What, exactly, is body reduction? Since glaze colors develop during the glaze reduction phase of the firing, why can't I skip this step?
Body reduction is the reduction of oxygen to the kiln relatively early in the firing. Typically I enter body reduction NO LATER than cone 010, and put the kiln into a slightly reducing atmosphere at 05, where it will stay until glaze reduction, around cone 8. It must occur prior to glaze sintering, because the function of body reduction is to act on the iron in the clay body (hence the name), which will become unavailable to the action of the flame once sintering has begun. That said, glazes are certainly not unaffected by body reduction. Remember that glaze is not a two dimensional surface. The interior of the glaze layer, which is to say, the part nearest the clay, also must be reduced prior to sintering, or again, the skin of the glaze will prevent the flame from acting upon it. Copper reds are particularly prone to fail if body reduction occurs too late, regardless of what happens during the final reduction phase. (Possibly they just tend to sinter at an earlier stage.) Depending upon how high you bisque, your clay body, and at what stage your earliest glaze begins to sinter, you could enter body reduction around 012, possibly; too early and your ware will experience bloating. Which is better than you experiencing bloating. Well, sort of. Your ware is ruined, your just have to eat better.
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