These last 2 weeks I have been getting over my fear and loathing of slabs, because I have finally figured out how to keep them from cracking during drying. So, lots of slab plates for starters, then a customer came in who wanted even more in different sizes for his sushi restaurant. This is good high end restaurant, and there is a very high chance of repeat business.
Anyway, I had been using someone else’s risers for my plates, but since I needed my own anyway, I spent a few hours today mixing up some fireclay and press molded my own risers. Below are pictures of the very simple process. These risers will be used during drying and also during firing to support corners and edges. The fire clay is very rough and easy to grind off if the glaze runs and sticks to it.
Here are some of the plates that have been made so far:
This post is a photo record with comments, in order to organize and retain my thoughts and observations about the 8th firing of the wood kiln. I always think I’ll remember until the next firing, but never do. So, if you get into this, beware there are a LOT of pictures, some of which may seem redundant. There are examples of most of the pots from the firing, minus teabowls and some others which are not photographed yet.
The 8th firing was a charm overall, with many good pots, some bad, and some that will get refired. Kiln was fired in oxidation primarily. We stoked once every 30 minutes for 2 1/2 days, then once every 20 minutes the last half day.
– Front was nicely melted, nice even gradient to the rear of first chamber, E-Karatsu was about ideal. Cone 12 at hottest front down to cone 6 at coolest rear.
– Front chamber behaves like anagama. No significant temp gradient from front to back of each setting. Next firing, will pack kiln by eliminating the space between settings, leaving horizontal space for flame travel. One long setting from front to back of first chamber.
– Ame yu better at low temps. Load rear to mid chamber.
– Ao yu needs more heat, mid to front chamber.
– Large platters go midway to back of front chamber. Too much ash up front.
– Firebox wares are very nice in a three day firing. Great place for Shino, under cover to avoid ash in pots.
– Rear chamber fires fast when preheated for 3 days. Jumped from 1044C to 1344C in four stokes, a little over an hour. Care must be taken to spend more time soaking at high temp, because rear chamber high temp wares not quite mature (surface changes only), clay not melted well. Spend 3 or 4 hours firing off rear chamber, slowly, to get inside the pots.
– Rear chamber fires very evenly front to back, and top to bottom. Pick a temp to fire at and load accordingly. Don’t expect a significant gradient.
Well, time to sit down with the last firing and figure out what went wrong, and what went right.
Firing and packing: Great. With this firing I figured out, FINALLY, the proper exit flue volume to use. And with it, the proper packing procedure. The kiln climbed steadily, without effort, for 45 hours and the flame in the front chamber reached front to back really nicely. It turned out to be a fairly even gradient from front to back, from cone 13 in front, to a good melted 8 in back. No sidestoking necessary.
I am going to go back to using a pyrometer for the next firing, because this firing got very reduced. I think the stoke intervals were too close, and a pyrometer may help me get a better idea of temp climb timing, and when each cycle goes into oxidation.
At one point, when we switched to stoking the 2nd chamber, the temp just started dropping. You could tell from the sound and color of the kiln. I jabbed a pyro into a peep hole and it told me right away that I was stoking to frequently. After the flame had receded into the chimney and peepholes, and the roar had died away was when the temp really started climbing in the chamber. I had been stoking when the flames receded and the kiln quieted down, but once I started waiting rather for the temp to start dropping after the rise, the kiln got back on track. This rise took longer than I imagined, sometimes up to 10 minutes. Had been stoking waaayyyyyy too much.
This firing was a watershed moment for me, regarding packing, air, and stoking. I think next time will finally be the load that makes us all smile.
Glazes: Glazed ware in the second chamber turned out really nice. Reduction was still too heavy, but we got nice surfaces. Still, I won’t aim for that in the future. Madara glazes were too yellow for my taste. The first chamber glazes were so reduced that whatever their intended color, everything turned out a semi matte green, from the excessive iron pulled from the body saturating the glaze and crystallizing. I think the stoking interval will fix this problem.
Clay body: Oops. Here is where the shit hit the fan. Too much iron, especially considering the temp and amount of reduction that happened. The clay I used for the large pieces has too much iron for that type of firing environment, which is why they almost all collapsed. Having worked with a gas kiln for so long, it is hard to get out of the mindset of adding iron rich clay to other whiter bodies in order to get a desired color. The white clays in this area have enough iron for good color in a wood kiln. I keep forgetting that, but perhaps the shock of this latest meltdown, and the subsequent pain of shelf grinding will serve to jog my memory whenever I’m mixing clay for the wood kiln.
Well, didn’t quite make the Feb. 29 date for firing the kiln. My wood ash supply ran out and it was hard locating more and getting it tested. Also had the top element go out on my electric kiln, so couldn’t use it for bisquing. Finally got that stuff done, so was able to mix up glazes and get to glazing/loading. Got the rear setting done this evening. My shipment of cones from Axner came in the nick of time, so here are the new self supporting cones in action. I have decided that I LOVE self supporting cones…
I mentioned that I’d run out of glaze ingredients. Some testing was required to ‘find’ my glazes again. This glaze is tough because it goes clear if too thin, or is too white and runny if too thick. Also, I’ve been trying to tweak my Madara to more closely resemble that of the old Karatsu pots, circa 1580. Theirs went on thin, but kept its color. A lot of silica, probably. Here are two test tiles with Madara glazes. They were fired in the tiny test kiln and you can see they went pretty clear, because they were directly in the flame path. The tile on the left is the tweaked version, with more silica/rice straw ash and less mixed wood ash. I think it is pretty close to what I want, and will have more color when not fired in the tiny kiln with that concentrated flame, and gets a slow cool.
This upcoming firing we will have our first attempt at hikidashi, which means pulling pots at high temp and letting them cool quickly, or quenching them in water or something flammable such as rice hulls, sawdust, etc… This time, all of the hikidashi will be hikidashi guro, or black glazed ware pulled at temp. In the picture of the rear setting above, all of those pots with the red glaze are going to be yanked out. From the side port, a pair of tongs about 1 meter long is just about long enough to get most of the ware. For pulling from the front stoke, tongs are just too short, so I spent some time this afternoon making a new tool. By drilling a hole into the side of the test kiln, I converted it into a simple forge. I took round steel stock and repeatedly heated it and hammered it to flatten the end then used a hammer and chisel to split it lengthwise from the end. A little creative hammering gave me a two pronged fork shape for scooping up bowls and pulling them out the front of the kiln. Can’t wait to give it a try.