Some Gritty Details, as it were, Part 1

Potters (“ceramists”, as they are sometimes called in order to differentiate them from “ceramicists”, the scientists and/or technologists who study ceramics, though some potters insist on using the longer version of the word) can deal with their materials in a “black-box” way, but they may do so to the detriment of their work. It’s not so much that there’s anything wrong with buying commercially prepared “clay”, slapping it on a purchased wheel, buying glazes, and so on; more that there is considerable satisfaction in knowing how it all works and what’s going on in there, and that when things go wrong, as they do from time to time, one has a marginally better chance of being able to take effective steps. Knowing your materials also helps you avoid badly behaved glazes in the first place, at least to some extent.

It also seems to me that the better informed you are about what you do the more guts it will have, the more depth and richness.

“Clay”, in the world of stoneware and porcelain, is not usually just a single material that people dig out of the ground and use “as is”. There are some examples of that in stoneware, but they are few, and far between. Mostly, when you dig up clay, it has roots and rocks and things in it, and must be washed or sieved, or both. Moreover, the characteristics usually have to be modified to suit the requirements of the potter.

Stoneware has to fire to maturity at a temperature that is generally on the order of 1150 to 1300 celsius. Some clays mature lower, some higher. A few can be used throughout a wide range of temperatures and firing conditions. It is, at least in principle, possible to mix clays, usually with other materials as well, to achieve the desired characteristics. ...But maturation temperature is not a simple thing. Some clays mature well over a wide range of temperatures, while others must be fired very carefully, because they have a much more narrow working range. Bone china, which contains a lot of calcium, is a good example of the latter.

Firing, too, is only one of the issues at hand. The stuff must be workable when it’s wet. There are several major ways of working with it, too, and each has its own requirements. I throw on a wheel. The pottery where I do my work, however, is by no means so limited. They throw, they ram-press, they may do some molding, and they do occasional slip-casting.

A few words of explication are probably relevant here.

As you can see, in one case the “clay” is a liquid, in another a very firm solid, and so on. For throwing, the clay has to be fairly firm, but if it is too hard, it becomes difficult to work, and of course if it is too soft, it doesn’t stand up well.

Potters adjust the texture by adding or removing water and, in the case of slip, by adding agents that reduce the attraction between the clay particles. This process is called deflocculation. Occasionally, a potter will use epsom salt or some other agent to flocculate clay that is too soft but has an appropriate amount of water in it. (In Pioneer Pottery, if I recall correctly, Cardew states that some clays have an appropriate moisture range that is only about 1% wide. That is, if you are working with a body for which the right amount of water for throwing is 27%, the same body with 29% water in it will fall apart on the wheel, and with 25% water it is so stiff that it cannot be kneaded or otherwise worked.)

Another way of stiffening and strengthening clay, extensively used in stoneware, is grog. Grog is any grainy or gritty material that is added to the clay body; it can range from sand to ground up pottery to perlite to fiberglass to “you name it”. (A word of caution: if you use fiberglass or anything else that is sharp and pointy, or poisonous, or otherwise nasty, WEAR GLOVES when you handle the material! If you can’t throw with gloves on, use the fiberglass-grogged stuff only for slabbing and other sculptural techniques.) Some potters like the feel of heavily grogged clay. I do not; grogged clay always feels like it is trying its best to scrape my palms off. That’s why I usually work with porcelain, which is not usually grogged.

Once you have thrown, cast, molded, slabbed, etc., and you have a piece of something that will eventually (you hope) become pottery, there is considerably more processing that it must go through. I’m going to leave some of that out until I have more time to write about it; when the stuff is “leather-hard”, for example, you get to trim off rough edges, create a base if the piece was thrown, and so on. After the piece dries, it is very fragile, and you usually cook it, but only partway. This process is called “bisque” or “biscuit” firing, and at least in the case of the word “biscuit”, refers to the fact that the wares get cooked twice. (“Biscotto” = “Twice cooked”, and in fact biscotti are baked twice, once to form the loaf, and once to dry and harden each biscotto so it will be nice and crunchy.)

If the piece survives drying and bisquing, you get to glaze it. Again, I’m going to leave this for later, because it is a very complex issue.

The next step, in the ordinary course of events, is to cook the piece to maturity, and the glaze along with it. Things are not always quite so simple, however. Some pieces are cooked to maturity first, and then painted with lower-melting glazes, after which they are cooked again. This can even be done several times, with several ranks of glazes, each rank melting considerably lower than the previous one. Needless to say, Murphy can strike anywhere in the entire process; the closer to the end you get, the more effort you’ve put into the piece... one of the joys of pottery is that there are many kinds of “accidents” in the kiln, but only some of them are disasters.

