Soda-glazing

Soda-glazing was developed in the 1970's as a chloride-free alternative to salt-glazing, soda-glazing involves introducing sodium carbonate or bicarbonate into the kiln at a high temperature to create soda-vapor. Various soda introduction techniques are used, including spraying a water and soda solution, dropping small amounts of sodium carbonate into the kiln, or introducing a solid plaster-like mixture made from sodium carbonate and bicarbonate, whiting and water. As with salt-glazing, the soda (Na2O) reacts with the alumina-silicate surface of the clay, creating a glaze. Despite the similarities with salt-glazing, the surface effects of soda-glazing can be quite different.

Source: ceramicstoday.com