Salt glaze pottery
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 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 results in a semi-vitreous state 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.
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