Simply put enamel is really colored glass composed of silicates. Glass, porcelain, earthenware, bricks, tiles and crockery are all silicates of varying composition. Silica, or sand, is colorless when pure. It has many peculiar qualities, one being that when heated with soda , magnesia, potash and lime or with oxides of lead or iron, it becomes a fused mass of glass or slag. Potash or soda plus lime in conjunction with silica in the proper proportions produces clear glass.
This is the process and materials necessary for the making of clear, colorless enamel called flux. Used alone over copper, flux results in a warm shade of tan. All of the colors, and there is no end to the number, are obtained by adding certain oxides of metals to this basic flux. Many of the rich colors used by the early limoges masters, and even those used by the Byzantine artists, were essentially the same as those used today. The deep transparent cobalt blue is made by mixing black oxide of cobalt with powdered flint glass. Other blues are obtained by using smaller proportions of oxide of cobalt. It is the one color people remember as an "enamel" color more than any other. Blue enamels are satisfactory over pure copper, but can't be compared to its brilliance and intensity over pure silver. Oxide of copper produces the turquoise shades as well as some of the grass greens. Red enamels, either opaque or transparent, are obtained from gold oxide. Other colors are produced by using the following oxides: platinum for soft grays, uranium and antimony for yellows, manganese for purples, oxides of tin for white, and iridium for a rich black. Opalescent enamels are compounded much the same as transparents, with the addition of more oxide of tin to increase the opacity.
Having combined the proper silicates with given amounts of oxides for coloring, the materials are placed into a melting furnace. after 15 hours of steady heat, a process known as fritting, the art enamel is usually poured onto cold iron slabs in pancake like forms.
If the craftsman prefers to grind the colors by hand, he may buy the frit in the unground , or chunk state. Commercially ground enamels are pulverized in large ball mills and then sifted or strained through sieves of various meshes. A convenient mesh for spatula work is 80 mesh. This means that the mesh or wire that the ground enamel is passed cross at the rate of 80 per square inch. 60 to 80 mesh is about as coarse as table salt. While 200 mesh would be like flour.