The ores are roasted with 8 per cent. of salt, or 400 lb. of salt for the
charge of 2½ tons of ore; the salt costs 70 cents, or 2s. 2d. per 100 lb.
So roasted the ores are only partially chlorinized, and their complete
chlorination is effected subsequently, during the process of amalgamation;
the chlorides are thus formed progressively as required, and, in fact, it
would almost appear that the success of the process virtually consists in
obviating the formation of injurious salts. All the sulphide ores in
Bolivia contain sufficient copper to form the quantity of cuprous chloride
requisite for the first stages of roasting, in order to render the silver
contained in the ore thoroughly amenable to subsequent amalgamation.

_Amalgamating_.--From the furnaces the roasted ore is taken in ore cars to
large hoppers or bins situated immediately behind the grinding and
amalgamating vats, locally known as "tinas," into which the ore is run
from the bin through a chute fitted with a regulating slide. The tinas or
amalgamating vats constitute the prominent feature of the Francke process;
they are large wooden vats, shown in Figs. 1 and 2, page 173, from 6 ft.
to 10 ft. in diameter and 5 ft. deep, capacious enough to treat about 2½
tons of ore at a time. Each vat is very strongly constructed, being bound
with thick iron hoops. At the bottom it is fitted with copper plates about
3 in. thick, A in Fig. 1; and at intervals round the sides of the vat are
fixed copper plates, as shown in Figs. 3 and 4, with ribs on their inner
faces, slightly inclined to the horizontal, for promoting a more thorough
mixing. It is considered essential to the success of the process that the
bottom plates should present a clear rubbing surface of at least 10 square
feet.

[Illustration: THE FRANCKE "TINA" PROCESS FOR THE AMALGAMATION OF SILVER
ORES.]