of the finer qualities of glass. We find more or less iron
in nearly all materials in use, and this substance is the
glass-maker’s constant enemy. So we use various decolorizing
 agents, principally oxide of manganese and nickel.
In the case of the finer qualities of glass, where we wish to
produce beautiful colors, we use various coloring agents—
such metallic substances as cobalt for making blue, with
the oxide of manganese for producing pink and gold, and
selenium for making rich ruby colors, as well as uranium
and cadmium for producing topaz and orange colors.
In its nature glass is peculiar, differing from almost
every other manufactured article. It is amorphous, having
 no line of cleavage, and, while crystalline in appearance,
the laws of crystallization do not in any manner apply to it.
To manufacture glass successfully and with uniform results,
 to make it commercially and meet the requirements
of the trade, require constant vigilance on the part of the
manufacturer, and sound judgment, founded on long
experience. No manufacturer, no matter how much he
knows himself as to how things should be done, and no
matter how trustworthy he considers his men, is ever
certain of results. When he fills his pots he can tell at
what hour any particular pot of glass should be ready to
work, but he can never be sure, because of the variable
conditions to which the process is subjected. Moreover,
when new pots are placed in the furnace, with walls from
three to four inches thick, his “melt” will be much longer
in preparation for working, than in the older pots whose
walls may be worn down say to an inch in thickness.
Then again, good glass may be spoiled by careless workmen
“gathering” the glass when too hot or too cold. This
produces streaky glass, or glass with blisters on it. Success
in glassmaking can be obtained only approximately, and
that by the exercise of constant vigilance and good judgment,
 and no manufacturer can say beforehand how much
glass of a good quality he is going to be able to turn out in a
given time.
The following are the principal manufacturers of glass
in the Pittsburgh district: