Available in a variety of colors to harmonize or contrast with the vision area, the ceramic enamel frit is applied to the surface of the glass. Ceramic enamel frits contain finely ground glass mixed with inorganic pigments to produce a desired color. The coated glass is then heated to about 1,150°F, fusing the frit to the glass surface, which produces a ceramic coating almost as hard and tough as the glass itself. A fired ceramic enamel frit is durable and resists scratching, chipping, peeling, fading and chemical attacks.
Spandrel glass can be installed monolithically, using insulated metal backpans, but is more often found as a component of an insulating glass unit. Reflective spandrel glass units are widely used when a uniform all-glass look is desired for the building exterior. Typical applications include commercial fixed windows, curtain walls, storefronts and wall cladding. Spandrel glass is traditionally an opaque material not intended for use in vision areas.
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Porcelain enamel has been around for 4000 years and shows no signs of disappearing any time soon. Although alternative materials like stainless steel, plastics and paints continue to gain market share and have completely taken over some markets, such as dishwashers, the cooking and laundry appliance markets remain strong, and total frit production remains about the same. However, some manufacturers believe a huge revolution in materials technology is needed to ensure the continued success of the industry. Both suppliers and manufacturers are doing their share to help ensure that this revolution takes place.
Porcelain enamel coatings are made from a frit based on low melting temperature (2000 to 2500°F) borosilicate glasses. After the glass raw materials are melted (generally in recuperative furnaces) at rates ranging from 5 to 50 tons per day, rapid quenching is used to shatter the resultant glass into small particles. Further particle size reduction is achieved by grinding. The coating is applied using wet suspension or dry electrostatic powder processes, and is then heated to about 1500°F to produce chemical bonding with the metal substrate.
Pre-milled frits now allow enamelers to custom blend their own enamel formulations without using costly milling equipment. The enameler can blend the exact amount required for the job, eliminating waste. Blends can be made almost “just in time,” eliminating the need for a large wet enamel inventory.
Many frit manufacturers have switched from air/gas to oxygen/gas combustion systems to lower their emissions, and this trend is expected to continue. Smelters have become more automated, and larger capacities are being used as product volumes increase through the increased standardization of frits.
For instance, unique appearance characteristics are under development, including metallic lusters to simulate copper metal or stainless steel appearances. Refinements in frit products are also being made to achieve “easy to clean” oven coatings, as well as infrared reflectivity for faster cooking. In addition, hybrid coatings are being investigated to take advantage of properties provided by both porcelain enamel and organic coatings.
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To attach a porcelain enamel coating to any substrate by enameling, both substrate and glass must be heated to the fusion temperature of the glass. However, fusion methods have not been successful for the more refractory materials. Because most refractory porcelain enamel coatings are amorphous or crystalline in nature, they have to be applied by relatively novel techniques.
Although most ceramic materials are refractory, some of them can be vaporized in an electric arc or hot vacuum. Thin porcelain enamel coatings of amorphous silica can be applied readily to relatively cool substrates by vaporizing metallic silicon or silicon halides in the presence of small quantities of oxygen. Apparently the transfer is accomplished largely as silicon monoxide, which recombines with oxygen on cooling. The process is used to obtain thin, protective, optically transparent films on lenses, certain electrical components, and metal reflectors.
Other porcelain enamel coatings may be produced by vaporizing one or more components of the coating. In this way porcelain enamel coatings of the respective carbides of silicon, boron, aluminum, and chromium can be deposited on graphite, silicon nitride can be formed on metallic silicon, and silicide coatings can be deposited on metals such as tungsten and molybdenum. These processes are necessarily expensive and are poorly adaptable to large specimens or complex shapes.
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