According to M. Frèrejean, proprietor of the great copper works of Vienne, in Dauphiny, too low a temperature or too much charcoal, gives to the metal a cubical structure, or that of divergent rays; in either of which states it wants tenacity. Too high a temperature, or too rapid a supply of oxygen, gives it a brick red colour, a radiated crystallization without lustre, or a very fine grain of indeterminate form; the last structure being unsuitable for copper that is to be worked under the hammer or in the rolling-press. The form which indicates most tenacity is radiated with minute fibres glistening in mass. Melted copper will sometimes pass successively through these three states in the space of ten minutes.
[Fig. 316.] represents a roasting mound of copper pyrites in the Lower Hartz, near Goslar, where a portion of the sulphur is collected. It is a vertical section of a truncated quadrangular pyramid. A layer of wooden billets is arranged at the base of the pyramid in the line a a.
C, a wooden chimney which stands in the centre of the mound with a small pile of charcoal at its bottom, c; d d are large lumps of ore surrounded by smaller pieces; f f, are rubbish and earth to form a covering. A current of air is admitted under the billets by an opening, in the middle of each of the four sides of the base a a, so that two principal currents of air cross under the vertical axis C of the truncated pyramid, as indicated in the figure.
The fire is applied through the chimney C; the charcoal at its bottom c, and the pile a a are kindled. The sulphureous ores d, f, are raised to such a high temperature as to expel the sulphur in the state of vapour.
In the Lower Hartz a roasting mound continues burning during four months. Some days after it is kindled the sulphur begins to exhale, and is condensed by the air at the upper surface of the pyramid. When this seems impregnated with it, small basins l l are excavated, in which some liquid sulphur collects; it is removed from time to time with iron ladles, and thrown into water, where it solidifies. It is then refined and cast into roll brimstone.
A similar roasting mound contains, in the Lower Hartz, from 100 to 110 tons of ore and 730 cubic feet of wood. It yields in four months about one ton and a half of sulphur from copper pyrites. Lead ore is treated in the same way, but it furnishes less sulphur.
There are usually from 12 to 15 roasting heaps in action at once for three smelting works of the Lower Hartz. After the first roasting two heaps are united to form a third, which is calcined anew, but under a shed; the ores are then stirred up and roasted for the third time, whence a crude mixture is procured for the smelting-house.