STOL® is the trade name for a naturally occurring siliceous volcanic rock called perlite.
The distinguishing feature of STOL® is that when heated above a suitable point in
its softening range it expands up to twenty times its original volume. In high
temperature applications, such as those encountered in the iron-and-steel and foundry
industries, the expansion of STOL® results in particles with large surface areas. This
expansion is also accompanied by a small cooling action. These effects combine
to coagulate slag, the unwanted by-product of metal smelting, and thereby form a
viscous, toffee-like mass that can be effectively removed from the molten metal
interface. An added advantage of perlite in this application is that no objectionable
smoke or noxious fumes are produced.