are fusible glasses rendered opaque by dissemination throughout the mass of a vitreous substance infusible at the temperature at which they are made, such as oxide of tin. Up to about 900 degrees C. the tin dioxide exists in a state of suspension in the enamel, but if that temperature be exceeded, real solution takes place and the opacity is diminished. Titanic oxide, zirconia, and other vitreous substances can be employed in place of the stannic oxide. The opacity is due to the difference in refractive index, and is obtained in some cases by the use of arsenious oxide, calcium phosphate, cryolite, etc. Platinum and iridium oxide are sometimes used to produce greys in enamels, and a great variety of other substances can be used according to the desired tint and other characteristics. One kind of enamel used for glazing castiron articles, such as saucepans, consists of powdered flints ground together with calcined borax, fireclay, and a little felspar, made into a paste and applied to the surfaces, which are then dusted over with a glazemixture composed of felspar, sodaash, borax, and a little oxide of tin, after which they are dried and fired at a red heat.
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