liability of iron to oxidation by heat or moisture, and to cor rosion even by the weakest acids, has led to many attempts to coat it with a protecting surface. Ordinary tin plate is the oldest and most familiar example of a partially successful method. Since the beginning of the present century, many attempts have been made to cover iron with a vitreous surface, and several patents have been taken for such methods of enameling. The chief difficulty in applying enamels to iron arises from the tendency of the metal to oxidize before it reaches the temperature at which the E. fuses, and to become brittle from the oxide combining with the silica of the enamel. This action being superficial, the mischief is the greater in proportion to the thinness of the iron. Therefore it is much easier to E. thick cast-iron vessels than thin vessels made of sheet-iron. A glass may be made by combining either silicic acid or boracic acid with a base; the latter fuses at a lower temperature than the former, but the glass is much dearer and not so durable as the silica glass. The enamels used for coating iron consist of a mixture of silica and borax, with various basic substances, such as soda, oxide of tin, alumina, oxide of lead, etc.
The best E. for such purposes with which we are acquainted, is that patented by C. H. Paris, and applied by Messrs. Griffiths and Browett of Birmingham. It consists of 130 parts of flint-glass powdered, 20 parts of carbonate of soda, 12 of boracic acid. These are fused to form a glass, then reduced to a very fine powder; the article to which they are to be applied is carefully cleaned with acid, then brushed over with gum water, and the powder dusted upon it. The gum water is merely to cause adhesion. This coating is then carefully dried, and heated just to the point at which the powdered glass will fuse, and by miming together, coat the surface. The E. is generally put on in two
separate layers or coatings, the first being dull gray, and the second or outer one of some brighter color. For sheet-iron hollow-ware, the latter is usually white, upon which a pattern is often printed with transfer paper by the process in use for earthen ware (see PorrittyWIlhesu light enameled iron vessels, from their' being practically indestructible (except when used for cooking), are made in large numbers for use on board ships, and for colonial markets. A great variety of articles, many of them beauti fully decorated in colors, such as grate-fronts, cloak-dials, panels of different kinds, tablets, and name-plates, are now executed in enameled iron at a comparatively moderate cost. It is also applied to corrugated roofing. Clarke's and other patent enamels have been successfully applied to saucepans, small cisterns, pipes, and other articles of cast iron. See HOLLOW-WARE.
The action of sudden heat is to expand the metal more than the E., and cause the latter to peel off. Acids find their way through minute invisible pores, which exist in the best E.; and when once they reach the iron, they rapidly spread between it and the E., and undermine and strip it off. This kind of action is curiously shown by filling an enameled vessel with a solution of sulphate of copper. The acid attacks the iron wherever pores exist, and little beads of metallic copper are deposited at all such spots; these beads go on growing until they are large enough to be very plainly seen. This is the severest test for trying the continuity of enameled surfaces, to which they can be subjected, as sulphate of copper will penetrate the glaze and body of ordinary earthen ware.