Applications and process of vitreous enamel

The word enamel comes from the High German word ‘smelzan’ and later from the Old French ‘esmail’. The Collins English Dictionary defines enamel as ‘a coloured glassy substance, transparent or opaque, fused to the surface of articles made of metal, glass etc. for ornament or protection’. Vitreous enamel is specifically on a metal base. It is thus defined as a vitreous, glass like coating fused on to a metallic base. In American English it is referred to as Porcelain Enamel.

It should not be confused with paint, which is sometimes called ‘enamel’. Paints cannot be enamel. They do not have the hardness, heat resistance and colour stability that is only available with real vitreous enamel. Beware of companies or products implying the use of enamel. Check their credentials and warranties.

The glass will be applied to the metal by a various methods either as a powder or mixed with water. This is followed by heating in a furnace to a temperature usually between 750 and 850 degrees Celsius. This ‘firing’ process gives vitreous enamel its unique combination of properties.

The smooth glass-like surface is hard; it is scratch, chemical and fire resistant. It is easy to clean and hygienic.

Vitreous enamel can be applied to most metals. For jewellery and decorative items it is often applied to gold, silver, copper and bronze. For the more common uses, it is applied to steel or cast iron. There are some specialised uses on stainless steel and aluminium.

The durability of the early advertising signs, still showing the brilliance of the original colours after a hundred years, is one of the best examples of the long-term colour stability of vitreous enamel. Compare them to signs, for example road signs, produced in less durable materials which fade and become shabby. Some of the early vitreous enamelled relics date back to the 13th Century BC and the colours are still as vibrant as the day they were produced (see our page on Enamelling History). If you want something where the colour will never fade – use vitreous enamel.

Following the disastrous King’s Cross fire, where combustible materials underground were the major cause, the specification of vitreous enamel for both decorative and functional parts in underground applications is now universal. It cannot burn, in contrast to paints and plastics. The famous London Underground station signs and maps are instantly recognisable uses of this unique product.

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Metals Suitable for Enameling

Enamels for metal normally used on copper work satisfactorily on good cast iron providing certain precautions are taken. Ideally the surface should be sandblasted, however in most instances it is sufficient to use an electric grinder to remove irregular surface areas such as lumps, projections, ridges, rust areas, etc. We do not recommend any casting whether it be cast iron, cast gold, cast silver, etc. be pickled. Castings usually have a few pores where the pickle solutions hide only to come out as blisters in the firing operation.

Best results are normally obtained if the firing temperature is as low as possible. Select enamels for metal which will fire at 1300-13500F for 20 minutes or more. Sifting is not an ideal method of applying the first coat. At these low temperatures individual enamel grains do not flow out easily. If the surface of the cast iron is exposed between grains of enamel it will oxidize during the long firing cycle. Although a second coat may flow out and cover the entire surface the oxidized areas may produce blisters. The best practice is to use Liquid Form – Water Base enamels for the first coat.

For small articles Thompson’s Overglaze Painting Enamel (dry powder) can be mixed with water and applied for the first coat. Cast iron does not require a cobalt bearing ground coat as described above for certain types of steel. Subsequent coats can be normal enamels for metal used on copper if they fire at these lower temperatures. They may be applied by the usual methods. Enamels for metal with expansions of about 240 to 340 are workable on cast iron.

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The Enamel Material

Enamel material comes in a number of forms: lump, string, liquid, and powder, as well as in the optical qualities of transparent, opaque, and opalescent. The important factor in selecting an enamel material is that it be made for the metal you are using. Enamel material expands as it is fired and then contracts as it cools. This is called thermal expansion. The metal on which the enamel material is fired must expand and contract at a slightly higher rate.

Enamels material arc sold in assorted lump forms and in meshes, probably as coarse as 10 mesh and as fine as #325. Some enamelists use the fines for a painting technique. I principally use 80 mesh powder, overglazes, and the 20 mesh in transparents for some jewelry.

Enamels material arc manufactured in soft, medium, and hard fusing, which refers to how they fire. The soft enamels material fire the most quickly. Some enamelists refer to the soft enamels material as delicate. In Thompson’s catalog, most of the 80 mesh enamels material for copper, steel, silver, and gold are listed as medium fusing.

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Industrial Enamel

A single-component, all-purpose, medium industrial enamel.

Designed for interior and exterior use, Industrial Enamel is an all-purpose enamel with a durable color pigment system. Recommended industrial environment uses include properly prepared steel, concrete, wood, plaster, primed aluminum and galvanized steel.

Benefits

  • Good exterior durability
  • High-gloss coating
  • Excellent application properties
  • Exterior/interior all-purpose enamel
  • 450 g/L (3.75 lb/gal) VOC
  • Suitable for use in USDA inspected facilities

Product data is a representative set of attributes and characteristics for this system or product line. Data for individual products may vary and is subject to change.

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