GLAZE (ceramic), a vitrified coating which gives to earthenware or porcelain its brilliance and impermeability.
Glazes are of three classes: The glaze proper (Fr. couverte), a soft glaze(Fr. vernis) and the enamel or opaque glaze (Fr. &toil). The glaze proper is a silicate of calcium, potassium and aluminum and is composed of feldspar chalk or whiting, kaolin and quartz. It is ap plied either to the clay ware (Chinese) or to the soft burned biscuit (modern); the whole piece is then burned to a high temperature (about 1,500° C.). Soft glaze comprises the vast range of earthenware and faience glazes and includes the glazes of bone china and soft porcelain. A soft glaze is either a silicate or a boro silicate, and the bases employed include the oxides and carbonates of the following ele ments: Lead, zinc, potassium, sodium, calcium, barium, magnesium, and as coloring agents the salts of iron, cobalt, copper, nickel, antimony, chromium and manganese.
The range of temperature is very wide. A simple lead glaze will fuse at 900° C. and a hard glaze for white earthenware may need 1,350° of heat. Enamels are sometimes used over other glazes and sometimes upon the biscuit body. Their essential condition is opacity.
Oxide of tin, alumina, calcium phosphate and calcium carbonate are used as opacifiers. The early wares made in Italy, Spain and Holland were of this type. (See MAJOLICA). For con venience of application glazes are ground in water and held in suspension, the article to be glazed being plunged into the liquid. For this reason only insoluble substances can be used, and where it is necessary to introduce alkaline salts and soluble boric acid or borates these are rendered insoluble by being melted with in soluble and readily combined reagents, such as whiting and barium carbonate. This melt is called a and the operation of melting is known as )f ritting.) Hence some glazes, mainly of the second class, are called fritted glazes, and glazes which contain no frit are termed ((raw) glazes. Fritted glazes are, as a rule, harder and clearer than those which contain no frit except in the case of porcelain glaze (cou verte), which is made from natural substances without frit.