Smoke firing is a simple method of firing pottery in which smoke becomes an integral part of the surface decoration. Inspiration has been drawn from "primitive" firing traditions still used today in many parts of the world. Examples of smoke firing can be found in the Neolithic Period (7000 - 3000B.C.)
To prevent damage during the heating and cooling process, a form with a constant wall thickness is skillfully constructed. Some pieces are covered with a layer of fine clay and burnished, others reveal the raw clay surface. The pottery is bisqued to 1000°C.
Pieces are covered with a variety of organic materials and placed in a bonfire. No glaze is applied. During the firing, a blackening or carbonization occurs on the pottery surface.
Because smoke fired pottery is fragile and porous, it is decorative and not designed to be used with food.
Crystalline glazes were first achieved by the Chinese during the Sung Dynasty, then "rediscovered" by Europeans during the 19th century. They require special glaze formulations, and exacting firing profiles. Slight variations in any of the components involved will affect the success of a firing.
The shapes visible in the glaze are zinc silicate crystals. The crystal nuclei form during the firing as the temperature climbs. Growth of the crystal occurs during a controlled slow cooling and holding process. Pieces are covered with a variety of organic materials and placed in a bonfire. No glaze is applied. During the firing, a blackening or carbonization occurs on the pottery surface.
Crystalline glazes are extremely fluid and have the tendency to flow off the vessel during the firing. Consequently, pieces are fired on pedestals that have been placed in individually constructed catch basins in the kiln. The vessel becomes fused to the pedestal during the firing process and the two must be separated after cooling. The excess glaze and all ensuing sharp edges are then ground smooth.
Crystal glazes are a constant challenge for the potter and at times the failures may outnumber the triumphs. It is the reward of removing a successful piece from the kiln that keeps the crystalline potter addicted to that quest - the search for those perfect circumstances that will grow the perfect crystal.