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Vitreous enamel, also porcelain enamel in US English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C (1,382 and 1,562 °F). The powder melts, flows, and then hardens to a smooth, durable vitreous coating on metal, or on glass or ceramics. The term "enamel" is most often restricted to work on metal, which is the subject of this article. Enameled glass is also called "painted". Fired enamelware is an integrated layered composite of glass and metal. The word enamel comes from the Old 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. Enameling is an old and widely-adopted technology, for most of its history mainly used in jewelry and decorative art. Since the 19th century the term applies also to industrial materials and many metal consumer objects, such as some cooking vessels, dishwashers, laundry machines, sinks and tubs, etc. "Enamelled" and "enamelling" are the preferred spellings in British English, while "enameled" and "enameling" are preferred in American English.
The ancient Egyptians applied enamels to pottery and stone objects, and sometimes jewelry, though the last less often than other ancient Middle Eastern cultures. The ancient Greeks, Celts, Georgians, and Chinese also used enamel on metal objects.
Enamel 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 by powdering colored glass, or by mixing colorless glass powder with pigments 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, enameled glass vessels needed to be fired at a temperature high enough to melt the applied powder, but low enough that 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.
Enamel was at its most important in European art history in the Middle Ages, beginning with the Late Romans and then the Byzantines who began to use cloisonné enamel in imitation of cloisonné inlays of precious stones. This style was widely adopted by the "barbarian" peoples of Migration Period northern Europe. The Byzantines then began to use cloisonné more freely to create images, which was also copied in Western Europe. The champlevé technique was considerably easier, and very widely practiced in the Romanesque period. In Gothic art the finest work is in basse-taille and ronde-bosse techniques, but cheaper champlevé works continued to be produced in large numbers for a wider market.
From Byzantium or the Islamic world the cloisonné technique reached China in the 13-14th centuries; the first written reference is in a book of 1388, where it is called "Dashi ('Muslim') ware". No Chinese pieces clearly from the 14th century are known, the earliest datable pieces being from the reign of the Xuande Emperor (1425–35), which however show a full use of Chinese styles suggesting considerable experience in the technique. It remained very popular in China until the 19th century, and is still produced today. The most elaborate and highly-valued Chinese pieces are from the early Ming Dynasty, especially the reigns of the Xuande Emperor and Jingtai Emperor (1450–57), although 19th century or modern pieces are far more common. The Japanese also produced large quantities from the mid-19th century, of very high technical quality.
From more recent history, the bright, jewel-like colors have made enamel a favored choice for designers of jewelry and bibelots, such as the 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 jewelers. A resurgence in enamel-based art took place near the end of the 20th Century in the Soviet Union led by artists like Alexei Maximov and Leonid Efros. In Australia, abstract artist Bernard Hesling brought the style into prominence with his variously sized steel plates.
Enamel was first applied commercially to sheet iron and steel in Austria and Germany in about 1850. Industrialization increased as the purity of raw materials increased and costs decreased. The wet application process started with the discovery of the use of clay to suspend frit in water. Developments that followed during the 20th Century include enameling-grade steel, cleaned-only surface preparation, automation, and ongoing improvements in efficiency, performance, and quality.
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Enamel may be transparent or opaque when fired; vitreous enamel can be applied to most metals. The great majority of modern industrial enamel is applied to steel in which carbon content is controlled to prevent reactions at the firing temperatures. Enamel can also be applied to copper, aluminium, stainless steel, cast iron or hot rolled steel, as well as gold and silver.
Vitreous enamel has many excellent properties: it is smooth, hard, chemically resistant, durable, scratch resistant (5-6 on the Mohs scale), long-lasting color fastness, easy-to-clean, and cannot burn. Enamel is glass, not paint, so it does not fade with UV light. Its disadvantages are its tendency to crack or shatter when the substrate is stressed or bent, but modern enamels are relatively chip and impact resistant because of good thickness control and thermal expansions well-matched to the metal. The Buick automobile company was founded by David Dunbar Buick with wealth earned by his development of improved enameling processes, circa 1887, for sheet steel and cast iron. Such enameled ferrous material had and have many applications: early 20th century and some modern advertising signs, interior oven walls, cooking pots, housing and interior walls of major kitchen appliances, housing and drums of clothes washers and dryers, sinks and tubs cast iron bathtubs, farm storage silos, and processing equipment such as chemical reactors and pharmaceutical process tanks. Structures such as filling stations, bus stations and Lustron Houses had walls, ceilings and structural elements made of enameled steel. One of the most widespread modern uses of enamel is in the production of quality chalk-boards and marker-boards (typically called 'blackboards' or 'whiteboards') where the wear and chemical resistance of enamel ensure that 'ghosting' or unerasable marks will not occur, as with polymer boards. Since standard enameling steel is magnetically attractive, they may also be used as magnet boards. Some new developments in the last ten years include enamel/non-stick hybrid coatings, sol-gel functional top-coats for enamels, enamels with a metallic appearance, and new easy-to-clean technologies.
