The sampling and assay of the precious metals; comprising gold, silver, platinum, and the platinum group metals in ores, bullion, and products

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1913 Excerpt: ... given on page 107. The samples are assayed in a gas-fired muffle furnace by the crucible fireassay method, using the following fluxes:--(1) "Silver" flux containing 40 per cent. each of sodium and potassium carbonates and 20 per cent. of borax glass; (2) litharge; (3) borax glass for cover. The charge for the crucible contains 0-2 assay ton of pulp, 30 grammes of "silver " flux, 60 grammes of litharge, and 1 gramme of flour. These are mixed by rolling in a rubber cloth, transferred to the crucible, covered with borax glass, and heated to a medium red heat for twenty minutes. The door of the muffle is then shut and the crucible heated to a bright yellow for thirty minutes, the contents poured into a conical iron mould, and when cold hammered free from slag. The metal is then cupelled at as low a temperature as possible, using a freely oxidising atmosphere without excessive draught; the temperature and draught control are important factors. The following table gives examples of results obtained by independent assayers:--1 "Sampling and Assaying of Silver Ores containing Cobalt, Nickel, and Arsenic," J. 0. Handy, Eighth Int. Cong. A/tpl. Chem., 1912, Sect. Iiia. (abstract, Journ. Soc. Chan. Ind., 1912, vol. xxxi. p. 880). Auriferous Tin Ore.1--Native gold is not infrequently found in tin ores and in the alluvial deposits of tinstone (cassiterite, tin oxide), the chief ore of tin. The samples obtained by panning alluvial deposits may be very rich in gold, and contain 25 ounces or more of gold to the ton. The crucible assay gives excellent results, both for silver and gold present; and as 25 grammes can easily be worked off, it is the most suitable method for most tin ores containing precious metal. Soda is the chief flux used, as ...