

1. 100% test clay. Back right in photo below.
2. One part test clay, one part Helmar kaolin. Back left in photo below.
3. One part test clay, two parts Helmar kaolin . Front right in photo below.
4. One part test, one part Helmar, 2/3 part silica, 2/3 part Custar feldspar. Front left in photo below.

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I poured each test out like a pancake on the wedging table with Silver watching and perhaps wondering if they were tasty cakes for cats. After a few hours, I was able to wedge the clay into four little balls. From these, I quickly whipped up eight pinch pots and marked the bottom of each piece with the number corresponding to the test clay. Each of the clays was pretty short and edges tended to break and separate.

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Sunday came like Christmas morning — a burning day — and I returned to the kiln great anticipation. Unfortunately, Saturday night was comparatively cool and the pieces had not dried — they were at the perfect leather hard trimming stage. I decided to fire them anyway. Besides, what better to fire wet pottery than a wet kiln?

I haven’t used the raku kiln in a long time and so when I pulled the tarp off the base bricks, they had all turned green with mold. Additionally, an entire colony of snails was living quite cozily in the moist damp undersides of the tarp. As an aside, I wonder if this has been the source of the large snail population this year. I find it disturbing to walk around in the grassy area behind the studio — it seems that every fourth or fifth step results in the sickening pop-crunch of a snail being smushed.

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After rescuing as many snails as I could find by putting them out in the tall grass areas in which I don’t walk, I set about loading the raku kiln. Because the pieces were substantially wet, I placed the shelf high above the burner port to reduce the risk of explosion. Then I candled the kiln at the lowest reliable flame, gradually increasing the temperature until the pieces appeared dry when I peered into the kiln. Once I got to that point, the fun began. I removed the regulator from the propane hose so I could a big jet of flame, and gradually began raising the temperature. Once I hit red heat, I let it roar. When that burner is tapping propane straight from the tank, it sounds like a jet engine.

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I knew I couldn’t get to cone 10 with the raku kiln, but during the firing I began to wonder how high I could go. I grabbed a spare unused pyrometer I had laying around and inserted the probe between the brick base and the fiber kiln body. Toward the end of the firing, I stoked a medium sized log into the kiln — chopped up into kindling of course — stoking two small sticks at a time. Eventually, I was able to get to 2200 F and held it there for a short time (given enough time, this would equate to about cone 5 when firing quickly). It was a hot day and the wood chopping, such as it was, made it seem hotter. I figured that was good enough for a test and called the firing.

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I was intending to open the kiln raku style right then, but I suddenly realized I was wearing shorts and had brought no pants, so I just closed up the kiln and decided to wait till it reached a temperature that wouldn’t blister my legs. Eventually, the temperature dropped and I was met with an amusing sight — a lovely puddle of glaze where the “pure” dug-up clay cup had been:

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Results:

#1: 100% dug-up clay. I love this glaze with the subtle variations of black and brown. A hot mud spring frozen while burbling — or a really flat cup (below):

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#2: 1:1 | dug-up:Helmar. This piece is slightly self-glazed (below):

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#3: 1:2 | dug-up:Helmar. This piece is quite dry (below):

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#4: 1:1:2/3:2/3 | dug-up:Helmar:silica:custar feldspar (below):

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#4 is rather interesting. It is self-glazing and incredibly porous — I know this because I broke it a little prying it off the shelf and the clay body is full of pinhead size holes (you can see the scar on the rim in the picture above). Perhaps it would make interesting insulating mugs. The porosity makes it feel extremely light though — so light that it feels “wrong” somehow.

Today, Rachel (green shirt), Tony (red shirt), Alan (picking piece), stopped by to fire off the raku kiln. Every since building the kiln, it has been living at Alan’s … but certain changes necessitated its move here to the anagama site. Today was its first firing here (discounting the one that didn’t work for lack of a wrench).

Alan’s new and unusual color:

I lost interest in raku quite some time ago — the results aren’t really what I’m after. I don’t mind white crackle so much but I became disenchanted with bright penny coppers and turquoise blues. Those results are pretty in their way — they merely aren’t what I want. However, I do like the actual orange and red colors in Alan’s new glaze.

Today I recovered some of the raku spark — could I “once fire” a piece? What the heck — no harm in trying. I went into my studio and grabbed something that hadn’t made it into the last anagama firing. It was an unbisqued (I don’t bisque) small slab bowl. I applied some white crackle glaze with a brush to the inside of the bone dry piece, and then set it on the kiln during the others’ firings in the hope it would dry sufficiently. It spent about two hours drying although I had to remove it between each kiln opening.

