The Ironmonger contains the following account of the improvements which have been made in this important branch of industry: Since the first introduction of this manufacture, now more than twenty years ago, and originating, if we are not in error, in Staffordshire, very considerable improvements have been made, both in the quality of the glassy covering and in the thinness and evenness of its application, and the force of adherence given to it, as applied upon the metallic surface, whether of cast or wrought iron. At first the coating was very thick, not far short of an eighth of an inch, very rough, and mammillated, far from free from minute cracks, and becoming discolored with rapidity over the fire, and quickly acted upon with acids, such as are used even in cookery. In fact, the glaze was a very fusible glass, so rich in lead and oxide of tin, and the temperature of applicationwas so low, that adhesion was imperfect. Some promising improvements have been comparatively recently made, but unfortunately only partially described. M. Ballouhey has described (in Wagners Jahres-bericht, 1868) his method of enameling cast and wrought iron. The enameling, or glazing, by known methods of cast and wrought iron has been now for some time introduced, and the applied process consists in covering the previously well-cleaned surface of the metal when strongly heated with an easily fusible enamel or glass in the state of fine powder, so as to melt it upon-the metallic surface, or burning-in the laid-on enamel or glass in a muffle or oven, whereby generally a better adherence is produced. A mixture of strongly basic silicates is used, so that this enamel cover cannot be very durable, but is easily affected by acid and saline liquids. In order to protect the iron in a more durable and complete manner, M. Ballouhey proposes a process quite different from this usual one. lie brings the surface of the metal, which is provided with a protecting covering, in contact with a mixture of such substances as are used for the manufacture of ordinary white glass, and then heats the whole till the temperature of vitri faction is reached. The iron in thiscase oxidizes superficially. The produced oxide of iron, probably having chiefly the composition of forge scales, combines with the silicic acid of the glass, and an iron glass sub-coat of silicate of iron is obtained, which forms with the metal beneath one body. This protecting cover may be made either thin or thick; it is, however, preferable to make it only very thin, because it will then better resist inequalities of expansion without cracking or scaling off. Trials are said to have been made to glaze, according to this method, iron plates to be used for covering iron-built vessels, in order to protect the iron against the influence of the air and sea-water, and against the attacks of marine molluscs and the growth of plants. Now, although we are disposed to believe this last method embraces a real advance in enameling- of iron; we are not so sure about the continued adhesion of the glaze; and, in any case, we pronounce the imagined application of such enameled sheets to the sheathing of iron-built ships a perfect delusion. The ingenious thought of so modifying the nature of the glaze, that the high heat needed to flux it shall supra-ficially cover the previously cleaned face of the iron with an excessively thin coat of black oxide before the glaze has had time to melt, and the employment of a glass of such a nature as shall seize upon the outer face of this thin parencliyme of oxide, and convert it partially into silicate, so that, as a result, this coat shall act as a tertium quid, and a solder, in fact, between the metallic iron and the external glass of the glaze, is very probably found not difficult to realize. But the final success in producing a really durable enamel on a metal plate depends upon the fulfillment of one or other of two con ditions: Either the glaze must be more or less flexible and extcnsiUe in all directions, so as to give with the expansion and contraction of the metal—just as the japan coating does upon an iron tea-tray; or glaze the must have a cc-efficient of expansion within the range of temperature to which the enameled article is to be exposed, precisely the same as that of the metal, iron, or cast iron, etc., which it covers. Precisely, we say, for the extensibility of any glass of whatsoever composition is so small, so nearly insensible, in fact, that unless so, the cracking, and finally parting company of the glaze from the metal surface is only a matter of a short time and the degree of change of temperature undergone. Now, this will not be nullified by any more effectual soldering of the glaze to the face of the metal, by a thin interposed film of forge scale oxide, which, perhaps, passes insensibly, though through a very shallow range in deptk of the iron, from (FeO-f-Fe2O3) on the outside