Thursday, June 9, 2011

Pyrometric Cones

Often we get people coming through our stores asking how we get our glazes so even. When you consider that our kilns are 27 cubic feet you can imagine how difficult it is to keep the temperature stable throughout the whole firing cycle. At Bison we fire our pieces twice...the first firing (Bisque) takes off the moisture and organic impurities in the clay body. The second (Glost ) is where we place the glaze on the vessels and fire them until the fine stoneware clay is vitrified. You can use electronic temperature readers but the tried and true method for many potters is to use  pyrometric cones. These little cones are placed in a small lump of clay and sit just behind the bung or observation holes in the kiln door. They are designed to bend when the heat work in the gas kiln reaches specific temperatures. We normally use one cone to let us know that the kiln is approaching maximum heat and the second one just bends slightly alerting us to turn off the kiln. The third cone is to show if we have gone above our normal firing range which can impact on the glaze finish of the pieces. For such a creative industry we certainly marry a lot of science together with design to get the end result. Kids love the cones as they look like dinosaur teeth...they also would make a great installation piece! In the second image you are looking through the kiln door at the cone in the red heat. Photos courtesy of David Plummer.

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