CRYSTALLINE GLAZES truly represent an artistic joining of chemistry and nature. Although difficult to produce, the rewards can be great. Glaze crystals grow in the molten glazes while the pieces are being fired in the kiln. The size and shape of the crystal is partially controllable through the experience and careful attention to the firing cycle; the placement and number of crystals is not, meaning that each piece is one-of-a-kind.
Crystals are not put in the glaze. The crystals actually form in the glaze in a chemical reaction during cooling and grow from small nuclei created during the melting process when silica and zinc come together to form zinc-silicate. The crystal glazed ware is fired to approximately 2000 degrees F. then the temperature is held in the kiln on cooling between 2000 F. and 1830 F. for approximately 3 to 6 hours depending on the glaze.
Each glaze composition, together with the firing schedule and glaze thickness, makes different forms and colors of crystals. For example, cobalt carbonate always gives you a blue, iron oxide usually results in gold or brown tones. Some of the most interesting glazes result when colorants are combined because often the crystals will be a different color than the rest of the glaze and many nuances and combinations of colors within the glaze are possible.
Crystal glazes run off the pot and need special containers to collect the running glaze so that it does not ruin the kiln shelves. Most artists use a variation of the pedestal and catch basin technique. The vase is placed on top of a thrown, porcelain ring, both of which are then placed in a shallow dish. During firing, excess gaze flows off the pot, down the pedestal ring, and collects in the catch basin. After the piece has cooled, the pedestal is carefully broken off the piece, and the sharp, glass-like glaze edge is ground smooth.