Glazes and Enamels

Glazes are the ceramic coatings done on ceramics, and enamels are done on metals. Both these coatings perform two functions—corrosion resistance and aesthetics. The substrates for both have a crystalline structure. 

The coatings in the form of glazes and enamels possess an amorphous structure, which is more resistant to corrosion than the crystalline one. Also, the surface of an amorphous material is smoother and provides a more pleasing appearance. Corrosion still takes place in amorphous materials. This was shown in the case of glasses. 

When the alkali ion goes into solution, an equivalent amount of hydrogen ion goes into the glass. That means an ion exchange reaction takes place, as shown in Equation 5.6 [27]. 

As H+ ions replace Me ions, a surface film is produced, resembling vitreous silica or silica gel, and having properties different from the parent glass [28]. This film swells, acts as a barrier to further reaction, and decreases diffusion rates into and out of the surfaces, thereby inhibiting further attack [8,29,30]. This is the mechanism of corrosion in neutral and acidic media. 

The presence of alkali ions in the solution tends to raise the pH to the point at which the silica-rich layer can itself be attacked by hydroxyl ions [27]. The reaction is represented by Equation 5.7. In this reaction, doubly bonded oxygen becomes singly bonded. 

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TABLE 26 Properties Required from Glass, Glaze, and Enamel... 

Uses. Cleaning compounds fertilizers manufacture of glazes and enamels... 

Uses. The metal is used in electroplating, in solder for aluminum, as a constituent of easily fusible alloys, as a deoxidizer in nickel plating, in process engraving, in cadmium-nickel batteries, and in reactor control rods. Cadmium compounds are employed as TV phosphors, as pigments in glazes and enamels, in dyeing and printing, and in semiconductors and rectifiers. 

The pure compound is used in glazes and enamels as an opacifying agent. It also is used to make colored glass, imparting yellow color to the glass. The crude form is used in flaming arc lamps. Another application of this compound is in the preparation of several other cerium salts. 

Cobalt(II) sulfate is used in storage batteries and electroplating baths for cobalt. It also is used as a dryer for lithographic inks in pigments for decorating porcelains in ceramics, glazes and enamels to protect from discoloring and as a additive to soils. 

Zinc fluoride is used in the manufacture of phosphors for fluorescent lights. It also is used in electroplating baths, in preservation of wood, in glazes and enamels for ceramics, and in fluorination reactions of organics. 

Uses. The most important areas of use of pigments are paints, varnishes, plastics, artists colors, printing inks for paper and textiles, leather decoration, building materials (cement, renderings, concrete bricks and tiles—mostly based on iron oxide and chromium oxide pigments), leather imitates, floor coverings, rubber, paper, cosmetics, ceramic glazes, and enamels. 

Zircon is used in the manufacture of refractories, as opacifier in glazes and enamels, as nucleant in glass-ceramics and also as a raw material for electrotechnical ceramics, chemically resistant glasses and enamels, etc. 

Alkali carbonates show comparatively low melting points and therefore find application as fluxes in the glass industry, and in the manufacture of glazes and enamels. 

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