When Tony, Alan, and Rachel were done, I put mine in the kiln. I decided to raise the height of the shelf so the piece would be more in the middle of the kiln away from the burner. I hoped that would provide an environment a little gentler than it would receive on a low shelf. We set the kiln shell in place and I covered the outlet hole with piece of metal to keep rain off the piece. I decided I’d let the pottery preheat by sitting in the warm kiln with the burner off for 10 minutes. The firing would end the moment we heard the piece break.

The piece didn’t break sitting in the warm kiln, so I turned on the heat a tiny bit and waited for that dull pop of exploding pottery. It never came, so I bumped the heat a tiny bit again. And again. And again. When the paint on the piece of metal started to burn off, I cranked it to a low roar. Still no breaking noise, and the paint crackled into fascinating patterns:

I bumped it up to full throttle. I could see the glaze melting and the piece glowing. After about 70 or 80 minutes, I stopped the burner, Alan and Tony lifted the kiln, and just like any other piece of raku, my once fired bowl went straight into a post firing reduction chamber. I’m in the browncoat (same one I wore to the opening night of Serenity in true geek fashion — I much prefer Firefly though).

In my impatience, I pulled it out too soon to douse it in water — it was at that point the foot fell off. No matter — I’m confident I can do once-fire-raku again. I won’t ever buy a bisque kiln.

Now, I also had a hole from getting dirt for the top of the kiln. Given a hole, a free paper, and a spike-pod piece that had been sitting around for a couple years because the tip of the spike got broken somehow, I decided Monday was the perfect day and this was the perfect project. It would give me something to open today and thus help me get through two days of the cool down period (ideally, at least ten days).

Let’s start with the hole:

I threw in some damp wood. This turned out to be a mistake because I couldn’t get it to light. What I needed was some thin kindling underneath the bricks that make up the floor — there is an air inlet underneath the top layer. I should have filled that with easily burned wood. As it is, I had to resort to the raku burner to get the thing going.

Then I filled it in with some millends. The millends are fairly damp too — they aren’t kiln dried, but being cedar, once they get going they burn like the devil:

Then I piled up more wood over top. They say that early kilns were simple pits with shards placed over top to help hold in heat. I don’t have any appropriate shards (I have lots of shards, but when I smash up stuff, I smash it into little bits), but I hoped the paper/slip wrap would function similarly. As it turns out, one Sunday paper isn’t quite enough to make a nice thick shell — it was only a double layer at the bottom and when things got going, it ripped:

Mixed up some slip (twice, needed 3x this amount):

Grabbed the paper:

And pasted it up, threw dirt on for good measure. Dirt is the cheapest most abundant refractory around. Great stuff:

I threw some hot coals down there and started waiting. Although it looks like they caught, when I’d pull them out the smoke would go away, so the smoke was from the wood I threw into the air inlet, not from the kiln wood:

To make a long and rain soaked story short, I eventually got it lit. But then it smoked like crazy — thick green smoke — the kind that looks like a liquid. The smoke was was bothering me a lot and although I knew speeding things along would damage the unbisqed pot, I was mostly interested in seeing some fire so I forged ahead anyway. I fanned the flames and indeed, heard some ominous pops from the kiln. At last it really took off and boy, what a sight:

Notice the cat in the middle to lower left. The kiln’s cats LOVE fire! Click the pic for large version:

Today, I retreived the pot. When I got to the pit kiln, I saw shimmering heat from the sides of the dirt walls. That surprised me … till I realized it was a shimmer shadow from the anagama chimney. I’ve never been much for pit fire and this piece hasn’t convinced me there’s anything there for me to do. Here’s the rather bland piece:

But then I turned it over. There’s glaze on the foot! Honest to goodness glass! I cursed a bit in a surprised and happy fashion. As arrogant as it sounds, I thought to myself, “if I can fire a hole in the ground to a glass finish, I can fire anything.”

Clicking on the following pictures opens a large view:

You can see the damage from pushing the fire, but so what? Glaze!

Here’s the worst hole:

The best was yet to come — something even more interesting than the pottery itself. When I dove into the pit to retreive my bricks before filling in the hole, I discovered the dirt had melted — really melted! I was simply amazed — it’s very brittle, but very cool. I’m thinking about incorporating dirt with firebox pieces in the anagama itself:

So there it is. I can melt dirt. What a great day and better yet, one more down in which I did nothing with the anagama. I have to work the next three days so that will help too.