Category Archives: Uncategorized

Sand and clay

There was a recent thread on clayart about sand vs grog, and sand has been on my mind since I  started making some teabowls for the upcoming firing. While I cannot speak to the truth of any of the statements made in the email thread, such as excessive cristabolite formation, sand creating micro pockets of space as the grains shrink, etc… I can speak a bit about the function of sand in Karatsu pottery.

I really goes back to the technology that the Koreans brought over with them. They commonly worked with porcelain stone which is not a clay but rather closer to a high alumina content feldspar. They knew how to work with stone as a clay body. Here in Saga there are a great many clays that have reasonable plasticity but they are so refractory that it is difficult to vitrify them, or they have such large particles that even if they do vitrify, they still leak.

How they solved these problems to create vitrified bodies is still a bit of a mystery, since the technology was all but lost, but recent research has indicated there may be significant additions of crushed sandstone in the clay bodies, OR the clay bodies may be entirely of crushed sandstone in some cases. I know of one contemporary potter here who does this successfully.

Of course sandstones vary and I’m sure some are not suited to add in large amounts, however there are many places here where feldspathic sandstone can be collected. It is very fine since it was run through a stamp mill. Here is some I collected in Sasebo, Nagasaki. There is about 1.5 kg there in the picture and it will get mixed in to about 4kg of clay.

You can add more, but it starts to affect workability pretty quickly. My teacher often adds up to 50%.  There are several benefits:
1. the clay body is more easily vitrified.
2. the body is strengthened and actually less prone to slumping at high temps. (this is kind of counter intuitive from no.1 above, but testing shows it to be true).
3. you get a much more interesting trimmed foot with much crinkling and tearing. With feldspar glazes, you get a nice crawling of the glaze on the trimmed surfaces, called “kairagi” or plum tree  bark, such as can be seen on the famous bowl Kizaemon.

The workability of the clay is affected, depending on the mesh size of the sand, and you may find that you need a different firmness in the clay to throw successfully. This hump was too firm because I let it sit too long on the plaster, and I had some problems with it.

I hate it when this happens.

I really hate it when this happens.

 Finally got one right here. It has a few of the important points for an Ido style bowl. The inside of the bowl is not round at the bottom but rather pointed, which is a better shape for preparing thick tea (koicha), and the inside surface is smooth, so that the tea will flow to the lips without sticking and hanging in bumps and ridges. Also, the inside lip surface doesn’t curve in or out, just extends into space, and the outside of the lip curves upward and in to meet the inside. This creates a ‘sharp’ edge which cuts the flow of liquid nicely against the upper lip. I never really got the importance of this last one, until I had a chance to drink koicha from an Ido style bowl. The whole process of drinking and wiping for the next drinker made the efficacy of this lip treatment very apparent. It was one of those “Oh, I get it now…” moments.

Bottom picture is taken in my nifty new pot mirror, 540 yen at Nitori. I love this mirror because I can see the profile of the pot without leaning over. When I lean over, I usually whack my head on the ware racks to my right. And no, I don’t seem to remember from time to time, so it is always an annoying surprise.

Lastly, the ‘rack that whacks’ with pots all lined up and happy. That rod sticking out horizontally at the center left is the one that is particularly dangerous. If you don’t hit that, then you hit a ware board edge, or worse yet, a nice fresh pot gets squished by your temple and you have to clean clay out of your hair.

Rake Out

Spent some time digging in the dirt today, getting the area in front of the rear chamber rake out hole squared away (literally).
It had been an eyesore, work blockage (wheel barrow always got stuck in the depression), and health hazard (rocks roll down inside and then you step on them, twisting your ankle).
I had some left over wood pieces and brick laying around, so I spent a few hours digging out the square and setting the frame in, then wedging it in with old brick fragments, etc… The digging was the hardest part, I was determined not to have to cut wood and as luck would have it it worked out that way.
At some point in the future I suppose I will lay some concrete in there sloping up and away from the hole, for maximum raking pleasure.

This gives you a vague idea of the original state of the pit.

Here it is finished, second pic with castable hole cover in place.

When not in use, a plank or two can be plunked down for easy passage, or alternatively punji stakes can (cheaper than bear traps) be set in the floor to catch unwary neighborhood kids who ignore warnings not to play in the kiln area.

Insulating castable is DONE

and I am thrilled to be done with it.

After all was said and done, 51 bags of insulating castable were applied to the kiln. 60lbs. per bag. All but 5 of those went on the rear chamber.  That’s 1150kg/2760lbs of castable sitting on the rear chamber.

First, the arch over the rear chamber door.

Next, the front and rear views of the rear chamber. Since I couldn’t do it all in one day, there are decidedly obvious lines where one day’s work started from the previous’ finished.

This rear side, I made the mistake of trying to work my way down from the top, then castable started falling down as the angle got steeper and steeper. Started from the bottom and worked my way up, but had problems with the mix sagging and settling, so set it down for a night and came back in the morning. The darker strip you see here is the last bit I filled in with. Makes the rear chamber looks like it is in the advanced stages of elephantiasis.

Thermocouple hole.

