Vitreous Virtues
October 1998

...and therein imagery grauen & enamelyd moste curouslie...

So wrote the English chronicler Robert Fabyan 500 years ago, when the science and art of vitreous (porcelain) enameling on metals was already 15 centuries old. Today vitreous enamel art is as vibrant as then; and the industry of enameling metal panels, which blossomed in the 19th and 20th centuries but later receded due to competition from other materials, is enjoying a resurgence thanks to new technological developments.

Vitreous enameling is the fusing of a thin layer of glass to a metal base, and is virtually impossible to achieve on large areas unless the glass has a high (up to 25 percent) boric oxide content. From the day of its invention, enamel has been used to beautify metal objects, and once the technology had developed to allow its application to large pieces, its durability and protective power could be exploited.

Traditionally an enamel surface is of two coats, base and cover, each of which had to be applied and fired separately. But 20 years ago the so-called 2c/1f process - two coats, one firing - was perfected. Not only did this decrease energy usage and process times dramatically, but more importantly, and together with advances in frit technology, the need for steel pre-treatment by pickling and nickel plating became unnecessary. In turn, the costs and potential environmental risks of effluent disposal were avoided. But in both processes the need for boric oxide, usually derived from Neobor® Pentahydrate Borax or Dehybor® Anhydrous Borax, is the same.

As a surface coating, especially of iron and steel, vitreous enamel is unassailable. It is highly durable, conferring a long service life. It resists scratching and attack by chemicals, and being easy to clean, is very hygienic.

But it has come in for stiff competition from paints, plastics, aluminium, and stainless steel. Even the London Underground, famed for the use of enamel panels in its stations, turned to laminates. The same considerations went for road tunnel linings and the industry faced a time in the doldrums - until new research discovered it actually improves food cooked in microwave ovens, new formulations that improve already excellent hygiene properties, and a new manufacturing process.

Microwave mismatches

Microwave ovens are usually lined with plastic, stainless steel or a painted surface. Okay for the basic process, but since integrated functions like a grill, air circulation and steam heating have been added, temperatures in the chamber can rise to 300°C or more, resulting in staining, yellowing and distortion. If an enamel lining is used, these faults are avoided altogether.

An added bonus, only recently identified, is that enameled steel, with half the thermal conductivity of stainless steel, actually improves food quality. This lower conductivity means less energy usage for the same cooking effect, hence shorter cooking times, and more vitamins are retained in the cooked food.

Keeping it hygienic

One of enamel's great advantages is its scratch resistance. Microbes have no place to hide and are easily cleaned away. Now, Japanese researchers at Ferro Enamels have discovered an enamel formulation which, while proven safe for humans, actively kills germs. By incorporating small amounts of copper and silver in the enamel, it has been found that the enamel has an in-use antimicrobial action. It seems that the metal ions either upset the germs' enzyme systems, or that oxygen is released, damaging their cell walls. Either way, they are killed.

Taking sides with ALUSI coats

One reason for the replacement of enamel by painted surfaces on white goods and less usage for architectural panels was, of course, cost - because to achieve rust resistance, both sides of the panel have to be coated. But there are new developments, with the ability to retain the rust resistance of sheets enameled on one side only. The trick is to aluminize the steel (on both sides) first.

This discovery of successful enamelling on aluminium/ silicon (ALUSI) coated steel sheets, recently described by scientists from Sollac and Ferro in France at the 1998 International Enamellers Congress, has brought other benefits. The enamel can be applied in one coat, on one side, and fired only once so saving costs. Remarkably the enamel coat is much more resistant to chipping and cracking if the panel is bent.

Going underground with enamels (again)

Having abandoned enamel panels in station refurbishment for a while, after suffering tragic fires, London Underground found itself facing very much tougher fire regulations. The solution is to bring back the decorative enamel panel which is tough, durable, easily cleaned and totally fire resistant.

The glass used to enamel metals is no ordinary glass. It must be very fluid when molten and have a low surface tension to enable it to spread evenly over the metal at a relatively low temperature. But most of all is the need to adjust the glass's thermal expansion.

The major hurdle in making large pieces are the different rates of expansion, on cooling and warming, of glass and metal. If they are not nearly the same, the enamel will crack, craze or flake off.

The answer is boric oxide: being up to 25 percent of an enamel mixture, it is responsible for the low melt temperature, viscosity and surface tension needed, and also has the remarkable property of altering the glass thermal expansion to suit the metal underneath. It also promotes a smooth surface and high luster. Either Neobor and Dehybor are used, Dehybor having the advantage that it has no water of crystallization to dispel in the furnace, reducing emissions and plant corrosion. Dehybor is also a more concentrated source of boric oxide, meaning productivity gains, and lower handling and freight costs for the manufacturer.

In practice, vitreous enamel usually marries two different kinds of glass. The first base coat includes cobalt and nickel oxides which help to form a very strong bond with the metal. A second base coat carries color and decoration.

The resultant bond is virtually inseparable, making for the extreme durability and rust resistance of enameled iron and steel. It is also the only metal coating process that can be guaranteed to resist severe thermal shock - for instance in water boilers, stoves, ovens and cookware.