ENAMELS are varieties of glass, gene rally opaque and colored, always formed by the combination of different metallic oxides, to which certain fixed fusible salts are added, such as the borates, fluates, and phosphates.
The simplest enamel, and the one which serves as a basis to most of the others, is obtained by calcining first of all a mix ture of lead and tin, in proportions vary ing from 15 to 50 parts of tin for 100 of lead. The middle term appears to be the most suitable for the greater number of enamels ; and this alloy has such an affinity for oxygen, that it may be cal cined with the greatest ease in a flat cast iron pot, and at a temperature not above a cherry red, provided-the dose of tin is not too great. The oxide is drawn off to the sides of the melted metal according as it is generated, new pieces of the alloy being thrown in from time to time till enough of the powder be obtained. Great care ought to be taken that no me tallic particles be left in the oxide, and that the calcining heat be as low as is barely sufficient ; for a strong fire frits the powder, and obstructs its subsequent comminution. The powder when cold is ground in a proper mill, levigated with water, and elutriated, as will be described, under Red lead. In this state of fineness and purity, it is called calcine, or flux, and it is mixed with silicious sand and some alkaline matter or sea-salt. The most ordinary proportions are, 4 of sand, 1 of sea-salt, and 4 of calcine. Chaptal states that he has obtained a very fine product from 100 parts of calcine, made by calcining equal parts of lead and tin, 100 parts of ground flint, and 200 parts of pure subcarbonate of potash. In either case, the mixture is put into a crucible, or laid simply on a stratum of sand, quicklime spontaneously slaked, or wood-ashes, placed under a pottery or porcelain kiln. This mass undergoes a semi-vitrification ; or even a complete fusion on its surface. It is this kind of frit which serves as a radical to almost every enamel; and by varying the pro portions of the ingredient, more fusible, more opaque, or whiter enamels are ob tained. The first of these qualities de pends on the quantity of sand or flux, and the other two on that of the tin.
The sea-salt employed as a flux may be replaced either by salt of tartar, by pure potash, or by soda ; but each of these fluxes gives peculiar qualities to the enamel.
A patent was granted to Thomas and Charles Clarke, of England, in 1639, for a method of enamelling or coating the in ternal surfaces of iron pots and sauce pans, in such a way as shall prevent the enamel from cracking or splitting off from the effects of fire. The specification prescribes the vessel to be first cleansed by exposing it to the action of dilute sul phuric acid. (sensibly sour to the taste) for three or four hours, then boiling the vessel in pure water for a short time, and next applying the composition. This consists of 100 lbs. of calcined ground flints ; 50 lbs. of borax calcined, and finely ground with the above. That mixture is to be fused and gradually cooled.
40 lbs. weight of the above product is to be taken with 5 lbs. weight of potter's clay ; to be ground together in water until the mixture forms a pasty-consis teneed mass, which will leave or form a coat on the inner surface of the vessel about one-sixth of an inch thick. When this coat is set, by placing the vessel in a warm room, the second composition is to be applied. This consists of 125 lbs. of white glass (without lead), 25 lbs. of borax, 20 lbs. of soda (crystals), all pul verized together and vitrified by fusion, then ground, cooled in water, and dried. To 45 lbs. of that mixture, 1 lb. of soda is to be added, the whole mixed together in hot water, and when dry, pounded ; then sifted finely and evenly over the internal surface of the vessel previously covered with the first coating or compo sition, while _this is still moist. This is the glazing. The vessel thus prepared is to be put into a stove, and dried at the temperature of 212° Fehr. It is then heated in a kiln or middle, like that used for glazing china. The kiln being brought to its full heat, the vessel is placed first at its mouth to heat it gradually, and then put into the interior of the infusion of the glaze. In practice it has been found advantageous also to dust the glaze powder over the fused glaze, and apply a second fluxing heat in the oven. The enamel, by this double application, becomes much smoother and sounder.