Firing Log

One of the ultimate joys of pottery is burning things. So when a friend of mine gave me a lump of clay that he dug up while in the process of building a foundation, I grew quite excited. Finally, I had an excuse to drag out the raku kiln, flare off some propane, and singe my eyebrows a bit.

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.

I built a raku kiln 4 or 5 years ago. Alan (you’ll see him below) taught me how to weld and donated some expanded steel. I zipped it up with a MIG welder — the welds are certainly amateurish and a pro would laugh, but it’s held up. Welding is a heck of a lot of fun — I wish I could come up with a good excuse to weld often but alas, that raku kiln has been my one and only time. Here’s the basic setup:

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.