The addition of lime,
oxide of iron, or alkali to silicate of alumina results in the formation
of a double silicate of alumina and lime, or of alumina and iron, &c.,
all of which are easily fusible. Similarly, if to a silicate of lime we
add oxide of iron, or soda, or even alumina, a fusible double silicate
will be formed. Thus lime, soda, oxide of iron, and clay, are _fluxes_
when properly used; but since lime, clay (and oxide of iron if there be
any tendency to form peroxide), are of themselves infusible, any excess
of these fluxes would tend to stiffen and render pasty the resulting
slag. So, too, soda, which is a very strong base, may act prejudicially
if it be in sufficient excess to set free notable quantities of lime and
magnesia, which but for that excess would exist in combination as
complex fusible silicates. There are many minerals which with but little
soda form a glass, but with more yield a lumpy scoriacious mass. There
are many minerals, too, which are already basic (for example, calcite),
and which, when present, demand either a less basic or an acid flux
according to the proportions in which they exist. For purposes of this
kind borax, or glass, or clay with more or less soda may be used, and of
these borax is by far the most generally useful. An objection to too
basic a slag (and a very important one) is the speed with which it
corrodes ordinary crucibles.
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