He glanced at her as he set the roasting pan on the stove top to cool.
Blende contains only about half as much sulphur as good pyrites, and this cannot be burned off as easily as from pyrites, but this" roasting "has to be done somehow in any case in order to prepare the ore for the extraction of the zinc.
The roasting of pyrites always takes place without using any extraneous fuel, the heat given off by the oxidation of the sulphur and the iron being quite sufficient to carry on the process.
The roasting of blende is nothing like so easy as that of pyrites, since the heat developed by the oxidation of the zinc sulphide itself is not sufficient for carrying on the process, and external heat must be applied.
This operation is both more costly and more delicate than the roasting of pyrites, but it is now perfectly well understood, and gas is obtained from blende furnaces hardly inferior in quality to that yielded by pyrites kilns.