Vitreous Enamels

Fluoridation of potable water suppHes for the prevention of dental caries is one of the principal uses for sodium fluoride (see Water, municipal WATER treatment). Use rate for this appHcation is on the order of 0.7 to 1.0 mg/L of water as fluoride or 1.5 to 2.2 mg/L as NaF (2). NaF is also appHed topically to teeth as a 2% solution (see Dentifrices). Other uses are as a flux for deoxidiziag (degassiag) rimmed steel (qv), and ia the resmelting of aluminum. NaF is also used ia the manufacture of vitreous enamels, ia pickling stainless steel, ia wood preservation compounds, caseia glues, ia the manufacture of coated papers, ia heat-treating salts, and as a component of laundry sours. 

No. 1 Busheling. This scrap is a clean prompt iadustrial scrap limited to about 30 cm (12 ia.) ia any dimension. It iacludes new factory busheling such as sheet clippings and stampiags (free of old auto body and fender stock) metals which have been coated, limed, or vitreous enameled and electrical steel containing more than 0.5% siUcon. 

The (Ti02) films are also appHed to glass or vitreous enamel for decorative purposes. Thin films enhance brilliance thicker films impart a silver-gray luster. Milk glass can be produced by mixing the titanate with a low melting enamel, which skaters when the coatkig is baked (424). 

Antimony trioxide has numerous practical applications (1). Its principal use is as a flame retardant in textiles and plastics (see Flame RETARDANTS Flame retardants in textiles). It is also used as a stabilizer for plastics, as a catalyst, and as an opacifier in glass (qv), ceramics (qv), and vitreous enamels... 

J. Wtatil, Vitreous Enamels, Borax Holdings Limited, London, 1984, p. 13. 

J. Tunnichffe and M. Rose, Vitreous Enameller DipI)., 1—5 (Spriug 1989). 

K. A. Maskah and D. W. White, Vitreous Enamelling. A. Guide to Modem Enamelling Practice, Pergamon Press, Oxford, UK, 1986. 

FIG. 5-12 Variation of absorptivity with temperature of radiation source. (1) Slate composition roofing. (2) Linoleum, red brown. (3) Asbestos slate. (4) Soft rubber, gray. (5) Concrete. (6) Porcelain. (7) Vitreous enamel, white. (8) Red brick. (9) Cork. (10) White dutch tile. (11) White chamotte. (12) MgO, evaporated. (13) Anodized aluminum. (14) Aluminum paint. (15) Polished aluminum. (16) Graphite. The two dashed lines bound the limits of data on gray paving brick, asbestos paper, wood, various cloths, plaster of parts, lithopone, and paper. To convert degrees Ranldne to kelvins, multiply by (5.556)(10 ). 

Wastes from the manufacture, formulation, supply and use (MFSU) of coatings (paints, varnishes and vitreous enamels), adhesive, sealants and printing inks Wastes from the photographic industry Inorganic wastes from thermal processes... 

Operations found in vitreous enameling, wall and floor tile, pottery. 

Aluminium coatings also provide a suitable key or pretreatment for subsequent coatings, e.g. aluminised steel provides a good base for vitreous enamel. 

A vitreous enamel coating is, as the name implies, a coating of a glassy substance which has been fused onto the basis metal to give a tightly adherent hard finish resistant to many abrasive and corrosive materials. The purpose of modern vitreous enamels is twofold, i.e. to confer corrosion protection to the metal substrate and at the same time to provide permanent colour, gloss and other aesthetic values. 

Frit may be milled dry or wet. The long established dry process is used for cast iron baths and for chemical plant. Vitreous enamel application by a dry electrostatic method is being used on an increasing scale. In these cases, the frit is milled alone, or with inorganic colouring or refractory additives. This is achieved in cylinders using balls of porcelain, steatite or more dense alumina, or with pebbles of flint, to produce a fine powder of predetermined size. 

Vitreous enamel is normally applied to the prepared metal or over a ground-coat by spraying or dipping. Alternative wet techniques are used, of which the most common has been electrostatic wet spraying. Electrophoretic deposition from the slurry has been found to be highly suitable for some components. 

In this section, chemical resistance will be divided into three parts, viz. acid, alkali (including detergents) and water (including atmosphere). Normally an enamel is formulated to withstand one of the corrosive agents more specifically than another, although vitreous enamel as a general finish has good all round resistance, with a few exceptions such as hydrofluoric acid and fused or hot concentrated solutions of caustic soda or potash. 

In the glass-bottle industry the bottles can be cooled in a dilute SOj/SO, atmosphere to increase chemical resistance. A similar effect has been noted with vitreous enamel. It has been postulated that a thin layer of —OH groups or — OH—HjO (hydronium) ions is adsorbed on the surface of a fired enamel. These ions are transformed into — OSO, or —OSO3 in the presence of oxides of sulphur which are more resistant to further acid attack. It is known that the acid resistance of a recently fired enamel improves on ageing, probably due to the enamel reaction with SOj/SO, in the atmosphere and it is quite common for the grading to improve from Class A to Class AA (BS 1344). 

It is recognised that vitreous enamel possesses good acid resistance, but an exception occurs with hydrofluoric acid. This is due to the relative ease of reaction between this acid and the silica (which is the largest constituent in the frit) to form silicon tetrafluoride. This reaction is made use of in some de-enamelling plants. 

The Institute of Vitreous Enamellers produced a report on detergent... 

---