Spend a good bit of time sorting and splitting wood for the next couple of firings today. Before the kiln was built, Craig Edwards mentioned on a couple of occasions that having the wood split and sorted makes for a much easier time firing. I understood the need for splitting, but sorting kind of stumped me. I couldn’t understand why sorting for length would help, thinking that one could estimate the volume of wood going in each stoke, more shorter pieces or less longer pieces, right?
This seems to work in theory, but when you start adding in factors like stoking under the grates vs over the grates, or doing the wooden door technique, or side stoking through a smaller hole that tilts down, then the size and length of the faggots gets pretty important pretty fast. Having to sort through a stack for the right size during the firing makes for a stressful time. Having the right size stacked in the right place so that it doesn’t need to be moved a lot really cuts down on the labor.
So, this time I culled out all of the longest wood and split it first, stacking it near the front of the kiln. Shorter fat pieces near the front as well, with thinner pieces toward the middle and rear for side stoking the front and rear chambers. Also a special section of extra long pieces for doing a wooden door type stoke to get over stalls, if they happen. Here are some pictures of the stacks, plus a little potter porn.
Well, usually at least. This last couple of days was an exception. I started dabbling in some nifty new technology (to me). This involved casting some new tools, so when the old ones wear out I’ll have some ready to go. This tool is called a Gyubera, or cow’s tongue rib, that is sort of indispensable when making a lot of Karatsu forms.
The problem with them is that it is awfully hard to find them in the right shape for what you need. There are a lot of cheap ones on the market which are, well, cheap. The one thing they have going for them is that they have a lot of extra wood on them so you can do a lot of fine tuning with a rasp and sandpaper. The good ones are good, but cost a small fortune. I paid about $80 for my large one that I use for teabowls. Smaller one was about $50. Am I the only one that thinks that is ridiculous? Well, they were handmade one by one, by a barber. As strange as that sounds, there was this one barber that made really nice gyubera. He is now retired, and though someone took over his business, the new guy needs practice.
However, the big problem with all the aforementioned gyubera is that they are made of wood and prone to cracking and rot. It’s kind of a ‘heads I win, tails you lose’ situation. If you keep them damp, they rot. If you let them dry out, get wet, dry, wet, repeatedly, they crack. I found a partial solution to this by soaking them in Minwax wood hardener, and applying a layer of epoxy. But the epoxy bubbles up with time and they wood hardener wasn’t enough to prevent cracking over the long run, although it did extend the life of the tool considerably.
Not wanting to have to buy more, because of the cost and the fact that the new guy still needs practice. I thought I’d just make some myself, but it didn’t solve the wood material problems. So, these last couple of days I spent making plaster molds of the original gyubera, and using dental grade acrylic resin to cast new ones. I got the resin from a friend who used to be in the business. It is a funny pink color because it is supposed to look like gums. Aesthetically speaking, not my favorite for clay tools, but on the bright side it is unlikely I’ll ever get my tools confused with someone else’s tools.
After a couple of tries, I got a pretty good working two piece mold of two gyubera side by side. Got it ready for the resin pour and got the resin poured without too much fuss. Once it was poured and set for about 20 minutes, it was ready to heat cure in a 50C hot water bath for 25 minutes. Strangely, all of this went fairly smoothly. The problem came with trying to get that mold apart later. I tried and tried but it was not happening, and I ended up breaking the mold. Still, the casts were covered in plaster, because the liquid of the resin had penetrated through the release agent into the plaster. Oops, guess I should have used the special acrylic resin release agent instead of the casting slip release agent. Live and learn.
The cleaned up casts will make usable tools, but they are pocked with bubbles and other spaces where the resin didn’t flow properly. I’ll have to try again, improve the craftsmanship.
Here are some of the first pots to get cleaned up after the firing last week. I’ll post more over the next few days as I get things cleaned up.
