dross, and throw away? That is the result of the mixture of the oxygen
of the air with the lead it comes in contact with; that is, so much of
the lead is oxidized. It is on its way to become litharge, or
red-lead, which is lead oxidized to the highest degree. To make the
oxide, they melt lead on the floor of a large oven. It becomes a
bright lake of melted metal, at first; it is stirred, and kept burning,
until the last appearance of anything like liquid lead is worked out of
it. Some glass-factories make their own red-lead; but ours comes from
Galena, in just this shape, as you see it."
"In what proportion do you mix your materials
for flint glass?"
"Three parts of sand, two of red-lead, and one
of alkali, is about as simple as statement as I can make of it. That
will make you a good strong glass. But there will be a tinge of green
in it, such as you see in a pane of common window-glass if you look
across the edge of it. That comes from a minute quantity of iron which
is contained even in the purest silica. A little arsenic and oxide of
manganese take it out, or, as we say, decolorize it. Too much lead gives
a yellowish cast to the glass. The oxides of other metals are used to
give different colors. In making different kinds of glass, the materials
may be varied infinitely. Boracic acid may take the place of silica.
Oxide of zinc may take the place of red-lead; in window and plate glass,
lime takes its place. A variety of other substances are used to produce
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