Saturday, December 17, 2011

Low & High Fire in the Same Studio

I've worked on several school kilns in the last month where the teacher is interested in doing some cone 6 work in addition to the low fire work they usually do. There is absolutely no problem with this, as long as the proper precautions are taken.

The greatest worry with keeping glazes and clays for more than one firing range is that the low fire materials may get into a high fire glaze load. Earthenware clay, which is formulated to mature around cone 04, will melt (yes melt, like into a puddle) at cone 5. When this happens it ruins the kiln shelves, and typically flows into the walls of the kiln, destroying whatever bricks and elements it comes into contact with. To prevent such a catastrophe, I recommend using a two different color clays for the low and high fire. If you are already using white clay for low fire, use a brown clay for high fire. If you are already using terra cotta for low fire, use a white clay for high fire. This way they can easily be identified. White clays all look the same once they have been bisque fired.

Glazes are another problem. While it's rare that an over-fired glaze will ruin the kiln, it can definitely ruin a kiln shelf and all the pots next to it. To keep from putting the wrong glaze on your pots, figure out a way to color code the glaze jars. I recommend putting a stripe of red tape on all glazes that go with the red/brown clay, and a white stripe on all the glazes that go on white clay. Keep it simple so that the students won't get confused.

Another concern is with the firing itself. It is not uncommon for someone to program their kiln for cone 5 when they meant to do cone 05, and then everything melts. One way to keep from getting confused is to simply use obviously different cones for your firings. Instead of cone 05 and cone 5, use cone 04 and 5, or 05 and 6. If you have a computerized kiln, once the firing starts hit the 'Review Program' button and make sure you programmed the correct cone temperature. If you have a manual kiln, color code your cone boxes the same as your glaze jars. Red tape for the red/brown clay, white for the white clay.

Last, any time time little shards of a pot end up on the kiln shelves or on the kiln bricks or elements, whether from blowing up in a firing or from breaking during loading and unloading, they must be vacuumed out. Just like an over-fired pot, those little shards of low fire clay will melt in a high fire. If that melted shard is on an element, it can ruin the element. If it is on a shelf, it will have to be chiseled off the shelf. Keep your kiln vacuumed out and clean and it will last a lot longer, and save you a lot of money in repair costs.

Hope this helps. Happy firing!

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