Titania opacified porcelain enamels

Porcelain enamels, also called Vitreous Enamelling, process of fusing a thin layer of glass to a metal object to prevent corrosion and enhance its beauty. Porcelain enamels iron is used extensively for such articles as kitchen pots and pans, bathtubs, refrigerators, chemical and food tanks, and equipment for meat markets. In architecture it serves as facing for buildings. Being a glass, porcelain enamels has the properties of glass: a hard surface, resistance to solution, corrosion, and scratching. Enamelware is usually quite resistant to acid and impact, but may crack if the base metal is deformed.

In general, base items consist of fabricated steel, iron castings such as bathtubs and stoves, or, for kitchenware, a good grade of low-carbon sheet iron formed in the shape of the utensil by pressing or drawing, by spinning, and by trimming, with handles, spouts, and ears welded in place.

The base items are cleaned by physical means such as sandblasting or by pickling in acid. Next a coating mixture of ground glass, clay, and water is applied and dried. The ware then is fired in a furnace. For cast-iron dry-process porcelain enamels, powdered glass is dusted over the hot ware; as it melts it forms a continuous layer of enamel. For wet-process porcelain enamels, a second liquid layer of cover enamel is applied.

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Porcelain enamels and ceramic coatings

Earlier definitions of ceramic materials usually stressed their mineral origin and the need for heat to convert them into useful form. As a consequence, only porcelain enamels and glazes were recognized as ceramic coatings until recently, when the principles of phase relations, bonding mechanisms, and crystal structure were applied to ceramic materials and to coatings made from them. In consequence, ceramic materials can now be most safely defined as solid substances that are neither metallic nor organic in nature, a definition that is somewhat more inclusive than older ones, but more accurately reflects modern scientific usage.

Most ceramics are metal oxides, or mixtures and solutions of such oxides. Certain ceramic materials, however, contain little or no oxygen. As a whole, ceramic materials are harder, more inert, and more brittle than organic or metallic substances. Most ceramic coatings are employed to exploit the first two properties while minimizing the third.

Low-Temperature Coatings

The outstanding resistance to corrosion of certain metals, notably aluminum and chromium, is attributable to the remarkable adherence of their oxide films. Aluminum does not corrode because its oxidation product, unlike that of iron, is a highly protective coating. It was once believed that some mysterious kinship between a metal and its own oxide was needed for this protection, but recent knowledge relating to the structure of metals and metal oxides has enabled metallurgists to develop alloys that form even more stable and adherent films. Methods for thickening or stabilizing these oxide coatings by heat treatment, electrolysis, or chemical reaction are widely accepted.

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