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.
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.
Gas kiln firing, 2020/03/16. Chosen Karatsu and Madara glazes. 3 types of Madara glaze from old buckets in which the exact contents were largely forgotten.
The normal madara glaze ended up being too low in silica, and going clear in most cases. Shirakawa madara pinholed in most cases. I’d forgotten about this behaviour. New Madara glaze test shows good color, but also, low in silica perhaps, needs more straw ash added (1kg).
Shida madara much too clear, only showed white where very thick. Maybe miscalculated when mixing the glaze? Add more Shida (2kg?) to overall batch. Test again.
When I was polishing work from the fall firing yesterday, I noticed two pieces that had been glazed, fired (on shells)glazed again, and fired again (on shells). This isn’t too unusual, but in the case of these two pieces, they were literally fired side by side both times and had the same glazes applied, twice.
The only difference between the pieces is the clay body. The teabowl is clay/sandstone 50/50, and the smaller cup is clay/sandstone 10/90.
Now probably it shouldn’t, but the difference in the fired glaze surface astounds me. And all other things being equal, it must be that 40% difference in clay content that has changed that glaze.
Since the glaze is collected, I don’t have an analysis, but my guess is that the higher alumina content in the higher clay body is responsible for making that high silica glaze go clear, by adding alumina to the glaze matrix and fluxing more of the silica.
Loading of the first chamber began this morning.
The stack starts at the bottom from back to front. This part of the firing process is very important because it determines where the fire and heat will go during the firing. A bad stack means you will have poor circulation and you’ve doomed the firing even before you’ve lit the fire.
The bottom of the stack is pretty much done, and the ware from 9 ware boards got stacked in there, which is about 80 pots give or take a few.
Here is my latest pot porn for you: freshly stamp milled, slaked, and filter pressed cakes of Izumiyama porcelain. 150kg.
Not for the faint of heart, Izumiyama is hard to work with. It is non plastic and likes to crack during drying, impossible for slab work, and deforms easily. And it costs more than twice as much as Amakusa porcelain from Kumamoto. Most porcelain artists in Arita and Imari switched to using Amakusa porcelain long ago because it’s much easier to work with, doesn’t crack, and fires whiter.
BUT! The beauty of Izumiyama in the wood kiln is absolutely undeniable. It fires to a soft ivory white and blushes in very subtle shades of peach. The surface is deep and translucent and vitrifies as low as around 1230C.
Tomorrow morning first thing, Peter the Pugger will be getting a major workout!
