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Clay is a complicated thing. Seems easy enough, after all it’s just dirt, right? Nope.
Of course it comes in different colors, and different textures (rough and groggy, or smooth), and these factors can influence the look of your piece and the way your glaze reacts. Especially at higher temperatures (cone 8 and above) the materials in the clay, such as iron, start to interact with the glazes and this is often used to good effect. Some clays (such as porcelain, or porcelain-like stonewares) are are made from materials with smaller particles and are more plastic (can be pulled and manipulated in wet form without cracking); the tradeoff is that these tend to shrink more than others.
All clay shrinks, first as the moisture evaporates, then as it reaches maturity and the materials “melt” together to become a cohesive solid. Manufacturers sometimes give shrinkage numbers so you can compare this between clays. An example stoneware will shrink 5-7% from wet to bone dry, another 3-5% in bisque, and another 4-5% in glaze firing. Overall about 12-14%, vs. up to 20% for porcelain (because it has smaller particle sizes it can pack denser.)
It is important for functional ware that clay reach maturity, which happens at a certain firing temperature. Commercial clay’s are actually a mixture of raw clays and other materials which are combined to create good working properties, and fire to a certain temperature. If a clay is rated cone 5, that is theoretically the temperature at which it becomes “vitrified”, or undergoes a chemical change and becomes almost glass like. The particles become densely packed, all the water is driven out, and at this point the clay is waterproof.
Many people think that glaze provides waterproofing for clay. In fact, with low fire clay it is about your only option since low fire clays never fully vitrify. But it’s effectiveness is limited. If relying on a glaze for waterproofness, you need a glaze with very good properties for waterproofness, and one which exactly fits the clay body so they expand and contract at the same rate. If not, the glaze will crack (craze) and even if it is microscopic crazing you cannot see, your piece will no longer be waterproof. If you think you have a piece with a good glaze fit, place it alternately in the freezer, and in boiling water. Or another test is to pour boiling water into a frozen cup. If the glaze doesn’t craze, it is probably ok.
So this is why stoneware is much more durable, because the waterproofness comes from the clay itself. If you fire a cone 10 clay to cone 5, it will be fine for sculptural or decorative work, but it will not hold up to daily use, microwaves, dishwashers, etc. because it has not matured (it is still too porous). Overfire a clay, and first it becomes brittle, then it starts to melt. So you want a clay that is made for the temperature you will be firing at.
Many manufacturers will give a firing range for a clay, for example cone 4-6. A small range is probably justifyable because of variations in the materials, and the inability to always achieve an exact cone level through a kiln due to hot or cold spots. However, some manufacturers take this to the extreme, marking a clay as cone 4-10. This is hogwash. There is a single, specific temperature at which that clay will become vitrified. It simply cannot happen over a large range. In this case I would bet it’s closer to cone 10 than cone 4, because they wouldn’t want to risk their clay melting in your kiln.
How do you determine whether a clay is matured? Do a porosity test. Take a piece of clay and fire it to temperature. Weigh it. Put it in boiling water for 5 minutes, and leave in the water as it cools. When cold, remove the piece and dry it off with a sponge. Weigh it again. The difference in weight divided by the original weight time 100 is the % porosity. An ideal clay would have 1-3% porosity. (Zero porosity, in addition to being practically impossible is also not as strong because it is more brittle.) If a clay is around 4-5% porosity, it is “waterproof” enough to be good for functional ware. In the above example of the clay with the large firing range, the cone 4-10 clay fired at cone 10 may have a 3% absorption, and at cone 4 may have a 13% absorption. So while technically it can be fired over that large range, it will not have the same properties when fired over that range. A cone 6 clay fired at cone 4 might be 6% absorption compared with 4% absorption, and either may be acceptable. Note that for a piece to be “microwave safe” the porosity should be very low. Otherwise, water which has leaked into the piece can quickly expand and cause it piece to crack.
Also, for oven ware use, the clay used should have low thermal expansion. This means that it doesn’t expand much as it gets warmer. If a clay expands a lot as it gets warmer, it will tend to expand more where it is hotter (closer to the elements) and less where it is colder (further from the elements), which creates stresses and can cause cracking. Some manufacturers list thermal expansion, or at least tell which clay bodies are good for oven ware because they have low thermal expansion.
A few other thoughts. According to Richard Zakin’s Electric Kiln Book, a similar temperature in reduction will mature clay slightly more than in oxidation. Also, manufacturers typically rate the clays on the high side, to avoid their clay destroying your kiln if your temperature is off slightly. So, for example, I have never had any problems firing cone 5 clay at cone 6 in oxidation. But always test! I haven’t tried them all! And there can be variation from batch to batch, and kiln to kiln. So in a way it’s nice to have at least a small margin for error.
My advice for functional work is to get clay with a maximum firing temperature that is the temperature of your glazes, and fire to that temperature, then you will be most sure of proper vitrification and good clay/glaze fit. Of course you will still have to test with your clay body and glaze.
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