Industrial porcelain enamel (also known as glass lining, glass-lined steel, or glass fused to steel) is the use of vitreous enamel for industrial, rather than artistic, applications. Porcelain enamel, a thin layer glass applied to a substrate of metal, is used to protect surfaces from chemical attack and physical damage, modify the structural characteristics of the substrate, and improve the appearance of the product. Enamel has been used for art and decoration since the period of Ancient Egypt, and for industry since the Industrial Revolution. It is most commonly used in the production of cookware, home appliances, bathroom fixtures, water heaters, and scientific laboratory equipment.
Characteristics The most essential characteristic of porcelain enamel, from an industrial perspective, is its resistance to corrosion. Mild steel is used in almost every industry and a vast array of products; enamel is an economical way of protecting this, and other chemically vulnerable materials, from corrosion. It can also produce extraordinarily smooth, glossy finishes in a wide array of colours; these colours will not fade when exposed to UV light, unlike paint. Enamel is highly heat-resistant; this allows it to be used in high-temperature applications where an organic anti-corrosion coating or galvanization may be impractical or even dangerous (see Metal fume fever). Porcelain enamel also sees less frequent employment of some of its other properties; examples are its abrasion resistance, where it may perform better than many metals; its resistance to organic solvents, where it is entirely impervious; its resistance to thermal shock, where it can resist rapid cooling from temperatures 500°C and higher; and its longevity.
Terminology The term porcelain enamel can be a source of linguistic confusion. Primarily used in the United States, the term is misleading because the material is not porcelain, indeed not any type of ceramic. It refers instead to vitreous enamel, which is a structurally distinct class of material. Outside the United States, the material is termed vitreous enamel, derived from the Latin word for glass. Production involves melting glass-forming minerals to create a granular substance known as frit. This frit is applied to a metal surface and fired in a furnace, causing the glass to melt and chemically bond with the metal. Porcelain is a solid, clay-based ceramic produced by firing refined clay, quartz and feldspar at high temperatures. Conversely, vitreous enamel consists of a thin glass coating fused to a metallic substrate, such as cast iron, steel or aluminium. From a materials science perspective, classifying such a coating as a ceramic is technically inaccurate. Whilst glass and ceramics are related, ceramics possess crystalline structures, whereas glass is an amorphous solid. Because the enamel coating is a glass, it lacks the crystalline arrangement of porcelain.
Applications
Porcelain enamel is used most often in the manufacture of products that will be expected to come under regular chemical attack or high heat, such as cookware, burners, and laboratory equipment. It is used in the production of many household goods and appliances, especially those used in the kitchen or bathroom area: pots, pans, cooktops, appliances, bathtubs, even walls, counters, and other surfaces. Porcelain enamel is also used architecturally as a coating for wall panels. It may be used externally to provide weather resistance and desirable appearance, or internally to provide wear resistance; for example, on escalator side panels and tunnel walls. In recent years, agricultural silos have also been constructed with porcelain enamelled steel plates to protect the interior from corrosion and the exterior from weathering; this may indicate a future trend of coating all outdoor mild steel products in a weather-resistant porcelain enamel.
Enamelling process The application of industrial porcelain enamel is a complex process involving numerous technical steps. All enamelling processes involve the mixture and preparation of frit, the unfired enamel mixture; the preparation of the substrate; the application and firing; and then finishing processes. Most modern applications also involve two layers of enamel: a ground coat to bond to the substrate and a cover coat to provide the desired external properties. Because frits are frequently mixed at higher temperatures than the firing process requires, most modern industrial enamellers do not mix their frits completely; instead, frit is most often purchased from dedicated frit producers in standard compositions, and any special ingredients are added before application and firing.
Frit For ground coats, the composition of a frit for any given application is determined primarily by the metal used as the substrate: different varieties of steel, and different metals such as aluminium and copper, require different frit compositions to bond to them. For cover coats, the frit is composed to bind to the ground-coat and produce the desired external properties. Frit is typically prepared by mixing the ingredients and then milling the mixture into a powder. The ingredients, most often metal oxides and minerals such as quartz (or silica sand), soda ash, borax, and cobalt oxide, are acquired in particulate form; the precise chemical composition and amount of each ingredient must be carefully measured and regulated. Once prepared, this powdered frit is then slumped and stirred to promote even distribution of materials; most frits are smelted at temperatures between 1150 and 1300°C. After smelting, the frit is again milled into a powder, most often by ball mill grinding. For wet application of enamel, a slurry of frit suspended in water must be created. To remain in suspension, frits must be milled to an extremely fine particle size, or mixed with a suspension agent such as clay or electrolytes.
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