Page 452 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 415

12.6 KINDS OF AMORPHOUS GLASS

The types and properties of glass can be readily varied by changing the relative amounts and nature
of ingredients. Soda-lime glass is the most common of all glasses accounting for about 90% of glass
made. Window glass, glass for bottles, and so forth, are all soda-lime glass. Soda-lime glass (75%
silica, 15% soda (sodium oxide), 9% lime (calcium oxide), and the remaining 4% minor ingredi-
ents) has a relatively low softening temperature and low thermal shock resistance limiting its high-
temperature applications.
Vycor, or 96% silicon glass, is made using silicon and boron oxide. Initially, the alkali-borosili-
cate mixture is melted and shaped using conventional procedures. The article is then heat-treated,
resulting in the formation of two separate phases—one that is high in alkalis and boron oxide,
and the other containing 96% silica and 3% boron oxide. The alkali–boron oxide phase is sol-
uble in strong acids and is leached away by immersion in hot acid. The remaining silica–boron
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oxide phase is quite porous. The porous glass is again heated to about 1,200 C, resulting in a 14%

shrinkage due to the remaining portions filling the porous voids. The best variety is “crystal”
clear and called fused quartz. The 96% silica glasses are more stable and exhibit higher melting
o
points (1,500 C) than soda-lime glass. Crucibles, ultraviolet fi lters, range burner plates, induction

furnace linings, optically clear filters and cells, and super heat-resistant laboratory-ware are often
96% silicon glass.
Borosilicate glass contains about 80% silica, 13% boric oxide, 4% alkali, and 2% alumina. It is
more heat-shock-resistant than most glasses due to its unusually small coefficient of thermal expan-

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sion (typically between 2 and 5 × 10 cm/cm/ C; for soda-lime glass it is 8–9 × 10 cm/cm/ C). It
is better known by such trade names as Kimax and Pyrex. Bakeware and glass pipelines are often
borosilicate glass.
Lead glasses (often called heavy glasses) are made by replacing some or all of the calcium
oxide by lead oxide (PbO). Very high amounts of lead oxide can be incorporated—up to 80%. Lead
glasses are more expensive than soda-lime glasses, and they are easier to melt and work with. They
are more easily cut and engraved, giving a product with high sparkle and luster (due to the high
refractive indexes). Fine glass and tableware are often lead glass.
Silicon glass is made by fusing pure quartz crystals or glass sand (impure crystals), and it is typ-
ically about 99.8% SiO . It is high melting and difficult to fabricate.

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Colored or stained glass has been made for thousands of years, first by Egyptians and later
by Romans. Color is typically introduced by addition of transition metals and oxides. Table 12.5
contains selected inorganic colorants and the resulting colors. Because of the high clarity of glass,
a small amount of coloring agent goes a long way. One part of cobalt oxide in 10,000 parts of
glass gives an intense blue glass. The most well-known use for colored glass is the construction
of stain-glass windows. In truth, there are many other uses such as industrial color fi lters and
lenses.
Glazes are thin, transparent coatings (colored or colorless) fused on ceramic materials. Vitreous
enamels are thin, normally opaque or semiopaque, colored coatings fused on metals, glasses, or

TABLE 12.5
Colorants for Stained Glass

Colorant Color Colorant Color
Nickel (II) oxide Yellow to purple Calcium fl uoride Milky white
Cobalt (II) oxide Blue Iron (II) compounds Green
Iron (III) compounds Yellow Copper (I) oxide Red, blue, or green
Tin (IV) oxide Opaque Manganese (IV) oxide Violet
Gold (III) oxide Red

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