Salt glaze pottery

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The type of pottery known as salt glaze or salted is created by adding common salt, sodium chloride, into the chamber of a hot kiln. This can be done by throwing salt into kiln peepholes or other openings. Sodium acts as a flux or melting agent, and reacts with the silica in the clay body. A dilute form of hydrochloric acid is given off as a vaporous by-product.

Salt fumes have a dramatic effect on clay under heat. Salt reacts at temperatures from about 900 °C (1660 degrees °F), the melting point of common salt, when a surface blush of color is formed on clays and clay slips, to over 1280°C (2350°F), the traditional temperature of high fired salt ware. At the higher temperatures, salt becomes an active vapor. A typical salt glaze has a glassine finish, usually with a glossy orange-peel texture, enhancing the natural color of the clay beneath it.

Salt fire techniques were first introduced in the 14th century, probably in the Rhineland of Germany. At that time, the process was used on a low fire earthenware clay which was "once fired", heated in a continuous arc from bone dry raw clay to finished ware. However, it was soon adapted to stoneware which is usually fired in two stages, a "bisque" fire and a final "high" fire. This two stage process heats clay to a semi-vitreous state (usually up to approximately 950°C - 1800°F ) and then allows the ware to cool to room temperature. Bisque ware is often decorated in some fashion before being returned to the kiln for firing to a near vitreous state.

Historically, salt kilns were used in many areas of Europe, particularly Germany, Scandinavia and the British Isles. Salt kilns were also popular during the colonial period in North America and in the early years of the United States. The use of salt as a glazing element generally reduced or eliminated the use of high silica glaze compounds by these craftsmen.

Modern Salt Ware

Although industrial salt firing continued until relatively recently the technique was promoted in the craft sector by English potter and artist Bernard Leach. It was introduced into Japanese craft pottery through Leach's association with Japanese potter, Shoji Hamada, in the 1950's. American studio potter Don Reitz introduced salt glazing into the curriculum at Alfred University, New York in 1959. Potters and salt kilns in both North and South Carolina are well known for producing modern salt fired ware, as are American universities with strong ceramic programs. These include Alfred University in New York and Utah State University in Logan, Utah.

Modern potters using traditional salt processes are working in many areas of the United States and Europe. The unique salt glaze finish entices modern potters as it provides an opportunity to capture spontaneous changes in atmosphere and color and is a tool for ongoing experimentation and creativity.

A related method, called soda firing, substitutes soda ash and/or sodium bicarbonate for salt and is a common alternative. Unlike salt, which will fume throughout the kiln, soda must be introduced in a manner that spreads it around the ceramic ware, such as by spraying. Soda glaze produces similar results to salt glaze, with subtle variations in texture and color.

Salt can also be used as a decorative element on individual pots. Bisque ware can be soaked in a brine solution to create salted patterns. Rope and other textiles can also be soaked in brine and wrapped around bisque ware. Salt can also be added, in solution, to colored clay slips and can be sprinkled onto bisque ware in pottery containers called saggars.

Other traditional pottery processes which have been revived or modified by modern potters include low-temperature pit firing, the Asian technique of raku and the use of saggar boxes in gas and wood fired kilns.

References

Hamer, Frank and Janet. The Potter's Dictionary of Materials and Techniques. A & C Black Publishers, Limited, London, England, Third Edition 1991. ISBN 0-8122-3112-0.

Rogers, Phil. 'Salt Glazing' pub by A&C Black Ltd. 1992 ISBN 0-7136-4821-x

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