Enamelling, the art of applying a coating of vitreous substance called enamel to a surface of glass or of metal, and causing it to adhere by fusion. In its homeliest application it is a sort of glazing, and as applied by modern methods to ornament and protect surfaces of cast or wrought iron, it may be considered simply a process of japanning. The facility with which colors might be introduced in vitreous compounds or applied to them, and become fixed by a subsequent fusion or baking, made the art in early times exceedingly popular, and in the middle ages it attained a higher rank even than it now holds, as one of the fine arts. The ancient Persians and Arabians appear to have practised it upon earthenware and porcelain; and the mode of coloring this ware at the present day is properly a process of enamelling. Articles of pottery enamelled in colors are found among the ruins of ancient Thebes, and in many of the cities of Egypt are buildings constructed of enamelled bricks taken from the ruins of older cities.

Wilkinson says: "It has been questioned if the Egyptians understood the art of enamelling upon gold or silver, but we might infer it from an expression of Pliny, who says, ' The Egyptians paint their silver vases, representing Anubis upon them, the silver being painted and not engraved;' and M. Dubois had in his possession a specimen of Egyptian enamel." From the Egyptians the art is supposed to have passed to the Greeks, and afterward to the Romans. Brongniart, however, in his Traite des arts ce-ramiques, traces its introduction into Italy from the Balearic isles by the Spaniards, who derived the art from the Arabs. The Romans introduced it into Great Britain, as appears from various enamelled trinkets that have been dug up there with other vestiges of the Roman conquerors. That the Saxons practised the art appears from an enamelled jewel found in Somersetshire, and preserved at Oxford, which according to its inscription was made by direction of the great Alfred. The gold cup given by King John to the corporation of Lynn in Norfolk shows, by the colored enamelled dresses of the figures with which it is embellished, that the Normans also practised the art. Among the Gauls enamelling upon metallic surfaces is understood to have been in use in the 3d century.

As practised upon earthenware in the style called by the French faience commune ou emaillee, and by the Italians majolica ware, it was carried to great perfection in the 16th century at Castel Durante and at Florence by the brothers Fontana d'Urbino. Other Italian cities adopted the art, and Faenza became famous for the works of Guido Selvaggio. The high style of art attained hardly outlived the artists who perfected it, and from 1560 it gradually deteriorated. Bernard Palissy, by practice of 25 years directed to the production of a cup like one of great beauty shown to him, sought to introduce the art in France, and his works became very famous, but his method died with him. His productions were interesting from being true copies of natural objects, in relief, and colored with exact faithfulness. Of late years the art has been revived in France, chiefly through the skill of Bron-gniart; and in Berlin also beautiful work of the kind has been executed by Feilner. - Painting in enamel, as practised upon plates of gold and copper, can hardly be regarded as applied to works of high art until the 17th century. Jean Toutin, a goldsmith at Chateaudun, appears about the year 1630 to have first made enamels of fine opaque colors, and applied them to portraits and historical subjects.

Other artists profited by his instructions, and several miniature painters attained great distinction in this branch. The art afterward fell into disuse, and was only applied to ornamenting watch cases and rings. In the early part of the present century it reappeared in some fine portraits by Augustin, and various French and English artists have since executed many portraits in this style, distinguished for the brilliancy of their colors, and the more valuable for their permanence. A piece of live inches in its longest dimensions was considered the largest that could be undertaken with safety; for with the increase of size the liability of injury to the enamel by cracking, and to the plate by swelling and blistering in the several processes of baking, rapidly increased; but by backing the metallic plate with one of porcelain, the work is now executed in pieces even 18 inches long by nearly as great a breadth. The process is usually conducted as follows : The plate is coated on both sides with white enamel, and on this the design is lightly sketched with a pencil. The colors, finely ground and mixed with oil of spike, are then laid on as in miniature painting.

By gentle heat the oil is evaporated, and in an enameller's fire the plate is next made red-hot to incorporate the colors with the enamel. The painting may then be retouched, and the colors again be burned in, and this may be repeated several times. But the greatest accuracy in the first drawing and coloring is essential for a perfect picture. In this department may be consulted the work of Count de Laborde, Notice des emaux exposes dans les galeries du Louvre. - In the ordinary processes of enamelling, the enamels used for the ground are opaque, and must bear a higher degree of heat without fusing than the colored enamels, which are afterward melted into them. They are made after a great variety of recipes, according to their uses. All those designed for metallic surfaces have a transparent base, which is rendered opaque by the substitution of combined oxide of lead and oxide of tin, in the place of the oxide of lead used as one of its ingredients. Five different mixtures of the two oxides are in use, the proportions varying from 3 1/2 parts of lead and 1 of tin to 7 parts of lead and 1 of tin. The two metals are melted together, and the combined oxide is removed as fast as it appears upon the surface.

