Porcelain enameling

Opacifiers are fine inorganic powders, usually white, that are used to reduce the transparency of ceramic gla2es and porcelain enamels. The coating becomes opaque because the particles of the opacifter scatter and reflect the incident light. When inorganic pigments are combined with white opacifiers, pastel colors are obtained. 

Finishes for aluminum products can be both decorative and useful. Processes in use include anodic oxidation, chemical conversion coating, electrochemical graining, electroplating (qv), thin film deposition, porcelain enameling, and painting. Some alloys respond better than others to such treatments. 

Porcelain enameling requires the use of frits and melting temperatures of 550 °C or below. Enamels are appHed over chemical conversion coatings that are compatible with the frit. AHoy selection is important to obtain good spall resistance. Alloys 1100, 3003, and 6061 are employed most extensively among wrought products and alloy 356 for castings. 

Another large appHcation is as an ingredient in the production of charcoal briquettes. The amount of sodium nitrate used in charcoal briquette manufacture depends on the type and amount of wood and coal used. Typically charcoal briquettes contain up to almost 3% sodium nitrate. Sodium nitrate is also used in the manufacture of enamels and porcelain as an oxidizing and fluxing agent. In porcelain—enamel frits used for metal coating, the amount of sodium nitrate in a batch varies with the various metal bases to be coated, typically from about 3.8 to 7.8 wt %. 

Antimony trioxide and sodium antimonate are added to specialty glasses as decolorizing and fining agents, and are used as opacifiers in porcelain enamels. Antimony oxides are used as white pigments in paints, whereas antimony trisulfide and pentasulfide yield black, vermilHon, yeUow, and orange... 

BaCl2 is used in heat treating baths because of the eutectic mixtures it readily forms with other chlorides. The melting points of some eutectic mixtures are BaCl2 KCl, 672—680°C BaCl2 N Cl, 39 mol % BaCl2) 654°C 631°C. BaCl2 is so used to set up porcelain enamels for sheet... 

The uses of boric oxide relate to its behavior as a flux, an acid catalyst, or a chemical iatermediate. The fluxing action of B2O2 is important ia preparing many types of glass, gla2es, frits, ceramic coatings, and porcelain enamels (qv). 

Porcelain Enamel. The slurry used in enameling is commonly composed of baH clay, frits, and coloring pigments (qv). The frits are finely... 

Any product that depends on aesthetics for consideration for purchase and use will be improved by the use of color. Hence, many ceramic products, such as tile, sanitary ware, porcelain enameled appHances, tableware, and some stmctural clay products and glasses, contain colorants. 

For both economic and technical reasons, the most effective way to impart color to a ceramic product is to apply a ceramic coating that contains the colorant. The most common coatings, gla2es and porcelain enamels, are vitreous in nature. Hence, most appHcations for ceramic colorants involve the coloring of a vitreous material. 

The resistance of these materials to firing temperature is definitely limited. They can be fired to about 1000°C. Hence, they are limited to use in porcelain enamels and in low firing artware gla2es. 

Gold purple, often caHed Purple of Cassius, is a tin oxide gel colored by finely divided gold (7). It has good coverage and brilliance in low temperature coatings such as porcelain enamels. It is a very expensive pigment, because of its difficult preparation as well as the price of gold. 

For conventional wet processing of sheet steel, the porcelain enamel frit is ball-milled using clay, certain electrolytes, and water to form a stable suspension. This clay-supported slurry of small particles is called the sHp and has the consistency of a heavy cream. The ingredients of the mill batch are carefully controlled. The amount and purity of all materials in the mill, including the clay and water, affect the rheological character of the sHp as well as a number of the properties of the fined enamel such as chemical resistance, reflectance, gloss, color, and abrasion resistance. 

Porcelain enamels meet a variety of performance characteristics required for different appHcations. The common characteristics of all enamels include good adherence to the substrate and good thermal expansion fit to the metal. Specific properties depend on usage for example, acid and alkaH resistance, hot water resistance, abrasion resistance, thermal shock resistance, high gloss, high reflectance, specific color, heat resistance, and cleanabihty. 

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