Vitreous enamel

Cloisonné enamel plaque, Byzantine Empire, ca. 1100

The Royal Gold Cup with basse-taille enamels; weight 1.935 kg, British Museum. Saint Agnes appears to her friends in a vision.

Vitreous enamel, or just enamel (or porcelain enamel in U.S. English), is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 degrees Celsius (1380 and 1560 degrees Fahrenheit). The powder melts, flows, and then hardens to a smooth, durable vitreous coating on metal, glass or ceramic. The fired enameled ware is a fully laminated composite of glass and metal. The word enamel comes from the High German word smelzan (to smelt) via the Old French esmail.^[1] 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 jewellery and decorative art, but since the 19th century applied to many industrial uses and in everyday day consumer objects. In British English there is a double "l" in "enamelled" and "enamelling" and other formations; American English uses "enameled" and "enameling".

History

The ancient Egyptians applied enamels to pottery and stone objects, and sometimes jewellery, though the last less often than other ancient Middle Eastern cultures. The ancient Greeks, Celts, Russians, and Chinese also used enameling processes on metal objects.^[2]

Enameling was also sometimes 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.^[3] Enamel powder could be produced in two ways; either through the powdering of colored glass, or the mixing of colorless glass powder with pigments such as a metallic oxide.^[4] Designs were either painted freehand or over the top of outline incisions, and the technique probably originated in metalworking.^[3] 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,^[3] though archaeological evidence for this technique is limited to some forty vessels or vessel fragments.^[3]

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 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.^[5] The Japanese also produced large quantities from the mid-19th century, of very high technical quality.^[6]

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 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.

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 twentieth century include enameling-grade steel, cleaned-only surface preparation, automation, and ongoing improvements in efficiency, performance, and quality.^[7]

Properties

St. Gregory the Great in painted Limoges enamel on a copper plaque, by Jacques I Laudin

Enamel may be transparent or opaque when fired; vitreous enamel can be applied to most metals. The great majority of vitreous enamel is applied to steel in which the carbon is controlled to prevent reactions at the firing temperatures. Enamel can also be applied to copper, aluminum,^[8] stainless steel,^[9] cast iron^[10] or hot rolled steel.

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.^[11] Its disadvantages are its tendency to crack or shatter when the substrate is stressed or bent, but modern enamels are chip and impact resistance because of good thickness control and thermal expansions well-match to the metal. Its durability has found it many functional applications: early 20th century and some modern 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. Commercial structures such as filling stations, bus stations and even Lustron Houses had walls, ceilings and structural elements made of enameled steel. 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.^[12]

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 2100 and 2650 °F (1150 and 1450 °C) into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit.^[13]

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.

There are three main types of frit. First, ground coats contain smelted-in transition metal oxides such as cobalt, nickel, copper, manganese, and iron that facilitate adhesion to steel. Second, clear and semi-opaque frits contain little coloring material for producing colors. Finally, titanium white cover coat frits are supersaturated with titanium dioxide which creates a bright white color during firing.

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 enamelling

Limoges? grisaille Stations of the Cross, Notre-Dame-des-Champs, Avranches

• 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.^[14]
• 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.^[15]
• 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.^[16]
• 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, the most famous European 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 surface. Grisaille and later Limoges enamel are types of painted enamel.^[17]
• Plique-à-jour, French for "braid letting in 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 only surviving medieval example.^[18]
• 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.^[19]
• 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.

Old German enamel street sign

Louis George enamel watch dial

A freehand enameled painting by Einar Hakonarson In the forest. 1989

Industrial Enamel Application

On sheet steel, a ground coat layer is put on first to create adhesion. The only surface preparation required for modern ground coats is a simply 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 1400 and 1640 °F (760 to 895 °C), iron oxide scale first forms on the steel. The molten enamel dissolves the iron oxide and precipitates cobalt and nickel. The iron acts 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.^[20]

Gallery

Early 13th century Limoges chasse used to hold holy oils; most were reliquaries.

Medallion of the Death of the Virgin, with basse-taille enamel

Silver, silver gilt and painted enamel beaker, Burgundian Netherlands, c. 1425-1450, The Cloisters

The Dunstable Swan Jewel, a livery badge in ronde bosse enamel, about 1400. British Museum

See also

• Nineveh
• Rostov the Great - A city renowned for its enamel work.
• Franz Ullrich - Founder of a german enamelware factory.
• Staffordshire Moorlands Pan, a 2nd century bronze trulla.
• Ceramic glaze

Notes

References

• Campbell, Marian. An Introduction to Medieval Enamels, 1983, HMSO for V&A Museum, ISBN 0112903851

External links

• Enamels on jewelry - historical
• Mechanical and Physical Properties of Vitreous Enamel
• IVE Institute of Vitreous Enamellers (UK)
• Glass on Metal Magazine Online (US)
• CIDAE Center of Information and Diffusion 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