When the oxidation is as thoroughly effected as practicable, the product is well washed to remove any particles of metal that may have escaped oxidation, as these would greatly impair the quality of the enamel; for the same reason it is essential that the metals themselves should be absolutely pure. One or other of the mixtures of oxides obtained by the method described is next melted with proper quantities of silica, saltpetre, and a little borax; the last gives greater fusibility as its proportion is increased, and no more is used when the enamel is to be applied upon copper or silver than upon gold. The plates are sometimes chemically acted on by the enamel, and if the gold of the gold plates is alloyed with too much copper, the appearance of the enamel is injured. An excellent enamel is prepared by mixing 30 parts of saltpetre, 90 of silica, and 250 of litharge; after fusion it is white, and may be used in taking photographs without collodion - gum, honey, and bichromate of potash being used instead.

For making colored enamels, either the opaque or transparent enamel serves as a base, and with it is melted a suitable proportion of some metallic oxide as a coloring matter; for a blue enamel, the opaque is used with oxide of cobalt; for a green, oxide of chromium, or bi-noxide of copper; for a violet, peroxide of manganese; for a yellow, chloride of silver; for a purple, purple of Cassius; and for a black, the transparent enamel is used with mixed oxides of copper, cobalt, and manganese. The different enamels, being prepared beforehand, are crushed to powder when wanted for use, and then kept at hand under water in vessels well covered to protect them from all impurities. The metallic surfaces to be coated are cleaned by boiling in an alkaline solution, and are then washed with pure water. The copper alloy in gold may be dissolved from the surface by boiling in a strong solution of 40 parts of saltpetre, 25 of alum, and 35 of common salt. - In the manufacture of enamelled earthenware, the white enamel is prepared by melting 100 lbs. of lead with 15 to 50 lbs. of tin, and adding to the oxides thus obtained the same weight of quartz sand, and about 30 lbs. of common salt.

The whole being well rubbed together is melted; and though it may appear dark, it becomes white when reduced to powder and baked upon the utensils. The proportions of the materials employed are very variable, and other ingredients are often introduced, particularly oxide of manganese, the effect of which in small quantity is to yield its oxygen to any carbonaceous impurities that may be present, and remove these in the form of carbonic acid. The colored enamels are applied by painting them when finely ground, and mixed with some vegetable oil, upon the white enamel, either before or after this has been once heated, and then baking them in. The ovens for metallic articles are muffles made to slide closely into the furnace, and furnished with a small aperture through which the progress of the operation may be observed. - The enamelling of cast-iron cooking utensils was practised at the close of the last century, and a number of different mixtures have since been in use. The use of lead must be carefully avoided in articles of this kind.

Vessels of wrought iron are also treated by the same process; and iron pipe for conveying water is advantageously protected by a clean silicious enamel not liable to affect the purity of the water. - The process employed for many years by the Messrs. Clarke of England consisted in the use of the following composition and method: 100 lbs. of calcined ground flints and 50 lbs. of borax calcined and finely ground, to be mixed, fused, and gradually cooled. Of this, 40 lbs. are mixed with 5 lbs. of potters' clay, and ground in water to a pasty mass. The vessel, first thoroughly cleaned, is lined with a coating of this about one sixth of an inch thick, and left for it to harden in a warm room. A new coating is next added, prepared from 125 lbs. of white glass without lead, 25 lbs. of borax, 20 lbs. of soda in crystals which have been pulverized and fused together, ground, cooled in water, and dried. To 45 lbs. of this 1 lb. of soda is added, the whole mixed in hot water, dried and pounded. A portion of it is sifted over the other coating while it is still moist, and dried in a stove at the temperature of boiling water. The vessel is then heated in a stove or muffle till the glaze fuses.

It is taken out, more glaze powder is dusted on the glaze already in fusion, and it is again subjected to heat. The Polytechnisches Centralblatt for 1872 recommends the following enamel for copper cooking vessels: 12 parts of white fluor spar, 12 of gypsum, and 1 of borax, finely powdered, and fused in a crucible; when cold the mass is again pulverized, and made into a paste with water, laid upon the metal, and fused. Small articles of enamel, as little toys imitating the figures of birds, etc, and also artificial eyes, are made by melting with the table blowpipe rods or tubes of enamel prepared for this purpose, and shaping them by hand, just as the glass blower works with tubes and rods of glass. - Enamelling of slates to imitate marble and malachite was introduced in London by Mr. G. E. Magnus. The art was first practised in the United States at Boston, and slates from Wales were imported for this purpose. Subsequently the slates of the Lehigh river were applied to this use in Lehigh co., Pa., and were also sent to Philadelphia to be there enamelled.

In Vermont the business is now carried on at West Castleton, where are extensive quarries of slate, and an establishment of the same kind is in operation in New York. A great variety of useful articles are produced, among which the most important are billiard tables, mantels, tubs for bathing, sinks, etc. The slates are sawed to proper shape, planed to uniform thickness, and rubbed smooth with polishing stones. The ground color adapted to the marble it is designed to imitate is then laid on, and after this the variegated colors. The slab is then placed in an oven heated to 200°, and allowed to remain over night. In the morning after cooling it receives a coat of varnish, and is returned to the oven till the next day. Other heatings and varnishings alternately succeed, with rubbing with pumice stone; and a final polishing with pumice stone, rotten stone, and the hand completes the process.