I went back and repaired the patches that hadn’t filled in properly around the top spy, as well as the stoke, getting rid of the chisel marks from Monday.

It’s hard to see from this angle but that castable is a full 6 inches thick at the stoke, and under that is a layer of fiber. That back wall is so well insulated, the stokers might have to wear coats and long underwear to keep from getting frostbite….

The bottom right section was done as sort of an afterthought (and looks like it) because I had extra mix left over and needed to put it somewhere. It will help support the right side of the wall, I think.

One of the trouble spots discovered in the first firing was where the arch met the front wall. As the kiln got hot, the arch expanded and created a place for cold air to leak in all up and down where the front wall meets the arch. I draped fiber down both sides and covered it up with castable, so now even with the expansion, the resulting spaces created will be covered and pretty airtight, I hope.

Next job is to go in and seal up the cracks in the first chamber covering mix, then apply some more over the top. I have clay left over from digging to make more mix, but my cousin came over the other day with some premixed red mountain clay/mica/straw/sand in a 50 lb. bag @ only 200 yen per bag., cheap. Normally it is used as underlay for roof tiles, but it may be just the thing for this job too.

Insulation on the second chamber

Finally getting around to finishing the insulation on the second chamber. There was a LOT of insulating castable left from the workshop, so I decided to use it. It’s quite a luxury to have access to so much for this job.

First, I did the front and rear walls. The front was easy because it was just framing the doorway, and I had plywood that could be clamped in place to hold the castable in. After draping fiber over the whole chamber and around the edges from the arch to the front and rear walls, started putting castable in the plywood ‘forms’.

Here you can see the castable draped over the edge, and the arch form over the door to finish that side. Both sides of the door are finished, with a little 30 degree angle built in to support the arch.

The rear wall was toughest, because the plywood wasn’t strong enough to support the castable going in, and started bowing out, which allowed castable in between the plywood and my spyhole, and stokehole placemarkers. So, the next morning when I removed the plywood I had to chisel out the holes, a little more than 3 inches where the stokehole was. Note the two test spots where I thought the stokehole was, before I checked from the interior.

This extra thickness in the rear wall is probably a good thing, since the stoking will take place there, and any extra heat is really uncomfortable. During the first firing there was no insulation, just bare brick, and the heat was unbearable. We draped fiber blanket from the arch just to create a heat barrier so we could have a place to stoke.

The rear wall got done in two days and you can see where the first and second pour connect. I did put in some mesh just in case that connection point gets iffy.  Also, I didn’t vibrate the mix enough around the peep hole and got some rough, unfilled space. I’m hoping this will not be a problem.

After the front and rear wall, I started in on the arch. The arch section curves over the edge to rest on the walls, which were finished fairly smooth. I expect the arch section to separate from the walls at this joint during firing as the arch expands.

 Last is the collection box behind the rear chamber. I put this layer of castable on mainly to seal it up, because it was leaking a lot of smoke during the first firing.

Doing the nearly vertical surfaces without forms was a challenge. I finally figured out the right amount of water on the second day. Very stiff mix, applied in softball sized balls to the wall, and pressed in slowly taking care not to shock or vibrate the built up wall of wet mixture. This worked well, and after things had stiffened up just a bit, I was able to go back over with a trowel to smooth the joints out.

 Well, that’s it for now. I expect another 2 days of work to finish out the chamber completely. Then I’ll move to the front wall of the first chamber, which I want to finish out much like the door side wall of the second chamber. I want to reduce the amount of air leaking into the front of the kiln when that arch expands, and help keep the stokers cooler.


Just a quickie here this morning before I head out the door. Here are some more jars that I coiled and paddled yesterday and the day before. I still need to work on speed. Having coils all stacked and ready to go really speeds things up.

One factor is stamina. My hands get tired on the big things, especially when I’m using that firm clay to speed up drying and keeping the pot stable.

 The largest jar at upper right is approx. 40 x 40cm. The others are all smaller. The smaller ones were were relatively easy to complete above the shoulder, where I usually get in trouble. The big one gave me some trouble at this point and was threatening to fall in on me when I paddled. Walls are about 4mm thick on all of them.

If you’re making a pot with a neck, it really helps to collar in for the neck. Once that is done the shoulder stiffens right up.

One last jar to add to this post, that was not finished before. This is the big one of the bunch and took longer because it needed to sit overnight. I think it was close to 50cm wide, but continues to shrink down as it dries. It has the low angle that I wanted, but I am not so sure it will survive the firing if I put it in a high flame area. So, this may be the only picture of it in one piece.  I kind of expect it to crack vertically near the foot, or perhaps the top will fall in, either way it should be fun to see when it happens.

This was a fun one to make, with a couple of close calls. A few more of these and I will have the Shinanju “Iron Pinch of Death” technique mastered.

Happy potting everyone!

The tragedy in Sendai

gets worse and worse by the day. I am feeling more and more lucky as I hear the accounts of damage in the quake and tsunami areas. Down here on Kyushu we haven’t felt any tremors, and are on the opposite side of the island from the tsunami danger areas.

Watching everything on tv, hearing about it from family and friends closer to the devastation, but it still seems somewhat unreal.

Things like this remind us to be thankful and appreciate what we have.