The process of salt vapour glazing (NaCl, sodium chloride), assisted with water, consists in the following:
(2NaCl + H2O) + heat → Na2O + 2HCl
The soda (Na2O) combining with the clay will form a glaze while the hydrochloric acid HCl will escape through the chimney. This vapour is hazardous and produces pollution in the form of white clouds. Hence the need for a more environmentally friendly alternative which was found with the use of soda carbonates (soda ash and baking soda).
Soda vapour glazing, done on a variety of ways, has the following scheme:
Na2CO3 + heat → Na2O + CO2
For vapour glazing to be effective, the salt or soda carbonates must be quickly decomposed to create volatile soda. The easiest way to achieve quick decomposition is to dissolve either salt or soda carbonates in water and to introduce it in the kiln either by spraying or drip-feeding it.
The high temperature in the kiln will instantly create steam thus breaking down the compounds.
In wood firing kilns the wood can be soaked with the dilutions.