Clay. It really is the single most important influence on your pots. So why is it that potters spend so much time developing their own ‘signature’ glazes and learning how to fire them, while at the same time (not always but in many cases) using a ‘one size fits all’ clay.
9 years ago in Dallas, my wife and I joined a clay class as something we could do together, and it was a lot of fun. Went through a lot of ‘clay class clay’, you know the stuff, cheap, easy to throw, very forgiving red stoneware clay. Used a lot of it, and didn’t really know there were other clays, other than the white B-mix that some of the other folks were using. But, something about it didn’t ring right, because at the time, the pots I admired and aspired to make were Japanese Shino, Iga, Bizen, and Shigaraki ware. I tried Shino, Tenmoku, Kaki glazes, but failed to get the surface I wanted. Couldn’t understand why. They were basically the same glazes right? Well, in retrospect, there were a number of things I wasn’t doing right: glaze application, glaze thickness, firing schedule, clay body.
At the time I didn’t know anything about firing, and very little about glazes, so I grabbed on to the one thing that was visually easiest to identify, and that was the clay body. Unfortunately, I knew even less about clay bodies than the other stuff. I bought Chappell’s Clay and Glazes book, and looked through the cone 10 brown stoneware section, selected a body that seemed simple, bought the dry bagged ingredients and went for it.
I remember marveling at the smoothness of the bagged clays as I measured them carefully into a receptacle and premixed the dry ingredients. They seemed like liquid already, even with no water added (I now know that this should have set off alarms in my head). Next I prepared a large tub filled halfway with water, and began sprinkling in the dry clay body (I wore a respirator the whole time). Also marveling at my own cleverness at realizing that the Shigaraki and Iga clays had ‘tooth’, I added to the body a very rough grog, something ridiculous like 10 mesh. May as well have named that claybody ‘Beltsander’. Still feeling clever, I made a bunch of pots and fired them, only to be shocked and disappointed at the finished work. The clay was pretty much the same as the ubiquitous clay class red stoneware, except with large dead, white, sharp, unmelted chunks of grog that looked completely out of place. Not at all like the surfaces of the pots I admired.
I’d like to say that first failure started me on the road to all kinds of experimentation with clay bodies, but I was a once a week hobbyist, and I didn’t have a clue about how to solve my toothy clay conundrum. It just didn’t occur to me that potters would add anything to their clay that didn’t come from a store/out of a bag. Shortly thereafter the company I worked for went very nearly belly up, and I found myself to be one of the many unemployed folks in the tech sector crash following the 9/11 attacks. Clay was forgotten for the time being, as other things like health insurance, shelter, and food came to the forefront.
Fast forward a few years, now back in Japan and spending a lot of time talking about Karatsu ware with my friend Tsuruta san. I first saw his pots on his website:
and was impressed by his flowing ash glazes, the Chosen Karatsu and Madara Karatsu. When I first held one water jar in my hands I noticed the little spots and bumps in the flow of the glaze and asked Tsuruta san what he put in the glaze to make it do that (It still didn’t occur to me to think about the clay), and he surprised me by telling me that it wasn’t the glaze, but rather the clay that was causing the interesting flow in the glaze. He was mixing Shirakawa yuseki (a fairly refractory weathered feldspar sand) into his clay body, and the reason he used that was because it softened and rounded over in the firing, without melting out completely and leaving voids in the clay like decomposed granite sand might do. At the time, he didn’t go any further with his various reasons for using the Shirakawa yuseki. Probably because I was still trying to wrap my mind around the idea of such small variations in the clay surface being the main reason for the beautiful, thick glaze surface, which I wouldn’t have thought to be such a strong influence.
In further conversations about considerations for clay body selection, I came to understand that Tsuruta san changed his clay bodies constantly, depending on what glaze would go over it, what temperature he’d fire, what sort of trimmed surface he was trying to acheive, what size the work would be, what the pot would be used for, etc… He used many different bodies even in one firing of his gas kiln. And he did it on a scale that was do-able for anyone if they thought about it and applied a little effort. Usually he doesn’t wedge more than 6 or 7 kilos of clay at once. This is something I now emulate, to save my wrists.
