This most useful and elegant material--now an article of almost
universal application, in various forms--is made on a very large
scale by fusing together sea-sand and alkali (either potash, soda,
or lime), and, in the case of “crystal,” or “flint-glass,” oxide of
lead. The following is about the composition of the chief kinds of
glass in use:--
FLINT GLASS. PARTS. PLATE GLASS. PARTS.
Pure white sea-sand 52 Pure white sand 55
Potash 14 Soda 35
Oxide of lead 34 Nitre 8
------ Lime 2
100 -----
100
CROWN GLASS. PARTS. GREEN BOTTLE GLASS. PARTS.
Fine white sand 63 Sea-sand 80
Chalk 7 Salt 10
Soda 30 Lime 10
---- ----
100 100
Flint-glass or “crystal” is very heavy, moderately soft (being
easily cut with a file), and very bright and white. It is used for
all table-glass, as decanters, wine-glasses, &c., and for the drops
or lustres of chandeliers. Plate-glass is that kind of glass now in
such general use for shop-windows, looking-glasses, &c. It is cast
on flat iron tables, and rolled out to the sizes required, then cut
and polished by machinery. In the rough state it is called “rough
plate,” and is the substance used in the Crystal Palace, and has of
late become almost a substitute for ground glass, which is simply
ordinary glass ground or roughened on the surface by means of sand,
so that it will admit light and yet not allow objects to be seen
through it. Crown-glass is the ordinary “window-glass.” It is made
in great circular pieces (see “Glass-blowing”), and cut up into the
sizes required. Crown or window-glass made some years back, had a
disagreeable tinge of green, which has been removed in modern glass
by the addition of a minute quantity of oxide of manganese.