What Is the Difference Between Plate Glass and Window Glass?
How is plate glass made? These questions are asked very frequently. The
two products are wholly unlike each other; and we wish to show wherein
lies the difference. We shall tell how plate glass is made; and we hope
to make it clear that great care, time and expense are involved in its
manufacture.
The raw materials may be said to be virtually the same in plate glass
as in window glass; the main difference being that in plate glass
greater care is exercised in selecting and purifying the ingredients.
Window glass is made with a blow-pipe. The work requires skill on the
part of the operator; but the process is quite simple and rapid. And
the result is, naturally, a comparatively ordinary and indifferent
product. On the other hand, the superb quality of plate glass is owing
to the elaborate method of producing it.
Commercial plate glass was first made in France somewhat more than two
hundred years ago; although glass in one form or another has been in
use for many centuries. Apparently glass was known in Egypt fully four
thousand years ago.
[Illustration: MINING SILICA]
The materials used are silica (white sand), carbonate of soda (soda
ash), and lime. Other materials, as arsenic and charcoal, are used in
small proportions, but the main ingredients are the first three named.
Probably it is little imagined that in the production of plate glass,
mining is involved in two or more forms (namely silica and coal), also
the quarrying of limestone, the chemical manufacture of soda ash on
a large scale, the reduction and treatment of fire clay to its right
consistency, an elaborate and expensive system of pot making; and the
melting, casting, rolling, annealing, grinding and polishing of the
glass.
In special uses, as in beveled plates and mirrors, two more elaborate
processes must be added--beveling and silvering--all of which are
performed under the direction of experts aided by a large amount of
labor and expensive machinery.
Pots of fire clay take so important a part in the successful
manufacture of plate glass that the subject deserves especial notice.
The different clays after being mined are exposed to the weather for
some time to bring about disintegration.
~THE CLAY MUST BE TRAMPLED WITH BARE FEET~
At the proper stage finely sifted raw clay is mixed with coarse, burned
clay and water. This reduces liability of shrinkage and cracking. It
is then “pugged,” or kneaded in a mill; kept a long time (sometimes
a year) in storage bins to ripen; and afterwards goes through the
laborious process of “treading.” Nothing has thus far been found in
machinery by which the right kind of plasticity can be developed as
does this primitive treading by the bare feet of men. The clay must be
treaded, not once or twice, but many times. The building of pots is a
slow, tedious and time-killing affair; but this is most essential.
~HOW MELTING POTS ARE MADE~
Without extreme care, some elements used in the making of the pots
might be fused into glass while undergoing the intense heat of the
furnace; or they might break in the handling. The average pot must hold
about a ton of molten glass, and the average furnace heat necessary is
about 3,000° Fahrenheit. The work is not continuous. Each workman has
several pots in hand at a time, and passes from one to another adding
only a few inches a day to each pot, so that a proper interval for
seasoning be given. After completion, comes the proper drying out of
the pots; and this is another feature in which the greatest scientific
care is required. No pot may be used until it has been left to season
for at least three months, and even a year is desirable. And after all
this trouble, the pot has but 25 days of usefulness. The pots form one
of the heavy items of expense in plate glass manufacture; and upon
their safety great things depend.
[Illustration: POT MAKING.]
[Illustration: MIXING THE CLAY.