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 decol- 
orizing 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, hav- 
ing 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 re- 
sults, 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 judg- 
ment, 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: