PORCELAIN ENAMELLING. Enamelling on metal is a very ancient art and has already been dealt with under the head ing "ENAMEL" (q.v.). The object of the present article is to describe the application of enamel in modern times to articles of domestic use, such as culinary utensils, baths, wash basins, etc.
Enamel is a vitreous substance fusible at a lower temperature than the metal to which it is applied, and while there are many variations in the constitution of enamels they are all naturally similar in character, consisting of silica, alkali and certain metal lic oxides.
Early in the 19th century, in Germany, enamel was applied to the interior of cooking pots ; but the enamels were subject to serious defects—firstly because their power of adhesion to the iron was weak, and secondly, because poisonous substances such as lead and arsenic were used in their composition. Very soon it was recognized that such an enamel was impossible for cooking utensils, and later on these poisonous elements were eliminated and a non-poisonous enamel was brought into general use. In 1839 the firm of T. and C. Clark, of Wolverhampton, obtained a patent for the application of a non-poisonous enamel to cast-iron vessels. The nature of the invention was not published, and very soon Messrs. Kenrick, their rivals in trade, entered into competition with them and legal proceedings ensued. In the course of the ac tion it transpired that enamelled cooking vessels had been made and sold in small quantities by the Eagle foundry, Birmingham, prior to 1839, and thereafter the contestants came to an agree ment to abandon the action and to maintain the patent ; which was kept in force until 1853. During that period both firms did a large business in enamelled cooking vessels—and later certain Scottish founders entered into the trade, so that enamelled pots and pans came into general use.
The chief constituents of the enamel used were silica, borax, and the carbonate of sodium—or the nitrate of sodium or potas sium—with probably a percentage of tin oxide as an opacifier, or whitener, to take the place of arsenic which had been used with the same object in the earlier enamels.
In the application of enamel it is necessary that the surface of the casting should be thoroughly cleaned. In the case of cook ing pots it was customary to turn the interior of the pot in a turning-lathe and so secure a perfectly suitable surface for enamel ling. In other types of articles the surface was cleaned by "pick ling" in an acid bath, or by sandblasting (the latter method became universal).
Perhaps the most important articles to which this so-called porcelain enamel has been applied are cast-iron baths. Several ironfounders, about the middle of the i9th century, made cast iron baths—but for the most part they were coated with what was technically called "metallic enamel" (which was merely sev eral coats of varnish paint dried in a stove and polished by hand to a smooth surface). This, naturally, was not very durable, and ironfounders who had been enamelling cooking pots, started to apply the same enamel to baths. This involved the building of large muffle furnaces for heating the baths, and mechanical ap pliances for handling them. This was done by a few firms on the continent of Europe and in England, but the product was not very good, and the sale was small. It was afterwards taken up by the Scottish founders and since then has made steady progress. It is common knowledge that the demand for baths has increased amazingly during the past 4o years, and now ironfounders all over the world are making porcelain enamelled baths in great quan tities. In America as recently as 1885, for example, no cast-iron baths were made, but they had succeeded in enamelling wash basins, sinks, and other such vessels. A director of one of the largest manufactories of such articles went to Scotland to in vestigate the Scottish methods of casting and enamelling baths. His firm purchased some castings, succeeded in enamelling them and thereafter proceeded with the installation of plant for the production of cast-iron baths. One American firm has now the largest output in the world, its production being approximately 1,500 baths per day. The total production of porcelain enamelled cast-iron baths throughout the United States in 1928 probably exceeded soo,000.
The German manufacturers have developed very largely their production of cast-iron enamelled baths, and now export them all over the world. Cheap labour has enabled them to produce at a very low cost, and to compete successfully with the manu facturers in other countries.
Enamelled cast-iron baths are also being produced in Australia, and in some parts of South America.
Every manufacturer has his own mixture of ingredients for his enamel—and, generally speaking, guards his secret well. Typical formulas may be found in Technologic Paper No. 142, U.S. Bureau of Standards, by Homer F. Staley.
Sheet-iron enamelling differs from cast-iron enamelling in this respect, that the first coat is burned on and the article then taken out to cool; when cold the final coat is brushed on, not powdered, and the piece is again put into the furnace to finish. There are many niceties of detail in the process that can only be gained by experience, and every enameller finds it necessary to work out these details for himself. (W. SH.)