Page:The American Cyclopædia (1879) Volume VI.djvu/598

 590 ENAMELLING 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 por- traits and historical subjects. Other artists profited by his instructions, and several minia- ture 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 pres- ent century it reappeared in some fine por- traits by Augustin, and various French and English artists have since executed many por- traits in this style, distinguished for the bril- liancy of their colors, and the more valuable for their permanence. A piece of five 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 enam- el, 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 con- sulted 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 com- bined 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 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 re- move any particles of metal that may have escaped oxidation, as these would greatly im- pair 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 quan- tities of silica, saltpetre, and a little borax ; the last gives greater fusibility as its propor- tion is increased, and no more is used when the enamel is to be applied upon copper or silver than upon gold. The plates are some- times chemically acted on by the enamel, and if the gold of the gold plates is alloyed with too much copper, the appearance of the en- amel is injured. An excellent enamel is pre- pared 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 bichro- mate of potash being used instead. For ma- king colored enamels, either the opaque or transparent enamel serves as a base, and with it is melted a suitable proportion of some me- tallic oxide as a coloring matter; for a blue enamel, the opaque is used with oxide of co- balt; 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 im- purities. 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 com- mon salt. In the manufacture, of enamelled earthenware, the white enamel is prepared by melting 100 Ibs. of lead with 15 to 50 Ibs. of tin, and adding to the oxides thus obtained the same weight of quartz sand, and about 30 Ibs. 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 fur- nished 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