Salt-glaze or salt glaze pottery is pottery, usually stoneware, with a glaze of glossy, translucent and slightly orange-peel-like texture which was formed by throwing common salt into the kiln during the higher temperature part of the firing process. Sodium from the salt reacts with silica in the clay body to form a glassy coating of sodium silicate. The glaze may be colourless or may be coloured various shades of brown (from iron oxide), blue (from cobalt oxide), or purple (from manganese oxide).^[1][2][3]

Except for its use by a few studio potters, the process is obsolete. Before its demise, in the face of environmental clean air restrictions, it was last used in the production of salt-glazed sewer-pipes.^[4][5][6] The only commercial pottery in the UK currently licensed to produce salt glaze pottery is Errington Reay at Bardon Mill in Northumberland which was founded in 1878.^[7][8][9]

The earliest known production of salt glazed stoneware was in the Rhineland of Germany around 1400;^[10] it was effectively the only significant innovation in pottery of the European Middle Ages. Initially, the process was used on earthenware. By the 15th century, small pottery towns of the Westerwald, including Höhr-Grenzhausen, Siegburg, Köln, and Raeren in Flanders, were producing a salt-glazed stoneware,^[11] with the Bartmann jug a typical product. In the 17th century, salt glaze gained popularity in England as well as in Colonial America.^[10] Westerwald Pottery was characterized by stamped medallions and the use of a cobalt oxide based colorant for decoration. Production of salt glaze pottery in Westerwald ceased because of environmental considerations in 1983.

In the UK during the 17th century and 18th century, high quality salt-glazed stoneware was produced in Derbyshire, Nottinghamshire, London and Staffordshire.^[12] Salt glazed pottery was also popular in North America from the early 17th century until the early 19th century,^[13] indeed it was the dominant domestic pottery there during the 19th century.^[14] Whilst its manufacture in America increased from the earliest dated production, the 1720s in Yorktown, significant amounts were imported from Britain until around the mid-19th century.^[15]

During the 20th century, the technique was promoted for studio pottery use by Bernard Leach. In the 1950s, it was introduced into Japanese craft pottery through Leach's association with Shōji Hamada. Don Reitz introduced salt glazing into the curriculum at Alfred University in New York in 1959, and it subsequently spread to other American universities with ceramic art programs.^{[citation needed]}

Due to concerns of significant amount of air pollution resulting from the process ^[16] environmental clean air restrictions led to the demise of widespread use of salt glazing.^[1] It was last used on any large scale for the production of salt-glazed sewer-pipes, and other than limited use by some studio potters the process is obsolete,^[1] although there are reports of it still being used for sewer-pipes in India.^[17][18]

The salt glaze is formed on the unglazed body by reaction of common salt with the clay body constituents, particularly silica, toward the end of firing. The body should ideally be richer in silica than normal stoneware, and iron impurities can help produce good salt glazes. A reduction atmosphere can be employed as the reduced iron silicates are very powerful fluxes.

The salting mixture of sodium chloride and water is introduced into the kiln when the appropriate temperature is reached, typically around 900 °C, or salt can be placed inside the kiln prior to firing.^[29] As the kiln reaches higher temperatures, typically 1100–1200 °C, the sodium chloride vaporizes and reacts with steam to form hydrogen chloride and soda. These vapors react with the silica in the body and other body constituents. A glaze of fairly high alumina content (0.6 molecular parts) and a relatively low silica content (2.6 molecular parts), and in which the main base is soda, is formed. Salt glazes have been improved by the addition of borax, and sometimes sodium nitrate, to the salting mixture. Colouring oxides can be incorporated in the salting mixture to give decorative effects, such as a kind of aventurine glaze.^[30]

Salt fumes in a firing atmosphere react in the following way:

2NaCl + 2H₂O → 2NaOH + 2HCl

2NaOH → Na₂O + H₂O

In soda firings the reaction is a little more direct, not requiring the presence of water vapor:

Na₂CO₃ → Na₂O + CO₂

Both the hydrogen chloride and the carbon dioxide are gases; they do not react with the sodium oxide that binds with the silica and other components of the clay body. Hydrogen chloride leaving the kiln will form a hydrochloric acid vapour on contact with moisture in the air or kiln exhaust gases. Any remaining sodium oxide will form salt by reacting with hydrochloric acid vapour as the gases exit the kiln. The sodium oxide (Na₂O) reacts with the alumina and silica in the clay body to form a sodium alumino-silicate glaze.^[1] The general reaction is shown below, with the values for x and y varying dependent on the amounts of sodium oxide, alumina and silica composing the glaze:

Na₂O + SiO₂ + Al₂O₃·(SiO₂)₂ → (Na₂O)_x·Al₂O₃·(SiO₂)_y

Salt can also be used as a decorative element on selected individual pots. Biscuit 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 biscuit ware. Salt can also be added, in solution, to coloured clay slips and can be sprinkled onto biscuit ware in protective, ceramic containers called saggars. A related technique, soda firing, substitutes soda ash and/or sodium bicarbonate for common salt. Whilst the application method is a little different, the alternatives need to be sprayed into the kiln, results are similar to salt glazing but for subtle differences in texture and colour.