A Guide to Glazing Chosen Karatsu

There is a new video up on the Karatsupots youtube channel. On it, I explain the process of glazing Chosen Karatsu ware, from applying the ame glaze, to trimming the ame, and subsequent application of the wara glaze.

Chosen Karatsu 11/17/2020

Today I want to post the results and thoughts about the latest CG firing. This firing was more successful than the CG firing in March, but I’ve uncovered some other problems, namely uneven reduction. My first thoughts on troubleshooting this are:
1. possible leaking kiln door fiber seal?
2. open the shelf placement to promote more air circulation?
3. Check the burners on the over reducing side, and possibly adjust secondary air?
4. look inside the car to see if any airways are obstructed?

Next, a look at the before/afters, with special attention to the thickness and trimming of the rice straw ash glaze:

Next, the keepers:

Thoughts:

-The glaze thickness was about right, and the interior glaze thickness was about perfect.

-The firing was too hot in the top half of the kiln, resulting in most of the loss. Also, over-reduced areas exacerbated this issue.

-Perhaps both glazes are a bit too runny at cone 11 and need adjustment.

-The middle and upper rear right was VERY over reduced, this needs looking into. Multiple possible causes.

– Use shells on the feet even when not in doubt about drips. Much of the loss in this firing could have been avoided if there had been something between the bisquits and the pots.

Gallery

Cool Color

This last firing of the gas kiln I used draw rings to get a better idea of when certain glazes matured. I’ve known this for a while, but was struck again at how colorless and boring the colors of the glaze and clay were when yanked and doused in a bucket of water. So colorless, in fact, that two different glazes appeared to be virtually identical when crash cooled. The sandstone body, also, is an uninteresting greyish white.

left: sandstone/ash 7/3 right: high silica ash / oak ash/ spar 6/4/3

Here are those same glazes after firing for 16 hours and cooling slowly in the kiln for 36 hours. (first two pics are same glaze and body as the ring on the left, above. Second two pics same as the ring on the right.)

Notice a difference? Not only in the quality of the glaze surface, which is distinctly more blue with patches of sugary white, but also with the clay body color development?

The changes in this glaze are even more pronounced. No longer a colorless clear, but a soft creamy white translucent.

Last is a black glaze which was a featureless glossy black on the test ring, but which given time, developed into a rich black/brown with some crystal growth.

So, with that in mind, I wonder how nice these glazes and clay body would look after a 96 hour cooling period in the wood kiln?

On an aside, here is one of five toggle buttons I made and fired in a saggar with rice husks and cockle shells. Same body as the cups above 70/30 sandstone/Izumiyama porcelain. Approx. 6cm

Scoring Goodies

I made a trip out to the mountains in Minamihata yesterday with my mentor to collect materials for pots and glazes, and we stopped by an old kiln site as well.

We were able to collect a good amount of feldspathic sandstone, weathered feldspar, and the ever popular grey stuff (don’t know what its real name is).

First, here is what we collected that will become pots and glazes this year:

Grey stuff. This is softer than rock and can be stamp milled or pot milled easily into a fine slurry. It has a lot of iron, and I use it as a glaze ingredient, as a slip, or as a pigment for decoration.

Sandstone. This has more iron than I’d normally like, but beggars can’t be choosers. It takes the place of both feldspar additions to clay bodies as well as grog. I like it because it helps the body vitrify, but gives it texture as well.

Glaze stone from near Okawachiyama. I’m not sure but I believe this is the glaze stone used in Nabeshima celadon. I look forward to testing this.

Closeup. You can see the feldspathic translucent pieces in the matrix, surrounded by the white powdery matrix. I think the white powdery part is high in silica, but again, need to test.

Found this in the same place as the Nabeshima glaze stone. This looks to be more pure feldspar. Very curious to see how it fires!

On the way to one of the collecting sites, we passed two old Karatsu kilns, Fujinokawachi and Kayanotani. I was surprised because they are separated by no more than 70 meters or so. These were big kilns. Kayanotani was a 22 chamber climbing kiln 52 meters long! Between them, there were probably hundreds of potters working. We couldn’t really access Fujinokawachi, but we were able to walk around on the hill where Kayanotani once stood.

Access to Kayanotani. You can see the faint grassy steps up the hill, just to the left and down a bit from the tree.

Sign board standing at the entrance to the kiln site. The shard pile extends up and down the hill to the left of the sign.

The outside surfaces of some coil and paddle flasks. Fujinokawachi and Kayanotani are both known for their fine coil and paddle work.

Interiors of the same. Note the different clays used and the different patterns created from the paddling tools.

Hillside littered with shards and chunks of kiln wall and kiln furniture.

The feet of some ash glazed bowls. The clay is quite light in color, and really vitrified.

Detail of some flask lips. On the one, you can see some bubbling in the glaze because the clay body started to bloat. The coil and paddle clay bodies were quite varied, but much of it seemed to contain high amounts of organic matter

More bowls. One in a light clay body, the other much darker. Again, vitrified and hard. All of the trimmed pieces I found were trimmed with an economy of movement. There is no wasted time in the trimming here.

Detail of another foot. Gorgeous clay, and lively trimming.

Interior detail of bowl directly above. Note the beautifully folded over and compress lip of the piece stuck to the inside here, and the glaze window.

Paddled Chosen Karatsu flask. I wish I could get those blues!

Overglaze tests with iron pigment

Here is the original test cup (r), and one pulled from the last firing (l). The first cup was refired to cone 5, the second to cone 6. I was worried with the extra heat that the overglaze brushwork would bleed or run, but it didn’t.
One thing I like about this overglaze deco is that it allows me a ‘second chance’ for decoration when refiring this kind of work. The other is that it allows for finer detail showing up in the final product. Underglaze iron tends to get absorbed into the glaze and/or clay body, so the finest lines become obscured or transparent.

2016-03-20 Glaze tests 釉薬の実験

Fired with cones 6,7,8 on top and bottom shelves. Pyrometer at middle shelf, didn’t display over 1215C.
Hotter on bottom than top. By cone, looks like cone 9+ on the bottom, cone 9 at the middle, and cone 8+ at the top.

Reduction strong at top shelf, weaker at middle and bottom.

棚三段、上、中、下。　オートンコーン6，7，8 使用、上:下置。温度計は中に。焼成中の温度計は1215Cを超えずが、上は8番コーンは曲がって、ヘタレ。下は8番フラット。　実際の温度は下：9+、ちゅう：9、上：8+　とみていいでしょう。
上は還元効いているようだが、中:下は中性気味。

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