It is often advisable to glaze the surface of articles made in clay, sometimes for appearance, but more generally in order to protect portions exposed to the action of the atmosphere, to sewage, or other destroying agents.
These glazes are either transparent - merely a film of glass - or opaque, like an enamel.
Transparent Glazes of several kinds are known in the trade. Two methods, adapted to the somewhat rough articles used by the engineer and builder, will now be described.
Salt Glazing is effected by throwing salt into the furnace when the articles it contains are at a high temperature. The heat of the fire volatilises the salt (chloride of sodium), the vapour being in the presence of oxygen and silica is decomposed, the chlorine goes off through the top of the furnace, the sodium combines with the silica in the clay to form silicate of soda, which again unites with the silicates of alumina, lime, or iron in the clay, to form a surface coating of glass.
This method of glazing has great advantages. The vapour of the volatilised salt gets into every crevice, however small, and coats it with an impenetrable film of glass.
Lead Glazing is carried out by dipping the article to be glazed (after it has been once burnt) into a bath containing oxide of lead and tin - or borax with kelp, sand, etc., ground to powder, and stirred in water to a creamy consistence.
The particles of these different materials adhere to the surface of the article when it is dipped. It is then withdrawn and re-burnt. The high temperature of the furnace causes the particles to run together and to form a film of glass over the whole surface.
This method of glazing is used for terra cotta, and sometimes for articles made from fireclay.
Lead glazing is also used for earthenware crocks, etc., which are made out of inferior clays such as would not stand the high temperature required for salt glazing. A lead glaze will generally chip off easily.
Opaque Glazes are required in cases where it is wished to give (to the whole, or to any portion of an article) an appearance superior to that presented by the ordinary burnt material.
The article to be glazed is dipped before burning into a slip formed of superior clay, very finely worked, dried, etc., and brought to the colour required.
Thus the pans of water-closets are often made white inside, and of a cream colour, or some other tint, outside.