CHAPTER XLII. GLASS AND POTTERY.

Examine white sand, calcium carbonate, sodium carbonate, smalt;
bottle, window, Bohemian and flint glass.

218. Glass is an Artificial Silicate.--Si02 alone is almost
infusible, as is also Ca0; but mixed and heated the two readily
fuse, forming calcium silicate. Ca0 + SiO2 = ? Notice that Si02
is the basis of an acid, while CaO is essentially a base, and the
union of the two forms a salt. There are four principal kinds of
glass: (1) Bohemian, a silicate of K and Ca, not easily fused,
and hence used for chemical apparatus where high temperatures are
required; (2) window or plate glass, a silicate of Na and Ca; (3)
bottle glass, a silicate of Na, Ca, Al, Fe, etc., a variety which
is impure, and is tinged green by salts of Fe; (4) flint glass, a
silicate of K and Pb, used for lenses in optical instruments, cut
glass ware, and, with B added, for paste, or imitation diamonds,
etc. Pb gives to glass high refracting power, which is a valuable
property of diamonds, as well as of lenses.

219. Manufacture.--Pure white sand, Si02, is mixed with CaCO3 and
Na2CO3, some old glass - cullet - is added, and the mixture is
fused in fire-clay crucibles. For flint glass, Pb304, red lead,
is employed. If color is desired, mineral coloring matter is also
added, but not always at this stage. CoO, or smalt, gives blue;
uranium oxide, green; a mixture of Au and Sn of uncertain
composition, called the "purple of Cassius," gives purple. MnO2
is used to correct the green tint caused by FeO, which it is
supposed to oxidize. Opacity, or enamel, as in lamp-shades, is
produced by adding As2O3, Sb2O3, SnO2, cryolite, etc. The glass-
worker dips his blowpipe--a hollow iron rod five or six feet
long--into the fused mass of glass, removes a small portion,
rolls it on a smooth surface, swings it round in the air, blowing
meanwhile through the rod, and thus fashions it as desired, into
bottles, flasks, etc. For some wares, e.g. common goblets, the
glass is run into molds and stamped; for others it is blown and
welded. All glass must be annealed, i.e. cooled slowly, for
several days. The molecules thus arrange themselves naturally. If
not annealed, it breaks very easily. It may be greatly toughened
by dipping, when nearly red-hot, into hot oil. Cut glass is
prepared at great expense by subsequent grinding. Glass may be
rendered semi-opaque by etching either with HF, or with a blast
of sand.

220. Importance.--Few manufactured articles have more importance
than glass. Without it the sciences of chemistry, physics,
astronomy, microscopic anatomy, zoology, and botany, not to
mention its domestic uses, would be almost impossible.

221. Porcelain and Pottery.--Genuine porcelain and china-ware are
made of a fine clay, kaolin, which results from the
disintegration of feldspathic rocks. Bricks are baked clay. The
FeO in common clay is oxidized to Fe2O3, on heating, a process
which gives their red color. Some clay, having no Fe, is white;
this is used for fire-bricks and clay pipes. That containing Fe
is too fusible for fire-clay, which must also have much SiO2. The
electric arc, however, will melt even this, and the most
refractory vessels are of calcium oxide or of graphite. Pottery
is clay, molded, baked, and either glazed, like crockery, or
unglazed, like flower-pots. Jugs and coarse earthenware are
glazed by volatilizing NaCl in an oven which holds the porous
material. This coats the ware with sodium silicate. To glaze
china, it is dipped into a powder of feldspar and SiO2 suspended
in water and vinegar, and then fused. If the ware and glaze
expand uniformly with heat, the latter does not crack.