ENAMELS AND ENAMELING. The term enamel is used for certain siliceous com pounds employed for coating metals. They are, in every sense of the word, glass, either trans parent, opaque, white or colored. The art Is a very ancient one, some claiming it originated with the Scythians, who are said to have in troduced it into China during the reign of a certain Emperor Thaiwonti, but India was ac quainted with the art before China. Extant evidence exists that the ancient Egyptians, Phoenicians and Assyrians used enamel coating in the decoration of jewelry, and of the clay beads and scarabs of the Nile-dwellers in our museums many are covered with colored glass. The subject of enamel work on the precious metals discussed in another article (see ARC ENAMELS), so we will confine this article to the technical side of enamel production and the industrial utilization of enamel.
Raw Materials.— The ingredients used in the production of the different kinds of enamels' are felspar, quartz, fluorspar, borax, boric acid, soda, potash, saltpetre, cryolite, clays, nium carbonate, stannic oxide and water. The coloring agents used are cobalt oxide, limonite, iron oxides, chromic oxide, cupric oxide, etc. The felspar composition most desirable sists of siliceous earth 65 per cent, alumina 18 per cent, alkalis 16 per cent. For the creation of white enamels it is essential that the felspar shall be as free from iron oxide especially, as possible. Quartz must be pure but it is fre quently replaced beneficially with a 'fine white river sand, which is pure quartz. Fluorspar or calcium fluoride of the purest quality only can be used, for white enamels especially. The borax needed for enamels is of the monoclinic system of crystallization (containing 10 mole cules water of crystallization) not the octahe dral crystals regular 'system, which contain ' only five molecules water. It is used as a flux or accelerator of fusion. Boric acid functions the same as borax but the former is more fre quently used than the latter. Soda and potash
have identical action in enamels and their pres ence heightens the lustre if it is not used too freely. The soda of commerce, being is mostly employed; it is first calcined to obtain its anhydrous condition, then finely Saltpetre (nitre) used is the sodium variety, ing cheaper than the potassium salt. It is used chiefly to decolorize;• little or none at all is needed in the blue enamels. Cryolite used in the enamel industry may be either the natural or the artificial. This sodium-aluminum-fluoride is utilized for • its double reaction of acting as flux and creating opalescence. Ammonium car bdnate is supposed by some to be a valuable in gredient to inhibit cracking or crazing of the enamel in firing on account of its making the substance more uniform. Some authorities, however, declare its use a waste of money and' useless. Stannic oxide' is expensive with the present prices of tin but its use continues as a necessity after many attempts with substitutes. Care must be taken in maintaining the purity of the metal while' in the oxidizing process or small black spots will appear in the baked' enamel from impurities or may occur from particles of metal not having become oxidized. This chemical produces perfect opacity, antimony substitutes do not, unless sufficient be used to cause •other defects. The poisonous' character of antimony has also caused its use to be forbidden in many countries. Lead oxide lends a wonderful brilliance to enamels; it acts also as a flux of great power. The lead oxides were formerly used considerably on earthen ware utensils to assist the glaze, but long since they have been eliminated by law on account of their poisonous nature, their solubility render ing lhem extremely injurious when used on vessels for cooking, or even containing, stuffs. The use of the lead oxides is, there fore, restricted to the glazes of ornaments and art work.