Starting and Finishing Firing in a Hot Kiln

Sometimes, in a classroom situation, we have no choice but to fire one batch after another without letting the kiln cool down between firings. Is this really a good idea?

If you are firing just copper without carbon (“open shelf”), it is not only a good idea but the only option. If you fire copper from cold without carbon, the oxidation layer will be very hard to remove. Moreover, internal oxidation may occur, which eventually may cause the copper to disintegrate.

(A little side note: it is not a good idea to fire copper without carbon, in a kiln or with a torch, more than once or twice. Eventually it will weaken and disintegrate. This happens not only to copper clay but also to pre-made copper sheet and tubing.)

However, if you are firing in carbon, whether copper or bronze, it is better to start in a cold kiln. The binder must burn off before the carbon is set on fire and starts consuming the oxygen in the kiln chamber. If you load the kiln at a temperature between 500°F – 1000°F, the binder has a very slight chance of burning, since it needs oxygen to burn. This will result in poor sintering or none at all. I learned this the hard way.

Is is a better idea to take pieces out of a hot kiln or let them cool down first?

Again, in a classroom situation there is sometimes no choice. But then cool the pieces immediately in water (unless they include stones). Otherwise, the exposure to heat and air will cause oxidation. The time spent on removing the fire scale may be better spent on waiting for the kiln to cool down to about 300°F.

Whether you fire small pieces for one hour in a fiber blanket box, or bigger pieces with more carbon for 2 hours, no oxidation will occur if you just let the pieces cool down before retrieving them from the kiln.

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