The word “kiln” (which can be pronounced “kil” or “kiln”) seems to come from the Latin word “culina”. I think it means “oven”; certainly that’s the obvious possibility. An ordinary oven, however, will not cut the mustard. Even bisque firing takes place around dull red heat; by the time you are “cooking” stoneware and porcelain you’re looking at 1250 or even 1300 celsius, and a few people fire even hotter. Temperature, however, is not a very good way to think about this; see below.

Because the inside of a pottery kiln is usually exceedingly hot when it’s in use, and because the temperature is only part of what’s going on, many potters use calibrated pyrometric cones (they’re actually tall triangular pyramids, but I won’t tell if you won’t), made of special ceramics, as heat indicators. You set up an array of three or four of these things where you can clearly and easily see them through the spyhole; when each one gets to its specified temperature, it slumps over.

The nice thing about a cone is that it doesn’t just fall over, bang, the instant it gets to temp; it’s real ceramic, just like the pottery being fired; and the heat has to work on it for a little while, just as it has to work on the pots.

The reason for the array of cones is to provide a warning when the interior of the kiln is getting close to the desired temperature, to let you know when it’s there, and to caution you if it gets too high. There are many tricky things going on in there, one of which is that (especially in the case of porcelain and stoneware) your pots may be partly melting, and if they get too hot they are likely to slump. This is usually not what you want.

Cones range from 022 (I think) to 01, in the “low-fire” range, and then continue from 1 to 42 or so in the “high-fire” range. Most potters rarely go any hotter than 12, and very few go beyond 14. You need a kiln built of rather special materials, and a good way to heat it, if you want to get higher than 14... we’re talking something like 2550 degrees F here, 1400 C. That’s white hot.

Cones from different manufacturers slump at slightly different temperatures. According to the Thames and Hudson book, an Orton #10 cone slumps at 1305 C. Even this, though, depends on how fast the temperature is rising. At 150 celsius degrees per hour, the cone can slump at a temperature perhaps 10 degrees higher than the temperature at which it slumps during a 50-degree-per-hour rise.

Turns out that different glaze materials work in different temperature ranges too; some glazes just boil if you try to fire them too high; some don’t melt except at very high temperatures. Different glazes have different characteristics of thermal expansion, too. That’s how you get a crackle finish, for example -- use a glaze that sticks nicely to the clay body, but shrinks a bit more as it cools. At least, that’s my current understanding. Grains of salt may be advisable.

Speaking of which, it seems that salt makes for some interesting glazes; but it’s pretty volatile, and it permeates the kiln, so you have to watch out where & when you use it. You don’t put the salt in the glaze; instead, you heave it into the heated kiln through a spyhole or something. If you once use a kiln for salt-glazing, you probably end up using it for nothing else, unless you want to make partially salt-glazed ware during the next few firings. Check the books on this.

It further turns out (no surprise) that different metallic ions make for different glaze or clay colors. This has complex implications, including the fact that if you use copper (usually the oxide or carbonate, ground to a powder) in a glaze and you fire it in an oxidizing atmosphere, you’re likely to get a green or blue color. If you fire it in a reducing atmosphere, on the other hand, you may get Ox-Blood red. (...Or you may not. If it were all that simple, there would be a lot more Ox-Blood red pieces out there.) Copper, or perhaps more accurately its oxide, is volatile at high-fire temperatures, by the way, and can migrate to other pieces in the kiln... (The nearly-invisible pinkness just below the lip of my original translucent example is copper that the piece picked up during firing.)

There’s a semi-infinite amount of tech about this, and at least an equal amount of artistry, so I’m going to leave it at that for now. If you want to know more, try your local library or some of the links.

Part Two: A Bit About What I’ve Been Doing

Because I’m interested in translucent porcelain, I read up on what people do when they want to mix porcelain to throw on a wheel. Let me give you a bit of background, in case you don’t already know about this. I’ll try to be swift about it, in case you do.

Clay (well, kaolinite -- there are also other kinds of clay) is typically a decomposition product of feldspar (or other similar minerals). Sometimes the feldspar is part of granite. Kaolin that is dug up where it was formed is usually composed of relatively large particles, and isn’t very nice to make shapes out of -- in potters’ terms, it has little plasticity.

Wind and water transport clay particles, sometimes over considerable distances, grinding them and/or sorting them (the heavier ones fall out first) along the way. So-called secondary kaolins, those that have been transported since they formed, usually have much smaller particles, and are much more plastic. Unfortunately, they also typically have lots more iron and titanium in them. This means that they melt at a somewhat lower temperature, and also that they are darker in color and considerably less translucent.

Even among primary kaolins there’s considerable variation, and we’ll get into that later.

One of the other clay tupes is montmorillonite. Among the montmorillonite clays there are montmorillonite itself, bentonite, and various others. I don’t know how these are formed.

