Enamoured by enamel - uses of vitreous enamel

Materials World magazine
,
1 Dec 2008

Vitreous enamel is one of the first metal finishes and it remains popular in the 21st century. Mike Collins, President of The Institute of Vitreous Enamellers, UK, considers its history and modern uses.

Vitreous enamel, or porcelain enamel as it is known in the USA, is a finish for metals produced by fusing specialist glasses onto metal (usually steel, cast iron or copper) at temperatures of around 800°C. This ensures full chemical adhesion between the coating and substrate. The glassy nature of the finish results in high durability, chemical and weather resistance, and colour stability.

One of the earliest uses of a type of vitreous enamel (14th century BC) is the mask of Tutankhamun, but this only used sintering. The earliest known fully fused vitreous enamels are gold rings from a Mycenæan tomb in Cyprus, from the 13th century BC which are decorated with layers of coloured enamel. The main application was on jewellery and decorative and religious art until the early 19th century, when its use rapidly grew into a major industry for cast iron cooking pots.

The process expanded to cast iron signs where it gave an unprecedented vibrancy and was the precursor to the enamelled sign industry. High-grade wrought iron as a substrate soon gave a wider range of signs and its availability led to a market for enamelled sheet iron cookware. But it was not until the early 20th century that suitable steel was developed, enabling the production of higher volumes. The durability of vitreous enamel is clear from the survival of these signs, some of which are well over 100 years old. Often referred to as ‘street jewellery’, they are collectable and still as bright as the day they were fired.

At the same time, the newly formed mass production cooker industry discovered vitreous enamel not only looked good but was also easier to keep clean. Consequently, the biggest volume of vitreous enamel is in the domestic appliance industry such as cookers and ovens. Modern enamels take advantage of improved mechanical and chemical durability.

Specialist continuous clean enamels contain a catalyst that breaks down absorbed fats. They are an important feature of modern cookers, reducing the need for frequent cleaning of the interior. The almost limitless range of colours available is exploited on appliances at the upper end of the market, such as the Aga. However, white remains the prime choice for the freestanding volume cooker market. Enamelled pan supports, often using cast iron as a substrate, will withstand direct flame impingement and thermal shock without deterioration, making them a popular option.

In the bathroom, an enamelled cast iron bathtub is a desirable object for a modern designer home. A more general use of enamel on sanitaryware is for pressed steel baths and shower trays.

Safety signs

The King’s Cross, London, UK, underground fire in November 1987 established that all materials used in underground systems should be Class 1 fire-resistant. As a result, vitreous enamel is used extensively for signage, fittings and station and access tunnel linings. Similarly, it is applied in road tunnel linings and some of the material for this application is produced on a continuous steel coil-to-coil line. A related market is architectural panels for the inner and outer parts of buildings, often with full colour graphics.

Graffiti has prompted a revival and rapid growth of the signage sector. As vitreous enamel is a glass, it is possible to use solvents or even paint stripper to remove the graffiti without affecting the material. This has been recognised by civil authorities, utility providers and rail networks, and extensive replacement programmes of other materials are taking place.

Growing resistance

The bacterial growth resistance of enamels is a benefit in sterile environments. Those that contain silver can control or even stop bacterial and microbial growth. This may lead to more widespread use of the material.

The chemical resistance of vitreous enamel is exploited in the production of storage tanks and silos, often used for storing and treating aggressive products. There has been a transition to enamel tanks for industrial storage of waste slurries, potable water, in sewage treatment plants and for biofuel applications. Chemical reaction vessels, pipework and valves are frequently glass-lined to give long-term resistance to reaction processes in the chemical and pharmaceutical industries.

A relatively new application is the use of screen-printed heating elements. An electrical resistance vitreous enamel is screen printed onto an existing vitreous enamelled stainless steel plate. This provides the source of heat for kettles, coffee makers and similar equipment that require a supply of boiling water in a short time. Gone are the elements that used to fur up with limescale – now there is a flat stainless steel plate in the bottom of the kettle which is hygienic and easy to keep clean.

Modern enamels are applied to cleaned steel by dipping or spraying, usually as a wet suspension produced by ball milling the vitreous frit, or a powder applied electrostatically. For volume production, parts are fired on a conveyorised furnace, while lower volume and specialist products are fired in a batch type box furnace. Further decoration is applied by screen or tampo printing, or transfer processes, and fired into the base enamel. It is even possible to reproduce a full colour image that is as durable and long lasting as the base enamel.

Further information: The Institute of Vitreous Enamellers