Karatsu potter Okamoto Sakurei just ended a show in Fukuoka this weekend at Gallery Ichibankan. For those of you who haven’t heard of him, Okamoto san is a very talented artist making Karatsu style wares. He is well known throughout the country and one of the top Karatsu ceramic artists today. He will also be doing a demonstration/lecture for the Workshop in Taku 2012: The Simple Teabowl.
During my visit we had a chance to discuss his upcoming presentation, as well as some wood firing diagnostics. Here are some of the pictures from the show. The gallery used a lot of natural light from its windows, which made for a very nice display, however it was not so camera friendly.
Since switching over to the new wood kiln, there has been a need to tweek my gas kiln glazes, as well as develop new glazes for wood firing. Problem is, it is hard to test new glazes because the wood kiln gets fired infrequently, and I fire the gas kiln much less now because most of the work goes into the wood kiln. So I need something that I can fire test tiles in, and after looking at some little electric test kilns, (and their prices) I decided to build a propane gas fueled test kiln.
I had the materials laying around, and scavenged some from previous projects. I came up with this little Itte-Koi (going and coming) style kiln. It is basically just a scaled down version of the first gas kiln I built 7 years ago. This one only required about 20 bricks, and the hardest part was carving the chimney bricks. Thin brick slices hold up the floor/shelf, the flame goes in the bottom front, climbs up the rear and comes back around down to the exit flue at the middle front.
The chimney brick shown strapped to the front was a mistake, I replaced it later with one that wasn’t cut through completely. I had carved this one through because my larger version had a passive damper here, but it seemed like too much trouble on this tiny kiln. Much easier to lay a slice of brick over the top of the chimney.
3 more bricks across the top finish the kiln. I later set the whole thing on a flat stand with wheels and fired it up with a raku burner I had laying around. Waaaaayyyy overpowered. Within 10 minutes I had red heat in the chamber, but there was a tall flame coming from the chimney because of all the unburnt gas igniting as it left the kiln. This with the regulator set about as low as it would go, so I need a smaller burner, probably a small weed burner or reduction burner for an electric kiln would be about right. I’ll post on this kiln again once I get a burner to fire it with and some glaze tests set to go!
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.
Madara Karatsu. Madara refers to the glaze, usually a high silica white which reacts with the clay to produce a mottled effect, with various shades of white, cream, yellow, brown, and subtle optical blues. Most modern potters use rice straw ash as the main ingredient in their madara glaze, but other types of high silica ash are sometimes used as well.
We don’t know exactly what was in these glazes traditionally, there are probably various combination of glaze ingredients and clay bodies that would yield the madara effect, and I suspect that the potters of old just used what was on hand, which would explain why some of the old kilns are known for specific types of pots. Hobashira-gama, one of the Kishidake Ko-garatsu kilns was probably the best known for its Madara Karatsu ware.
Hakuo is one of my favorite tea bowls, and a sort of collection of ideal Madara Karatsu traits. The glaze is not too thin, not too thick, the clay is vitrified nicely, but quite sandy so that it reacts well with the glaze. Below is another piece of a tea bowl from Hobashira-gama. This is a shard that was given to me by a friend.
You can see the mottling , including the blue spots, probably from ash that settled and melted in the bowl.
There is a lot of sand in this clay body. You can see here that the glaze is not that thick. Many modern potters apply the rice straw glaze thicker, or too thick (often the case with me), resulting in a white glaze. Too thin and you end up with clear.
When you touch this shard you can feel that it is well vitrified. It does not absorb water. There is a lot of speculation about what the body really is. Recent examination of shards like these under an electron microscope indicate that the clay body is not really clay at all, but rather powdered stone. When people talk about the technology brought to Japan by Korean potters, they always mention the climbing kiln, and glazes. Perhaps more importantly, they also brought over the technology of processing stone into a usable body for making pots. They had this technology for processing porcelain stone, but did not have a source immediately, and made pots for many years before discovering the porcelain stone deposit at Izumiyama, Arita. Suitable types of feldspathic sandstone probably served as alternative ‘clay’ bodies in the meantime.
Who got the twisted James Taylor reference in the last post? Sorry about that, physical anthropology humor from a previous life.
Anyway, a friend came over yesterday and gave me a crash course in plaster casting. In one of his previous lives, he was a dental prosthetic technician and did casting in various materials for a living. My knowledge of plaster handling and casting jumped about tenfold in 4 hours.
We poured the mold similarly to before, with a couple of differences: he applied whipped cream consistency plaster to the backs of the originals, let it set, then turned them over and planted them in a freshly poured base, so the working side was face up. We cleaned up that surface in running water before it set completely, then applied separating agent and poured a different colored super hard type of plaster over the working surface of the originals. Then we leveled everything off with regular plaster. Mold finished.
Once completely set, I took the mold and soaked it in hot water for an hour to let it absorb as much water as possible, which helps the separating agent to work, and also keeps the plaster from absorbing liquid from the resin mix, which keeps the resin more fluid. Once the separating agent was applied to all surfaces, I put the mold together and tied it with old bicycle tire rubber, stood it up and poured in the resin. Then after letting the full mold sit for about 15 minutes, put it in a 50C bath for 30 minutes to cure the resin. This time I literally used the bath, and set the hot water temp for 55C. Much easier than a pot on the stove.
Once the curing was done, I let the mold cool, popped it open (which was easy with the proper separating agent) and out came two very nice gyubera. After trimming off the excess, and sanding them with a flap wheel and some 180 grit wet paper, they were finished. Victory! There were some small air bubbles but nothing like before, and the mold is in great condition for next time.
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.