The Process, Continued - Pitfalls
in the firing that can actually adhere to the other pots in the kiln and cause those other pots to have an unintended finish. If a pot is glazed on one side only, the pot is much more likely to break during or after the firing.
Even if the glaze doesn't drip, crystallize, crawl, or just plain look bad, it can also pinhole, craze, pit, blister, or peel. It's a wonder it ever turns out well!
If the firing is too fast or too slow, or if the atmosphere in the kiln isn’t correct, the pots may not survive, or may be ruined. For example, firing with a copper red glaze requires that the atmosphere in the kiln be starved of oxygen at a specific temperature (called firing in reduction). In a 12-hour firing, if I’m even 10 minutes late in achieving that reduction, I may get a white finish on those pots instead of a red one. If I don’t have enough reduction in the firing, I might get a white or even a green pot, instead of the beautiful copper red I wanted.
If the firing is too fast, the glaze finish may have pinholes or crazing (cracking). Most kilns have some areas that are somewhat cooler than others, or have spots in the kiln where reduction or oxidation is higher. So, where a pot is placed in the kiln can effect how it turns out. In my kiln, a copper red glaze works best in the center of a kiln shelf, but can turn white if the piece is near the edge of a kiln shelf.