Vitreous enamel, also known as porcelain enamel, is a type of glass coating. It is typically bonded to a metal or another enamel, then fired at around 850 degrees Fahrenheit (454 Celsius). Coating metal with vitreous enamel can prevent it from corroding, add a decorative flair and make it easier to clean. The inorganic coating provides resistance to abrasion and wear, making it a practical technology for cooking utensils and cookware, as well as jewelry and decorative accessories.
A powdery mixture, typically made of borax, quartz and feldspar, makes up a ground coat. A cover coat consisting of titanium dioxide, quartz and dehydrated borax completes the enamel powder. The powder is applied to a substrate, or, the material being used. Common substrates are metal, ceramic and glass.
The powdered glass is fused to the chosen material by firing, which melts the powder and coats the substrate. Vitreous enamel turns out smooth, durable and hard as glass. After firing, the powdered mixture can result in a transparent or opaque coating or in brilliant colors. Sometimes, a translucent quality is achieved.
The colors in vitreous enamel are obtained by adding minerals that come from cobalt, iron, praseodymium or neodymium. The minerals create varying shades of purples, reds and grays. New enamel colors cannot be mixed as with paint. The powder can be so finely ground that minute dots of two colors form what appears to be one color.
Vitreous enamel cannot burn but can shatter upon strong impact. Its durability offered a way to improve functional items throughout history. In the early 1900s, it was used on signage for advertisements. Oven walls, kitchen pots, bathtubs, sinks and the outsides of kitchen appliances benefited from vitreous enamel coatings. Commercially, farm silos and coal chutes began taking advantage of its long-lasting abrasion resistance.
Many artists admired the bright colors and began to make jewelry and decorative items. The Faberge eggs by Carl Faberge and the popular enamel boxes are two examples of enameling. The French used gold or copper wiring to separate areas, or cells, of color with vitreous enamel to create cloisonne pieces. French designers also popularized champleve pieces, or "raised fields" of metal around which the enamel is fired. The famous Limoges ceramic ware featured vitreous enamel paint used before firing.
Enameling was used by ancient Egyptians, who chose pottery and stone as substrates. Ancient Greeks and Chinese, Russian and Celtic artists used metal substrates. The Romans decorated glass vessels using enameling techniques.