_Roasting_.--From the stamps the stamped ore is taken in small ore cars to
the roasting furnaces, which are double bedded in design, one hearth being
built immediately above the other. This type of furnace has proved, after
various trials, to be that best suited for the treatment of the Bolivian
silver ores, and is stated to have been found the most economical as
regards consumption of fuel, and to give the least trouble in labor.

At the Huanchaca mines these furnaces cost about 100_l_. each, and are
capable of roasting from 2 to 2½ tons of ore in twenty-four hours, the
quantity and cost of the fuel consumed being as follows:

Bolivian dollars at 3s. 1d.
Tola (a kind of shrub), 3 cwt., at 60 cents. 1.80
Yareta (a resinous moss), 4 cwt., at 80 cents. 3.20
Torba (turf), 10 cwt., at 40 cents. 4.00
----
Bolivian dollars. 9.00, say 28s.

One man can attend to two furnaces, and earns 3s. per shift of twelve
hours.

Probably no revolving mechanical furnace is suited to the roasting of
these ores, as the operation requires to be carefully and intelligently
watched, for it is essential to the success of the Francke process that
the ores should not be completely or "dead" roasted, inasmuch as certain
salts, prejudicial to the ultimate proper working of the process, are
liable to be formed if the roasting be too protracted. These salts are
mainly due to the presence of antimony, zinc, lead, and arsenic, all of
which are unfavorable to amalgamation.