Vitreous enamel
In a discussion of art technology, enamel (or vitreous enamel or porcelain enamel in U.S. English) is the colorful result of fusing powdered glass to a substrate by firing, usually between 750 and 850 degrees Celsius. The powder melts and flows and hardens to a smooth, durable vitreous coating on metal, glass or ceramic. According to some sources, the word enamel comes from the High German word smelzan (to smelt) via the Old French esmail. Used as a noun, "an enamel" is a usually small decorative object, coated with enamel coating, such as a champleve or a cloisonne.

# History
Enamelling is an old and widely-adopted technology.  The ancient Egyptians applied enamels to pottery and stone objects.  The  ancient Greeks, Celts, Russians, and Chinese also used enameling processes on metal objects.
Enamelling was also used to decorate glass vessels during the Roman period, and there is evidence of this as early as the late Republican and early Imperial periods in the Levantine, Egypt, Britain and the Black Sea. Enamel powder could be produced in two ways; either through the powdering of colored glass, or the mixing of colorless glass with colorants such as a metallic oxide . Designs were either painted freehand or over the top of outline incisions, and the technique probably originated in metalworking.Once painted, enamelled glass vessels needed to be fired at a temperature high enough to melt the applied powder, but low enough that the fabric of the vessel itself was not melted. Production is thought to have come to a peak in the Claudian period and persisted for some three hundred years, though archaeological evidence for this technique is limited to some forty vessels or vessel fragments
From more recent history, the bright, jewel-like colors have made enamel a favored choice for designers of jewelry and bibelots, such as the fantastic eggs of Peter Carl Faberge, enameled copper boxes of Battersea enamellers, and artists such as George Stubbs and other painters of portrait miniatures.  Enameling was a favorite technique of the Art Nouveau jewellers.

# Properties
Enamel powder often is applied as a paste, and may be transparent or opaque when fired; vitreous enamel can be applied to most metals. It has many excellent properties: it is smooth, hard, chemically resistant, durable, can assume brilliant, long-lasting colors, and cannot burn. Its disadvantages are its tendency to crack or shatter when the substrate is stressed or bent. Its durability has found it many functional applications:  early 20th century advertising signs, interior oven walls, cooking pots, exterior walls of kitchen appliances, cast iron bathtubs, farm storage silos, and processing equipment such as chemical reactors and pharmaceutical chemical process tanks.
Color in enamel is obtained by the addition of various minerals, often metal oxides cobalt, praseodymium, iron, or neodymium.  The last creates delicate shades ranging from pure violet through wine-red and warm gray.  Enamel can be either transparent, opaque or opalescent (translucent), which is a variety that gains a milky opacity the longer it is fired. Different enamel colors cannot be mixed to make a new color, in the manner of paint.  This produces tiny specks of both colors; although the eye can be tricked by grinding colors together to an extremely fine, flour-like, powder.

# Techniques of Enamelling
- Basse-taille, from the French word meaning "low-cut".  The surface of the metal is decorated with a low relief design which can be seen through translucent and transparent enamels.
- Champleve, French for "raised field", where the surface is carved out to form pits in which enamel is fired, leaving the original metal exposed.
- Cloisonne, French for "cell", where thin wires are applied to form raised barriers, which contain different areas of enamel applied above the original metal form.
- Painted enamel, a design in enamel is painted onto a smooth surface.  Grisaille and Limoges enamel are subategories of painted enamel.
- Grisaille, French term meaning "greying", where dark, often blue or black background is applied, then limoges (Limoges porcelain) or opalescent (translucent) enamel is applied on top, building up designs in a monochrome gradient, paler as the thickness of the layer of light color increases.
- Limoges enamel, made at Limoges, France, the most famous European centre of vitreous enamel production.
- Limoges porcelain, named after the town in France where it was invented, is the technique of "painting" with a special enamel called "blanc de limoges" over a dark enamelled surface to form a detailed picture, often human figure. It is a form of Grisaille.
- Plique-a-jour, French for "braid letting in daylight" where the enamel is applied in cells, similar to champleve, but with no backing, so light can shine through the transparent or translucent enamel. It has a stained-glass like appearance.
- Ronde bosse, French for "round bump".  A 3D type of enameling where a sculptural form is completely or partly enameled.
- Stenciling, where a stencil is placed over the work and the powdered enamel is sifted over the top.  The stencil is removed before firing, the enamel staying in a pattern, slightly raised.
- Sgrafitto, where an unfired layer of enamel is applied over a previously fired layer of enamel of a contrasting color, and then partly removed with a tool to create the design.
- Counter enameling, not strictly a technique, but a necessary step in many techniques, is to apply enamel to the back of a piece as well - sandwiching the metal - to create less tension on the glass so it does not crack.

# "Enamel" paint
Some paints are called "enamel paints".  This is a commonly used, yet fanciful term, implying that an ordinary latex or oil-based paint has the same properties as true, fired enamel.
Bicycle frames and similar steel objects are traditionally stove enamelled in countries such as the UK.  The paint is baked on but the temperatures are much lower than for true vitreous enamel - approximately 200 degrees Celsius.  The process should not be confused with powder coating as the enamel paint is sprayed on "wet".