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Old German enamel street sign
LOUIS GEORGE genuine handmade fire enamel watch dial


In a discussion of material science, 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 champlevé or a cloisonné.

History

Enamelling is an old and widely-adopted technology. The ancient Egyptians applied enamels to pottery and stone objects. The ancient Greeks, Celts, Russiansmarker, and Chinesemarker 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 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.


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. Commercial structures such as gas stations, bus stations and even Lustron Houses had walls, ceilings and structural elements made of porcelain-enamel steel.

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

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.
  • 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 (subsequently applied) enamel.
  • 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 Limogesmarker, 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-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.
  • 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.


Industrial Porcelain Enamel

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.

The key ingredient of industrial porcelain 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.

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 porcelain enamel coating material. First, wet process porcelain 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.

Most industrial porcelain enamel is applied to ASTM A424-compliant enameling steel. The carbon in enameling-grade steel is controlled to prevent reactions at the enameling firing temperatures. Some porcelain goes onto aluminum, cast iron or hot rolled steel. 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 mildy alkaline solution.

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. 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 fired enameled ware is a fully laminated composite of glass and metal. The porcelain enamel coating has excellent chemical resistance, corrosion resistance, scratch resistance (5-6 on the Mohs scale), long-lasting color fastness, cleanability, and is non-flammable. Porcelain enamel is glass, not paint, so it does not fade with UV light. Modern porcelain enamels are chip and impact resistance because of good thickness control. Typical domestic applications of porcelain enamel are on ovens, clothes washers, sinks, bathtubs, glass-lined water heaters, cookware, bakeware, and bbq. Industrial applications include boilers, heat exchangers, architectural panels, and electronic circuits. Some new developments in the last ten years include enamel/non-stick hybrid coatings, sol-gel functional top-coats for porcelain enamels, enamels with a metallic appearance, and new easy-to-clean technologies.

See also



Notes

  1. Gullick, J.T. and Timbs, J., "Painting Popularly Explained," Kent & Co London, 1859, quoting M. de Laborde, "Notice des Emaux du Louvre"
  2. 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.
  3. Gudenrath, W., Enameled Glass Vessels, 1425 BCE - 1800: The decorating Process. Journal of Glass Studies, 2006. 48


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