The key ingredient of vitreous enamel is a highly friable form of glass called frit. Frit is typically an alkali borosilicate chemistry with a thermal expansion and glass temperature suitable for coating steel. Raw materials are smelted together between 2,100 and 2,650 °F (1,149 and 1,454 °C) into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit.
Color in enamel is obtained by the addition of various minerals, often metal oxides cobalt, praseodymium, iron, or neodymium. The latter creates delicate shades ranging from pure violet through wine-red and warm gray. Enamel can be transparent, opaque or opalescent (translucent), which is a variety that gains a milky opacity with longer firing. 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.
There are three main types of frit, usually applied in sequence. A ground coat is applied first; it usually contains smelted-in transition metal oxides such as cobalt, nickel, copper, manganese, and iron that facilitate adhesion to steel. Next, clear and semi-opaque frits that contain little coloring material for producing colors are applied. Finally, a titanium white cover coat frit, supersaturated with titanium dioxide which creates a bright white color during firing, is applied as the exterior coat.
After smelting, the frit needs to be processed into one of the three main forms of enamel coating material. First, wet process enamel slip (or slurry) is a high solids loading product of grinding the frit with clay and other viscosity-controlling electrolytes. Second, ready-to-use (RTU) is a cake-mix form of the wet process slurry that is ground dry and can be reconstituted by mixing with water at high shear. Finally, electrostatic powder that can be applied as a powder coating is produced by milling frit with a trace level of proprietary additives. The frit may also be ground as a powder or into a paste for jewelry or silk-screening application.
Techniques of artistic 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. The 14th century Royal Gold Cup is an outstanding example.
- Champlevé, French for "raised field", where the surface is carved out to form pits in which enamel is fired, leaving the original metal exposed; the Romanesque Stavelot Triptych is an example.
- Cloisonné, French for "cell", where thin wires are applied to form raised barriers, which contain different areas of (subsequently applied) enamel. Widely practiced in Europe, the Middle East and East Asia.
- En résille (Émail en résille sur verre, French for 'enamel in a network on glass,') where enameled metal is suspended in glass. The technique was briefly popular in Seventeenth-century France which was re-discovered by Margret Craver in 1953. Craver spent 13 years re-creating the technique.
- Grisaille, French term meaning "in grey", where a dark, often blue or black background is applied, then a palescent (translucent) enamel is painted 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, a famous center of vitreous enamel production. Limoges became famous for champlevé enamels from the 12th century onwards, producing on a large scale, and then from the 15th century retained its lead by switching to painted enamel on flat metal plaques.
- Painted enamel, a design in enamel is painted onto a smooth metal surface. Grisaille and later Limoges enamel are types of painted enamel. Most traditional painting on glass, and some on ceramics, uses what is technically enamel, but is often described by terms such as "painted in enamels", reserving "painted enamel" and "enamel" as a term for the whole object for works with a metal base.
- Plique-à-jour, French for "open to daylight" where the enamel is applied in cells, similar to cloisonné, but with no backing, so light can shine through the transparent or translucent enamel. It has a stained-glass like appearance; the Mérode Cup is the surviving medieval example.
- Ronde bosse, French for "in the round", also known as "encrusted enamel". A 3D type of enameling where a sculptural form or wire framework is completely or partly enameled, as in the 15th century Holy Thorn Reliquary.
- 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.
- Serigraph, where a silkscreen is used with 60-70in grade mesh.
- 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.
Industrial Enamel Application 
On sheet steel, a ground coat layer is applied to create adhesion. The only surface preparation required for modern ground coats is degreasing of the steel with a mildly alkaline solution. White and colored second "cover" coats of enamel are applied over the fired ground coat. For electrostatic enamels, the colored enamel powder can be applied directly over a thin unfired ground coat "base coat" layer that is co-fired with the cover coat in a very efficient two-coat/one-fire process.
