Monday, June 6, 2011

Kilns, Smoke & Flame

by Mary Ellin D'Agostino
Technical Advisor

Frequently, when someone purchases a new kiln and uses it for the first time, they call or email in a panic about their kiln catching on fire or the inside turning black. Usually, they have turned the kiln off. In every case, after they have answered a few questions, I have been able to assure them that everything is fine; that they should turn the kiln back on and continue firing.

When you fire metal clays, there will always be some smoke and flame. This is perfectly normal. We call it “firing” because we heat the metal clay up, burn off the binder, and hold it at a very hot temperature to sinter (fuse the particles of metal together). The binder has to catch on fire. There will be smoke and flame. If you are firing at a very low (for metal clay) temperature, some of the soot might even remain on the inside of your kiln, but if you fire again at a hotter temperature, the carbon soot will continue to burn and your kiln will return to its natural white state.

If you open the door of the kiln while it is hot, but before the binders have burned completely away, the binder in the pieces may instantly catch on fire and emit an alarming amount of smoke and heat. When you open the door to the kiln, you introduce oxygen into the firing chamber. The oxygen combines with any remaining (hot!) binder and starts a flash fire. While alarming, this is perfectly normal. A flash fire of this type is slightly dangerous since the person who opened the kiln door can get a face full of smoke and heat. Generally, I recommend that you don’t open the kiln door during this period of firing—especially if your pieces include burnable cores since the cores add a lot more fuel to the flame.

I did do this some years ago without thinking (even though I knew better) and had my own exciting moment. Since the kiln was full of beads with cork clay cores, there was a lot of flame and smoke! The soot stains on the outside of my kiln remain to remind me of the consequences of opening the kiln during the binder burn-off stage. I probably could clean them off, but I kind of like the reminder and it is a great teaching tool because I can point to the soot stains as part of my kiln safety and use lecture in classes.

If the chamber of the kiln remains dark after a low temperature firing, it is because the kiln was not maintained at a hot enough temperature for a long enough period of time to burn out the soot. Most often, this happens when people follow the directions for pre-firing their new fiber kiln shelves. Simply re-fire the kiln to a hotter temperature and it will return to it’s normal white state. If you are firing low-fire silver clay and the chamber remains dark after firing, you may want to check your firing schedule as it may have not gotten hot enough, long enough to sinter the metal clay. Even the lowest firing temperature (1110°F) should be sufficient to burn out the soot if held for the recommended 45 minute firing time.

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