Deep Blue Jewelry Jar 1Porcelain, hand thrown, jewelry jar with a deep blue crystal glaze. Every piece is thrown and trimmed on the wheel using Coleman porcelain. This jar was fired in an oxidation atmosphere in an electric kiln. Crystal glazes are fired to cone 10 (2350 F) until they turn molten and begin to run down the surface of the pot. The temperature is then cooled, stopping the glaze from running, and causing crystals to grow.