The frit in the ground coat contains smelted-in cobalt and/or nickel oxide as well as other transition metal oxides to catalyze the enamel-steel bonding reactions. During firing of the enamel at between 760 to 895 °C (1,400 to 1,643 °F), iron oxide scale first forms on the steel. The molten enamel dissolves the iron oxide and precipitates cobalt and nickel. The iron acts as the anode in an electrogalvanic reaction in which the iron is again oxidized, dissolved by the glass, and oxidized again with the available cobalt and nickel limiting the reaction. Finally, the surface becomes roughened with the glass anchored into the holes.
See also 
- Ceramic glaze
- Rostov the Great - A city renowned for its enamel work.
- Staffordshire Moorlands Pan, a 2nd century bronze trulla.
- Franz Ullrich - Founder of a German enamelware factory.
- Oskar Schindler
- Fred Uhl Ball - American enamelist who created the largest known enamel mural.
- Campbell, 6
- Andrews, A.I. Porcelain Enamels, The Garrard Press: Champaign, IL, 1961 p. 1.
- Rutti, B., Early Enamelled Glass, in Roman Glass: two centuries of art and invention, M. Newby and K. Painter, Editors. 1991, Society of Antiquaries of London: London.
- Gudenrath, W., Enameled Glass Vessels, 1425 BCE - 1800: The decorating Process. Journal of Glass Studies, 2006. 48
- Sullivan, Michael, The arts of China, 4th edn, p. 239, University of California Press, 1999, Page 239
- Sullivan, Michael, The arts of China, 4th edn, p. 239, University of California Press, 1999, ISBN 0-520-21877-9, ISBN 978-0-520-21877-2, Google books
- "Japanese Cloisonné: the Seven Treasures". Victoria and Albert Museum. Retrieved 2009-08-30.
- Andrews, Andrew Irving, Porcelain enamels: the preparation, application, and properties of enamels, Garrard Press, 1961, Page 5
- Andrews, A.I. Porcelain Enamels, The Garrard Press: Champaign, IL, 1961 p. 5.
- Judd, Donald, “Porcelain Enameling Aluminum: An Overview,” Proceedings of the 59th Porcelain Enamel Institute Technical Forum, 45-51 (1997).
- Sullivan, J.D. and Nelson, F.W., "Stainless Steel Requires Special Enameling Procedures", Proceedings of the Porcelain Enamel Institute Technical Forum," 150-155 (1970).
- Pew, Steve, "The Who, What, Why, Where, and When of Cast Iron Enameling," Advances in Porcelain Enamel Technology, 177-186, (2010).
- Fedak, David and Baldwin, Charles, "A Comparison of Enameled and Stainless Steel Surfaces," Proceedings of the 67th Porcelain Enamel Institute Technical Forum, 45-54 (2005).
- Gavlenski, Jim and Baldwin, Charles, "Advanced Porcelain Enamel Coatings with Novel Properties," Proceedings of the 69th Porcelain Enamel Institute Technical Forum, 53-58, (2007).
- Andrews, A.I. Porcelain Enamels, The Garrard Press: Champaign, IL, 1961 p. 321-2.
- Campbell, 7, 33-41
- Campbell, 7, 17-32
- Campbell, 6, 10-17
- "Craft: Jewelry: Brooch". Luce Foundation Center for American Art. Smithsonian American Art Museum. Retrieved 29 March 2013.
- Campbell, 7
- British Museum collection database, "Scope note" for the term "enamelled"; other sources use different categories.
- Campbell, 38-42
- Campbell, 7, 42
- Feldman, Sid and Baldwin, Charles, "Surface Tension and Fusion Properties of Porcelain Enamels," Proceedings of the 69th Porcelain Enamel Institute Technical Forum, 1-10 (2008)
- Campbell, Marian. An Introduction to Medieval Enamels, 1983, HMSO for V&A Museum, ISBN 0-11-290385-1
|Wikimedia Commons has media related to: Enamel|
- Enamels on jewelry - historical
- Mechanical and Physical Properties of Vitreous Enamel
- Glass on Metal Magazine Online (US)
- CIDAE Center of Information and Difusion of the Art of Enamelling (ES)
- Society of Dutch Enamellers (NL)
- The Enamelist Society (US)
- Guild of Enamellers, UK
- Porcelain Enamel Institute (US)
- International Enamellers Institute
Collection Highlights: Art in the Islamic World. Beaker. Smithsonian Institute: 2013. Dimand, M. S. An Enameled-Glass Bottle of the Mamluk Period. Metropolitan Museum of Art. Papadopoulous, Kiko. Venetian Eastern Trade: 11th to 14th Centuries. 20 Jan. 2012.