Ceramic - Pottery Dictionary

by Susan Mussi




Salt glaze. Pottery referred to as salt glazed or salted is created by adding common salt, sodium chloride, into the chamber of a hot kiln. Sodium chloride acts as a flux and reacts with the silica and clay in the clay body. A typical salt glazed piece has a glassine finish, usually with a glossy and slightly orange-peel texture, enhancing the natural color of the body beneath it.

Technical process
Salt fumes have a dramatic effect on clay under heat. When kiln temperatures reach the melting point of common salt, approximately 900° C (1660 degrees° F), granulated or rock salt can be introduced into a kiln through peepholes or other openings. This resulting in a surface blush of color forming on the ware body. At higher temperatures, over 1280° C (2350° F), the traditional temperature of high fired salt ware, salt becomes an active vapor throughout the kiln interior. A dilute form of hydrochloric acid is given off as a vaporous by-product.

First introduced in the 14th century, the process was initially used on earthenware which was fired from green (unfired) to finished ware in a long, slow cycle. However, the process was soon adapted to stoneware which can either be fired in a one fire cycle or in two stages, a “bisque” fire and a final “high” fire. This two stage process results in a semi-vitreous state at a lower bisque temperature. Ware is then allowed to cool to room temperature for decoration before a further firing.

Salt can also be used as a decorative element on selected 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 saggers.

A related method, called soda firing, substitutes soda ash sodium carbonate ( Na2CO3 ) and/or sodium bicarbonate (NaHCO3) for salt and is an increasingly 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 results similar to salt glaze, with subtle variations in texture and color.

From: The Free Dictionary

Salt glaze; The data on the first known production was around the fifteenth century, it was an anonymous source in Germany. A potter threw salt in a kiln, while it was firing and when he opened it he found it had a crystal clear finish. It was a reaction between the salt and the silica of the clay. During the seventeenth century English potters imported the technique from Germany and made it thrive. In the same century some German potters immigrated to the United States and applied their knowledge on it on earthenware rivaling the production produced in England. The salt glaze, became known in Japan through the collaboration of the potters Shoji Hamada and Bernard Leach Britain.