Building Design and Construction Handbook - Merritt - Ventech!

4.96 SECTION FOUR
indoor and outdoor applications because of its hardness, durability, washability, and
color possibilities. For building purposes, porcelain enamel is applied to sheet metal
and cast iron, the former for a variety of purposes including trim, plumbing, and
kitchen fixtures, and the latter almost entirely for plumbing fixtures. Most sheet
metal used for porcelain enameling is steel—low in carbon, manganese, and other
elements. Aluminum is also used for vitreous enamel.
4.78 PORCELAIN ENAMEL ON METAL
Low-temperature softening glasses must be employed, especially with sheet metal,
to avoid the warping and distortion that would occur at high temperatures. To obtain
lower softening temperatures than would be attainable with high-silica glasses, boron
is commonly added. Fluorine may replace some of the oxygen, and lead may
also be added to produce easy-flowing brilliant enamels; but lead presents an occupational
health hazard.
Composition of the enamel is carefully controlled to provide a coefficient of
thermal expansion as near that of the base metal as possible. If the coefficient of
the enamel is greater than that of the metal, cracking and crazing are likely to
occur, but if the coefficient of the enamel is slightly less, it is lightly compressed
upon cooling, a desirable condition because glass is strong in compression.
To obtain good adhesion between enamel and metal, one of the so-called transition
elements used in glass formulation must be employed. Cobalt is favored.
Apparently, the transition elements promote growth of iron crystals from base metal
into the enamel, encourage formation of an adherent oxide coating on the iron,
which fuses to the enamel, or develop polar chemical bonds between metal and
glass.
Usually, white or colored opaque enamels are desired. Opacity is promoted by
mixing in, but not dissolving, finely divided materials possessing refractive indexes
widely different from the glass. Tin oxide, formerly widely used, has been largely
displaced by less expensive and more effective titanium and zirconium compounds.
Clay adds to opacity. Various oxides are included to impart color.
Most enameling consists of a ground coat and one or two cover coats fired on
at slightly lower temperatures; but one-coat enameling of somewhat inferior quality
can be accomplished by first treating the iron surface with soluble nickel salts.
The usual high-soda glasses used to obtain low-temperature softening enamels
are not highly acid-resistant and therefore stain readily and deeply when ironcontaining
water drips on them. Enamels highly resistant to severe staining conditions
must be considerably harder; i.e., have higher softening temperatures and
therefore require special techniques to avoid warping and distorting of the metal
base.
Interiors of refrigerators are often made of porcelain-enameled steel sheets for
resistance to staining by spilled foods, whereas the exteriors are commonly bakedon
synthetic-resin finishes.
4.79 PORCELAIN BIBLIOGRAPHY
F. H. Norton, ‘‘Elements of Ceramics,’’ Addison-Wesley Publishing Company, Cambridge,
Mass.