Tag Archives: karatsupots

Gyubera, attempt #2

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.

Hey baby, I’m your Homo Habilis…

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.

The First Pots: Chawan and Guinomi

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.

First Chamber Loaded

Finally! After seemingly endless days of loading, the front is finished. The large pieces in front are most refires with the exception of the large jar to the right. The big white piece in the middle isn’t a pot, it is a large piece of  Shirakawa Toseki (porcelain stone from the Shirakawa area near Arita dam). The square right behind it is a piece of sandstone that I cut in two and hollowed out to make a box.

The Kiln

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:

http://karatsupots.com/workshop2012/2012home.html

 

The series starts not with ‘the kiln’, but  ‘the hole’.

Kiln design and expert workshop guidance by Craig Edwards of Minnesota.

Continuing Loading… 窯詰めの続き

Finished the middle setting today. It is mostly saggared ware. I picked up these nifty saggars that are big enough for larger pieces like flower vases and mizusashi. 今日は真ん中のセッティングを完成した。殆んどはボシに入った水指と花入れです。

There is a reason for doing it like this. I’m hoping that the saggars will effectively split the front chamber into two chambers, allowing the front to achieve temp more easily. ボシを使う理由は二つ:独特な効果を得るためと共に壁になって欲しい。大きな火立てになり、部屋が二つに分けられ、手前の部屋は温度が上がりやすくなると想定している。

Just behind the saggars are teapots along the floor, they will be covered in partially to fully covered in embers from the wood stoked directly behind them. The 4 square frame shaped kiln posts are there to prevent the stoked wood from bouncing/rolling down onto the pots. ボシの直ぐ裏には搾り出し急須が置かれている。ここですぐ裏の焚口からの薪が熾きになり、その熾きが急須の周りと上に溜まり、炭化効果が得られると思う。薪が作品の上に転ばないよう、支柱を急須と焚口下の間に置いてある。

Two more things: got a new ember rustling tool which is a shovel head welded onto a pole. this should come in handy for tossing embers and ash around.