Category Archives: pottery

Building a Test Kiln

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!

Made a dent in the wood pile today…

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 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.

Madara Karatsu Chawan. Name: Hakuo

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.

inside bottom of a tea bowl

You can see the mottling , including the blue spots, probably from ash that settled and melted in the bowl.

detail of clay/glaze interaction

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.

foot of tea bowl

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.

detail of foot

25 Pound Pots

Just this last week I got my hands on some clay which is ideal for very large coil and paddle work. It had been sitting in the corner, calling out to me for a few days before I figured out what to make with it. Perfect practice shapes, new planters for the root-bound palm trees in our living room. They are waaayy overdue for a re-potting and looking a little peaked. Once they have new homes they should be really genki again.

Each of these pots is one 10kg pug of clay. Wet, they are about 47cm tall by 47cm wide. I made an important discovery this time: my electric Shimpo Whisper makes a good wheel for coil and paddle of big diameter work, because when it is disengaged, the wheel spins freely like a banding wheel. So, I can use it for paddling, then engage it for using ribs on the shape, or forming the lip. You can see the kickwheel in the photo, the flywheel is at a bad height for my body and difficult to kick. My other kickwheel is in the corner and there is not enough room around it for large work. So, the electric wheel solution was a stroke of luck.

These pots were constructed from one base slab and 3 wall slabs, stacked and paddled. These are great shapes for practicing the slab and paddle method, because they don’t require any technically difficult shaping, ie. forming a tapered shoulder or neck, and they don’t get much wider than the base. Once I get comfortable with these, I’ll go back to storage jars with more belly and more taper at the neck. Having a few of these around will be really convenient for storing glazes too.

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.