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.