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.

The ores are roasted with 8 per cent. of salt, or 400 lb. of salt for the charge of 21/2 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 21/2 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.]