to metallic iron in the interior, and from a basic glass of earthy and alkaline bases outside all, into silicate of iron, and of earthy bases at the junction with the glaze. Let the glaze itself expand more or less than does the iron, and it must and will split off sooner or later. The coating of glaze, if it cannot get loose at the metallic face, wi 11 do so in some stratum above that, slitting through the silicate of iron, or the glaze itself somewhere, and in planes more or less approximating to thosi of the iron plate. The first condition for success, then, in improving this manufacture, is to be found in the production of a glaze of exactly equal co-efficient of expansion with the iron; and for this end nothing would suffice, or be so rapid in yielding practical fruit, as a train of exact experiments upon the expansion of long rods of enamel of various, but known, composition, all being of such a nature as otherwise to permit their being fused on to iron. The material once obtained as respects this point, adhesion might be procured from glasses of probably very various composition, and from others than such basic glasses as require the production of the coat of oxide on the iron, or silicates of iron as the cementing inter-coat. But yet it seems to us that this silicate of iron coat promises well, and would be advisably kept to. The main difficulty in every case, hitherto, has been to get the glazed coating fused and spread, without so far oxidizing the iron, by the prolonged and high heat to which it is exposed, that when again cold, the coating shall not fall oflj bringing a thin scale of oxide of iron along with it. Now, it appears to us that this might be almost complete-ly avoided, and all difficulty as to oxidization of the iron a voided, and j ust as much or as little oxidization as we require given to it, by burning-on the glaze in one of Siemens ges furnaces. We can there employ a pure, oxidizing, neutral, or a deoxidizing flame to heat up and nearly bring to tho fusing point the material spread on the iron for the glazf; when the latter approaches incipient fusion, we can use tho oxidizing flame, and so just get that extremely thin film of hammerscMag, or black oxide (like that on the surface of the sheets of fine Russian sheet iron of commerce) which we here want. Or possibly, sheets already oxidated up to that point might be put at once into such a furnace, and by a pun1 neutral flame the glaze be burnt on, i.e, fused and caused tu adhere. Many useful applications for this ware, were it CJ:;V perfect as a manufacture, might be found. For lining tho inUrior of water pipes and cisterns, it was proposed and tried in tliis country before 1852, but failed because the necessary conditions for durable enamel did not exist. Upon tho ouf er surface of thin iron roofing tiles it may be a valuable addition, and might enable iron slating or tiling to be used in damp countries as well as at present in dry ones, and wills i lie advantage of having brilliant and party col ration through the erauiel given to the tiling. So far no real use has born made of enameled iron ware but for culinary or. like vessels, and tlie manufacture of these in France and Belgium, an the Exhibition at Paris of last year showed, yields a far better and handsomer product than ours in England. Enameled cast-iron name plates, or single letters, for streets, etc., were proposed at least thirty years ago, or even earlier, and have been employed greatly in Germany and 155 France, and to gome extent in Great Britain. So far as OUT j observation goes, they have nowhere proved permanent; frost and oxidation are rival destroying agents. Wherever such enameled iron plates are to be seen, after some years of ex- posure, tliey are more or less defaced. j The labels of the streets of Paris, we believe, are lettered upon plates of porcelain. The application of glass-faced plates, however, to sheathing ships is perfectly Utopian. Ships iron sheathing, like that ef copper or its alloys, must not only sustain more or less fully the chemical action of the sea-water and air, but the mechanical friction of the water when the ship moves, and all the violent and constant accidental abrasions to which its surface is subject in the Daijrating of every ship. Enameled iron cannot do either; and as to fouling, the glazing itself is a first-rate nidus for the deposit of marine vegetable and animal organisms, quite as good, if not better, than the coat of rust, which is the usual and indispensable forerunner of fouling. If iron ships are not to be sheathed with certain suitable copper alloys (copper alone is mo3t suitable), insulated by wood from the iron body of the ship, then sheath with wood alone; but whatever is to be employed, enameled kon ought not to be.
This article was originally published with the title "Improvements in Enameling Iron Ware" in Scientific American 20, 10, 154-155 (March 1869)