To return to the issue at hand: here in The West (that is, outside of China, Japan, and Korea) we make porcelain by mixing kaolin, feldspar, and silica. Unfortunately, if you just mix those three minerals, what you get is typically unusable. I tried this: called up Clay Art Center, in Tacoma, and had them send me ten pounds of what amounts to a classic high-fire porcelain. It contained 50% Grolleg kaolin, 25% Custer feldspar, and 25% silica. It looked like flour or face powder.

The first thing I did when it arrived was to put on my NIOSH/MSHA safety mask and mix a little of the powder with water. Then I laughed. A lot. I could sorta roll out a little snake shape, but if I pulled on it or tried to bend it, it just pulled apart. There was no way I was going to put it on a wheel and make teacups out of it.

What pottery houses usually do when they want to make porcelain that people can throw, is to replace some of the kaolin with other clays that are much more plastic. Many throwing porcelains contain 15% or more of a very plastic material called ball clay, which was apparently carted out of the mines in balls because it was easy to form and move that way. Some commercial throwing porcelains (and probably stonewares as well) also contain several percent of bentonite. You don’t want to put too much bentonite into your porcelain, though, because some bentonite turns into slime when it gets wet (people use it to line ponds, in fact), and all bentonite seems to have rather high shrinkage when it is fired.

Both ball clay and bentonite tend to fire out rather dark, because they contain contaminants like iron and titanium. I prefer food-grade bentonite, which is much lighter in color, and I have occasionally used a percent or so in some of my mixes; but at the beginning of this project, I decided that my best course would be to eliminate all mineral additives, and use only nontoxic organic materials to enhance the plasticity of the porcelain. When I say “organic”, by the way, I am using a slightly loose version of the chemist’s definition: materials that contain carbon.

(While I was working on it, I realized that what I was doing was modifying the behavior of the water in the “clay” rather than swapping out some of the minerals for others that were more plastic. To be sure, it’s widely known that various microorganisms grow in clay, and that clay tends to become much more plastic with age. There are even reports that the ancient Chinese used to age some of their porcelain for up to 50 years before using it! Some people even try adding things, yoghurt, for example, to their porcelain in an attempt to improve it. I’m told that yoghurt works, but that after three months of, uhh, maturation, the porcelain smells astonishingly bad. In any case, it’s clear that modifying the behavior of the water at least ought to be a viable approach.)

If I may diverge for a moment, I hope that at least some readers are now asking themselves why potters bother. Let me make it worse: porcelain is similar in texture to creamcheese, and the original version was about as easy to throw. Modern throwing porcelains are closer to what the French used to refer to as “grès”.

...So why do we mess with it, if it’s such a pain in the neck and is so ill-behaved? Well, first of all, it feels really great on your hands. Most stoneware is filled with little particles of fired clay, which are there to strengthen it during throwing. (They’re called “grog”.) Grog is probably responsible for most of the speckly look that much stoneware has. Grog also makes the stuff very abrasive when it’s wet. I once saw a novice potter actually bleeding at the end of her first lesson, though to be sure that is extremely unusual.

I do throw stoneware, even heavily grogged stoneware like the stuff that hurt the novice I just mentioned, but I don’t regard it as a peak experience in kinesthetic mode. Porcelain, on the other hand, is very sensual stuff. In addition, it can be translucent, which stoneware just ain’t. It has a delicacy that other clay bodies lack. It... it’s porcelain. I love it.

Enough diversion. Back on the track here:

When I started, it was not clear to me whether I could succeed. I only knew that there really should be a way to do this, and that I was determined to find at least one such method.

It seemed to me that I wanted something slimy or slippery, and perhaps somewhat sticky as well. One of the things I tried early on was liquid dishwashing detergent; that was slippery, but not sticky, and what I got was mineral cottage cheese.

I also tried one or two starches, which I cooked in water and then mixed into the dry porcelain. This resulted in the amazing discovery that library paste with clay in it is... library paste.

Now, I’m working from memory here, so I’m not necessarily giving things in the order in which I actually tried them. I do, of course, have all of this written out, but it’s elsewhere. Besides, you probably don’t want every last detail. But some of it is pretty amusing, or at least I think it is.

After a while, I decided that I really had to try an eggwhite. Great, porcelain that you have to store in the freezer, because it grows salmonella (or stinks horribly) if you leave it out. Well, foo. There was nothing for it, I hadda do it, I did it.

I still have the bowl I threw from the eggwhite stuff. One part of the bottom is very thin and is notably translucent, even though the bowl is not glazed. If you tap the bowl with your fingernail, it rings like a little bell. The mineral content is 50% kaolin, 25% feldspar, 25% flint: pure porcelain, no mineral additives. When I threw that bowl, I knew I was going to make it.

I also knew that I had to find another way, because eggwhite was not good enough.

For now, I’m going to take another break. I’ll continue this deranged saga (it gets much worse) when I have time.

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Last modified: Mon Nov 17 19:51:22 EST 2014