Vitreous enamel

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Cloisonné enamel plaque, Byzantine, ca. 1100

In a discussion of art technology, enamel (or vitreous enamel, or porcelain enamel in American English) is the colorful result of fusion of powdered glass to a substrate through the process of firing, usually between 750 and 850 degrees Celsius. The powder melts and flows to harden as a smooth, durable vitreous coating on metal, glass or ceramic. It is often applied in a paste form and may be transparent or opaque when fired. Vitreous enamel can be applied to most metals.

Also, an "enamel" is a decorative object, usually very small, having an enamel coating, such as a piece of champlevé or cloisonné.

Vitreous enamel has many excellent properties: it is smooth, hard, chemically resistant, durable, can take on long-lasting, brilliant colors, and cannot burn. Disadvantages are its tendency to crack or shatter when the substrate is stressed or bent.

The durability of enamel has given it many functional applications, including: early 20th century advertising signs, interior walls of ovens, speckleware cooking pots, exterior walls of high quality kitchen appliances, cast iron bathtubs, storage silos on farms and process equipment such as chemical reactors and tanks for the chemical and pharmaceutical process industries.

Modern German enamel street sign

Enamelling is an old and widely-adopted technology. The ancient Egyptians applied enamels to pottery and stone objects. Other practitioners include the ancient Greeks, Celts, Russians, and the Chinese.

The bright, jewel-like colors have also made enamel a favored choice for designers of jewelry and bibelots, such as ancient beads, the fantastic eggs of Peter Carl Fabergé, 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.

According to some sources, the word enamel comes from the High German word smelzan (to smelt) via the Old French esmail. Some techniques of enameling:

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.
Champlevé, French for "raised field", where the surface is carved out to form pits in which enamel is fired, leaving the original metal exposed.
Cloisonné, French for "cell", where thin wires are applied to form raised barriers, which contain different areas of enamel applied above the original metal form.
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 porcelain, named after the town in France where it was invented, is the technique of "painting" with an especial 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-à-jour, French for "braid letting in daylight" where the enamel is applied in cells, similar to champlevé, 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.

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.

[edit] "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".

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Enamel

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