More than 500 years ago, the English chronicler Robert Fabyan described the science and art of vitreous (porcelain) enameling on metals, a practice that was already 15 centuries old. Today, vitreous enamel art is as vibrant as then. The industry of enameling metal panels, which blossomed in the 1800s and 1900s, had receded but is enjoying a resurgence thanks to new technological developments.
The Science of Vitreous Enamel: Boric Oxide Is Required
Vitreous enameling—the fusing of a thin layer of glass to a metal base—is virtually impossible to achieve on large areas unless the glass has a high (up to 25%) 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 two coats, base and cover, each of which had to be applied and fired separately. But in the 1970s, the so-called 2C/1F process—two coats, one firing—was perfected. This technique decreases energy usage and process times dramatically. With advances in frit technology, the need for steel pre-treatment by pickling and nickel plating became unnecessary.
As a result, effluent disposal costs and potential environmental risks are mitigated. But both processes still need boric oxide, usually derived from Neobor® Pentahydrate Borax or Dehybor® Anhydrous Borax.
Enamel Glaze: Versatile and Beautiful
As a surface coating, especially iron and steel, vitreous enamel is unassailable. Highly durable with a long service life, it resists scratching and chemical marring. It is easy to clean, and therefore, very hygienic.
But paints, plastics, aluminum, and stainless steel offer stiff competition. Even the London Underground, famed for the use of enamel panels in its stations, turned to laminates.
The industry took a downturn until research:
- Improved food cooked in microwave ovens
- Discovered new formulations that improve already excellent hygiene properties
- Updated manufacturing process
Microwave ovens are usually lined with plastic, stainless steel, or a painted surface. Okay for the basic process, but since integrated functions such as a grill, air circulation, and steam heating have been added, temperatures in the chamber can rise to 572°F (300°C) or more. Higher temperatures can result in staining, yellowing, and distortion. If an enamel lining is used, these faults are avoided altogether.
An added bonus is that because enameled steel has half the thermal conductivity of stainless steel, it actually improves food quality. This lower conductivity means less energy usage for the same cooking effect, shorter cooking times, and more vitamins 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.
In another development, Japanese researchers at Ferro Enamels discovered an enamel formulation which, while proven safe for humans, actively kills germs. By incorporating small amounts of copper and silver in the enamel, scientists 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
Cost was one reason for replacing enamel with painted surfaces on white goods and reduced use of enamel for architectural panels. To achieve rust resistance, both sides of the panel have to be coated, which increases cost.
But new developments enable rust resistance of sheets enameled on one side only. Aluminizing the steel (on both sides) first was described by scientists from Sollac and Ferro in France at the 1998 International Enamellers Congress. Enameling on aluminum/silicon (ALUSI)-coated steel sheets has other benefits, including:
- Lower production costs because the enamel can be applied in one coat, on one side, and fired only once
- Better able to resist chipping and cracking if the panel is bent
Going Underground with Enamels (Again)
For a time in the late 20th century, the London Underground abandoned enamel panels in station refurbishment to reduce costs. However, after suffering tragic fires, the Underground found itself facing much tougher fire regulations. The solution was to bring back the decorative enamel panel, which is tough, durable, easily cleaned—and totally fire resistant.
Boric Oxide: Key Ingredient in Enamel Glaze
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 important is the need to adjust the glass' thermal expansion.
The major hurdle in making large pieces is the different rates of expansion of glass and metal during cooling and warming. If the rates are not nearly the same, the enamel will crack, craze, or flake off.
The answer? Boric oxide. Comprising up to 25% of an enamel mixture, it is responsible for the low melt temperature, viscosity, and surface tension needed. Boric oxide also has the remarkable property of altering the glass’ thermal expansion to suit the metal underneath.
Boric oxide also promotes a smooth surface and high luster. Either Neobor or Dehybor can be used. Dehybor has 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, resulting in 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, creating 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 the severe thermal shock in water boilers, stoves, ovens and cookware.
From home appliances to fire resistant signs in the subway, vitreous enamels are making everyday life safer and more beautiful.