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. 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. References Further reading [[Phil Rogers (potter)|Rogers, Phil. Salt Glazing']]'. A&C Black Ltd. 1992 Cochrane, Rosemary. Salt-Glaze Ceramics Crowood Press, 2002. Nichols, Gail. "Soda, Clay and Fire". American Ceramic Society, 2006. Nelson, Glenn C. Ceramics: A Potter's Handbook. 1966, Holt, Rinehart and Winston, Inc., New York. Minogue, Coll & Robert Sanderson, "Wood-Fired Ceramics: Contemporary Practices", University of Pennsylvania Press, 2000. Hamer, Frank & Janet. The Potter's Dictionary of Materials and Techniques.'' A & C Black Publishers, Limited, London, Third Edition 1991. . Glass compositions Ceramic glazes Types of pottery decoration Ceramics of medieval Europe