Karatsu clays in general are quite sandy and have a wide range of particle size, mostly on the large end of the scale, so not terribly plastic. The large enough amounts of large grain sand prevent the body from sealing, even after you’ve fired it high enough to make it deform and bloat (I’ll get back to this in a minute). Tsuruta san taught me the trick of adding 10 percent Amakusa porcelain stone to the standard red or white Karatsu body to waterproof it at cone 10, or to add stamped Shirakawa feldspar to the same bodies to seal them at cone 6. At one point, when I showed him a few bloated pots, he mentioned off hand that I might try mixing in some crushed sandstone to strengthen the clay body. Who knows, he continued, I might also come closer to the mottled blue color in my rice straw ash glaze that I so long for. Fool that I am, I sort of dismissed this without trying it right off the bat like I should have, because in my mind I just pictured more large grains in my clay making it less plastic and more prone to leaking. Will I never learn?
When the Karatsu revival occurred during the ceramics boom of the early to mid 20th century, many of the potters came to believe (except for a few, who played it rather close to the chest), from using the natural local clays, that Karatsu ware just naturally leaked, until one used it enough to seal the pores, or did something to treat it, like boiling it in togijiru (the white liquid left over from washing rice). So, this became a sort of characteristic of Karatsu ware, and people learned to accept and live with it. The very old pieces didn’t leak, but that’s because they were used for so long, right? Until recently, no one could test this, but with modern devices and microscopes, it is now coming to light that the clay bodies used by the potters 400 years ago DID NOT LEAK. What?! Well, now that we can see better what was in those clays, guess what we’re seeing? Pounded sandstone. Sometimes more than a little. Sometimes maybe even 100%. As ridiculous as it sounds, yes 100%. I even know one potter who does this. He also does other impossible things like firing glazed greenware to cone 8 or 9 in 5 hours, and the stuff doesn’t crack, but I digress.
Ok, that got me thinking, why didn’t I listen to Tsuruta san’s advice and take it to heart immediately? I got out and collected some fairly white sandstone from a spot outside of Imari, took it home and pounded it in my home stamp mill which consists of a large stone mortar and an old fence post pounding stick. To my surprise, the resulting powder was not grainy at all, in fact it was quite fine, but with a range of particle size up to about 1mm (the bulk of it was much finer).
I wedged it in to my clay body (20%) which had been distorting pretty badly during firing and lo and behold, the clay held its shape. Not only that, the body was completely sealed, no drips, no sweating. Imagine that. And just to apply a little gold leaf to the lily, my madara (rice straw ash glaze) looked fantastic, better color than ever before, much closer to those gorgeous chawan in the museum with the off-white surfaces, mottled with subtle chun blue effects. And to top it all off, the sandstone greatly contributed to the chirimenjiwa effect (fine tearing and wrinkling) in the trimmed areas of the pots, just like the 400 year old Karatsu ware, and it didn’t detract from the plasticity and workability of the clay.
Now, I’m not saying everyone should start putting sandstone, or Amakusa, or Shirakawa feldspar in their clays, but it does pay to experiment with materials close at hand. A good additive(s) will do wonders to a commercial clay body (the standard red and white Karatsu bodies mentioned above are ‘store bought’, but still dug from a hill and raw processed, not mixed in mass quantities from fine, bagged ingredients) I do dig a lot of my own clay, but keep it for special stuff.
What I’m trying to say is, think about what you want to make and custom design your clay body for the glaze and the form and the function of the pot. One glaze on 5 different clay bodies is 5 glazes, without all the extra glaze ingredients. About as close to instant gratification as you can get.
Here’s a standard red Karatsu body and I’m mixing in 20% sandstone. In the forefront is one that’s already mixed, going to go on plaster overnight to firm up, and spiral wedged in the morning. The large shallow platter is indispensable for mixing, keeps everything in one place while the loose stuff is getting mixed in. Otherwise you have to fight with it to keep it on the table, and have to constantly scoot it back under your clay.