20 to 24 20 to 25 2.5 grammes (38.58 grains, from

to 15

10

To to A. T.) are used and repeatedly scorified.

Charge same as above.

Lead matt

12 to 15

basic

Gun-metal.

Kiehnstöcke

18 to 20 1 to 5

Charge 2.5 grammes (38.58 grains, to1⁄2 A. T.).

8 to 9

8

10 to 20

0 to 20 See p. 22.

20 to 24 20 to 25 Charge 2.5 grammes (38.58

Cobaltiferous ores

Cupriferous ores.

The quantity of test lead required according to the foregoing table is measured or weighed off, and divided approximately into two parts. The accurately weighed assay sample is mixed with one-half of the lead in the bottom of the scorifier, and the mixture is covered over with the remaining part of the lead, and finally the borax is added. The charged scorifier is placed in a strongly heated muffle, the mouth of which is closed, and a strong draft kept up for the purpose. The lead will soon commence to fuse, and in sinking down absorb silver from the ore. This, on rising to the surface, is roasted off, and is strongly oxidized by the lead oxide formed at the same time. During the oxidation slag is formed from the edges of the scorifier, by the combination of another part of the lead oxide with the metallic oxides and earths that are present, and with the borax. The time required for this

66

first heating (roasting and fusing) is from 25 to 30 minutes when a completely fused ring of slag, without adherence to the edge of the scorifier, will show itself. (Refractory ores, such, for instance, as contain zinc, cobalt, and nickel, or which contain considerable lime, require the strongest heat, and, should they not completely fuse even then, a sufficiently large addition of borax must be made before the final heating.) The second period is that of the scorification." The fire is checked, and the mouth of the muffle is opened, until, by the continued oxidation of the lead and foreign metallic compounds, the entire surface of the lead is covered with slag. This will require from 20 to 30 minutes. The mouth of the muffle is then closed, the heat raised, and a final heating of 10 to 15 minutes is given to render the slag completely fluid. The scorifier is now taken from the furnace, and allowed to cool in the scorifier or poured off. After cooling off, the lead button is carefully freed from slag and hammered into the form of a cube, with truncated edges and corners. The time required for the entire operation will be from to 1 hour, according to the degree of fusibility of the When the ores are very poor, a number of the lead buttons which have been obtained are placed on a scorifier, either with or without borax, and scorified as indicated above. If necessary the concentration is repeated until finally one button containing the entire percentage of silver is obtained. A second scorification is also advisable, in case the button be too large, or when it contains much antimony, arsenic, or copper. A percentage of nickel will exert a disturbing influence in cupellation. A cupel will usually absorb about its own weight of litharge, from which the proper size of the button may be estimated.

ores.

Hungary: Two samples each of 2.5 grammes (38.58 grains, about A. T.) are each charged with 8 to 16 parts of granulated lead, in such a manner that one-third of it is mixed with the ore and some silver flux (2 parts of melted Villach litharge and 1 part of calcined borax), and covered with the remaining two-thirds. Lower Harz: 5 grammes (77.16 grains, ab. A. T.) are mixed in the scorifier with 50 grammes (771.61 grains) of granulated lead and 0.75 to 1 gramme (11.57 to 15.43 grains) of borax, and covered with 0.5 gramme (7.71 grains) of borax. The cupels consist of 3 parts wood-ash and 1 part bone-ash.

With chloridized ores: A charge of 5 to 10 grammes (77.16 to 154.32 grains, to A. T.) of the ore is scorified with ten times its weight of lead, and cupelled to determine the percentage of silver. In a second sample the silver chloride is dissolved out by lixiviation with sodium hyposulphite, and the residue is scorified, etc., for the estimation of the unchloridized silver.

2. Crucible assay.-In this method of assaying, the ore is fused with lead oxide (litharge, white lead), in order to decompose the metallic sulphides (p. 78), with fluxing agents (potash, borax), for slagging off oxide and earths, and with some carbonaceous substance (charcoal powder, flour, argol, black flux), for reducing the lead which then collects the silver. The quantity of the reducing agent will depend on the reducing power of the ore. It should be so gauged that the lead button produced shall not be too large in order to prevent a notable loss of silver in cupellation. Ores containing large percentage of antimony, arsenic, and zinc should be previously roasted, to prevent the formation of oxysulphides, etc., which are difficult to decompose, and which carry silver along with them into the slag; 5 grammes (77.16 grains, ab. A. T.) of the finely powdered ore are mixed in a crucible with 40 grammes. (617.29 grains) of a flux consisting of 1.5 parts of lith

1 B. u. h. Ztg. 1871, p. 254.

arge, 0.15 part of potassium carbonate, and 0.08 part of flour. This is covered with 25 grammes (385.80 grains) of litharge, and this in turn with about 4 grammes (61.73 grains) of borax. The crucible should have smooth sides, a diameter of 45 millimeters (1.77 inches) at the top, and of 30 millimeters (1.18 inches) at the bottom, an inside height of 145 millimeters (5.70 inches), and outside of 165 millimeters (6.49 inches). The charged crucible is then placed in the furnace upon a bed of glowing coke, which should cover the grate to a height of from 100 to 150 millimeters (3.93 to 5.9 inches), and is then surrounded up to its rim with wood charcoal. The furnace is left open

for the first quarter of an hour. After the coal has been replenished, the cover of the furnace is put on and the fusing is continued for a quarter of an hour longer. The crucible is then taken out, allowed to cool, and the lead button, which should weigh from 20 to 25 grammes (308.64 to 385.80 grains) is freed from slag. (Chile.) The same quantity of litharge as used for the assay is fused at the same time, but without ore, with fluxing agents. The lead button is freed from slag and cupelled, to determine the percentage of silver, which must be deducted from the assay-button. Or, 5 grammes (77.16 grains, ab. A. T.) of ore are fused with 50 grammes (771.61 grains) of lithage, 2 grammes (30.87 grains) of argol, 12 grammes (185.19 grains) of sodium carbonate, with a covering of common salt, and the resulting lead button is cupelled. White lead is sufficiently free from silver.

Mexican charge:1 20 grammes (308.64 grains, about A. T.) of ore, 66 grammes (1018.53 grains) of litharge, the same quantity of sodium carbonate, and 3 grammes (46.30 grains) of charcoal powder are mixed in a crucible of the above dimensions and covered with 20

grammes (308.64 grains) of common salt. 40 assays are put in the

1 B. u. h. Ztg. 1874, p. 86.