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We get many questions about clay firing temperatures. People want to know if they can fire a certain clay to a certain Cone.
First it is important to know that the maximum Cone rating of a stoneware or porcelain clay is the temperature at which it vitrifies. This is the hardening, tightening and finally the partial glassification of the clay. Vitrification results from fusions or melting of the various components of the clay. The strength of fired clay is increased by the formation of new crystalline growth within the clay body, particularly the growth of mullite crystals. Mullite is an aluminum silicate characterized by a long needle-like crystal. These lace the structure together, giving it cohesion and strength.
While Stoneware and Porcelain clays (clays fired at about Cone 1 and above) vitrify, low fire clays never fully vitrify. This is why they are never as strong, and are not as desirable for dinnerware. Low fire clays are typically only fired to a maximum of Cone 04, although some go a bit hotter. Fire them too much hotter, and they will still melt, even though they do not vitrify.
You should never fire a clay above it's maximum Cone rating. If it says Cone 6, you can fire it to any temperature UP TO Cone 6, but NOT over. (This is unlike glaze which must be fired to the exact specified temperature range.) If clay over-fires, it will first slump and bloat, and then will melt and potentially cause a lot of damage to your kiln. Therefore, the safest clay to have in your studio is Cone 10 clay because you will never risk over-firing it.
You want to fire to the temperature that your glaze needs, as long as that is at, or below, the maximum Cone of the clay.
It is generally, but not always, best to fire a clay to its vitrification temperature.
Why is it better?
When clay vitrifies it gets very strong. This is especially important for dinnerware where pieces are exposed to a lot of abuse. Vitrification also makes the clay's porosity low. Glaze itself will not waterproof a piece, so if the clay is porous it will absorb water and then crack when microwaved or frozen.
When clay vitrifies, the glaze will "melt" into the clay, melding them together. This makes the piece more difficult to chip. (Ever notice how store-bought low-fire mugs chip a lot, and stoneware mugs don't?) It also makes the glazes look more "earthy" and look like they are a part of the piece, instead of being on top of the piece.
When is it not better?
Sometimes you do not want your clay to shrink much. For example, when firing sculptures where the clay has various thicknesses, if it shrinks too much it's more likely to crack/break. In this case, you might use high fire clay, and low fire it, to avoid shrinkage.
Some studios and schools fire many different types of clay and they don't want to risk over firing low-fire clay. Therefore, they only allow high fire clay in their studio. Usually it works fine to apply low fire glazes to high fire clay. Yes, the clay isn't vitrified. But remember, low fire clay never vitrifies anyway. The only problem you will sometimes encounter is more glaze crazing, because of "fit" issues between high fire clay and low fire glaze.
Stoneware clays tend to be easier to work with than low fire clays. That is another reason people sometimes use high fire clays with low firing and low fire glazes.
A final note. Not all clays have the same characteristics. Some clays, when vitrified will still have 10% porosity. This is true of many sculpture clays, for example, because of their coarse particles. Good, tight clay bodies, suitable for dinnerware, will have a fired porosity of less than 5%. Less than 3% is most desirable. (In comparison, low fire clays usually have as much as 20% porosity.)
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