Overall the firing went well, much better than the first two certainly. I’m finally starting to get a grip on how the kiln climbs, and learning not to worry about it too much. We fired both chambers this time and it took roughly 30 hours. Cone 10 flat in front, cone 9 touching in the rear, and cone 13 almost touching in the second chamber. The second chamber is a pleasure to fire, very relaxing after stoking the front. I think I can still get more aggressive with the front, taking the temp up even more, probably somewhere around cone 12 would be good for the rice straw ash glazes.
The intention was to drop cone 6 in the rear and call it quits, but it ended up getting hotter than expected, and the ware in front ended up a little too shiny. However, the middle and rear of the setting came out just about perfectly. Temp from top to bottom was quite even. We stopped stoking the second chamber when cone 11 dropped and clammed everything up after letting some of the excess heat out. Still, when I peeked in the next morning, that second chamber was still glowing, and cone 13 was mostly down. Guess that 1250kg of insulating castable does its job well.
Most of the failures this time around were caused by bad glazing, not the firing. Rice straw ash glazes were universally too thick, and an ash glaze I whipped up hoping to be a nice runny green ended up being a crusty reddish brown. Oops.
On and off I’ve had requests for pictures of the kiln, so here are some selected photos of it from beginning to completion. Building this kiln was the subject of the first Workshop In Taku, in 2010. The second in the series, Workshop in Taku 2012: The Simple Teabowl, will happen from May 12 – 18, 2012. Full details here:
The series starts not with ‘the kiln’, but ‘the hole’.
Kiln design and expert workshop guidance by Craig Edwards of Minnesota.
Although I dig a lot of my own clay, every now and then I find a bagged clay that I like, and use it for certain projects. Since getting the big wood kiln, I’ve done more of this, because I can blow a lot of clay in one firing, and wasting collected clays is a real waste of time and effort. Once I’ve nailed down how to fire the woodie, I’ll go back to my collected clays since my loss rate will be much less (or at least here’s hoping).
Shoko Todo in Ureshino has a couple of clays I like, one of them is called Karatsu Kishidake because the base clay in the formula is from the Kishidake area. It also contains a lot of very fine sand, which is something I really like. Still, I like to add things to bagged clay to improve it, and this is no exception. If a lot of fine sand is good, then a lot lot must be better, right? Perhaps not.
I started by adding about 15% of my own sand to the clay, and to my surprise it improved the workability, giving the clay more backbone. It also gave the trimmed surfaces more character.
For the next hump, I added about 25% extra sand. I should have known I was in trouble when I couldn’t even wedge the stuff without it splitting apart and having chunks fall out, sticking to the table more than the clay lump. When I started throwing the hump on the wheel, pots would split vertically as I pulled the walls, and I couldn’t get near as thin as with the 15% sand/clay mix. Most of the bowls I managed to finish still had rips in them that I had to go back and fix later. When trimming, this clay gave a very rough texture, and I really liked it, but not enough to go through the frustration of throwing the stuff again. Although, for small things like guinomi this clay is the bomb.
Below in the pictures are two trimmed feet for comparison. One is the 15% mix and the other 25%.
With the wood kiln and trying to fill it, there is not nearly enough shelf space (or ware boards) in the studio. My neighbor has a small grove of bamboo up the hill, and gave me permission to cut down a few for building a ware board frame. Here in the pictures it is mostly finished, but still needs about 2 more tiers added higher up.
I moved most of the new pots out here, and it freed up all kinds of space in the studio, but there is still a shortage of ware boards. Cheap plywood is still about $10 per sheet, but it looks like I’ll have to bite the bullet and buy some soon.
The oval dishes are the next run of food dishes. They look really good with Chosen Karatsu glazing, and also with iron brush deco under a feldspar glaze. The clay is a new one I’m experimenting with, bought from a clay specialist, but after trimming these I don’t think I’ll use this again, at least unaltered. Wedging in some sand might perk things up a bit, but the basic clay body is pretty boring by itself. We’ll see, it might have really great color in the wood kiln and that makes up for a lot.
I had a lump of left over sandy clay that I wedged in near the end of the run. From the looks of some of the trimmed feet, I could have done a better job…