This section is from the book "Leaching Gold and Silver Ores. The Plattner And Kiss Processes: A Practical Treatise", by Charles Howard Aaron. Also available from Amazon: Leaching Gold And Silver Ores.
166. 5th Assay.Ore.......................... 1/2 ounce.
Sodium bicarb............... . 720 grains.
A little flour and pulverized glass will do no harm, and in some cases are beneficial. Place in a No. 7 or 8 crucible, with a very little borax and three large nails; cover with salt. Fuse slowly; then give a white heat for half an hour or more. If, on seizing one of the nails with the tongs, rinsing it in the fluid slag, and tapping it against the side of the pot, it is found to be free from adhering lead globules, the other nails may also be removed; but if the nail shows adhering lead, which cannot be washed off, more time is required.
The matte and slag should retain no lead in any form; hence the weight of the button should correspond with that of the litharge used, or may be in excess, if the ore contained lead. It must be soft, and separate well from the matte. If it does not, more soda is probably required. 300 grains of pure litharge contain 278.5 grains of lead, but five per cent may be allowed for volatilization.
This is a very convenient assay, requiring but little modification for the different sulphuretted ores. It is not liable to boil over, nor is it so destructive to the crucible as are the other methods described. If properly made, it gives as high results as can be obtained by any other method, in most cases, both as to gold and silver. Arsenical mattes cannot be assayed successfully by this method, and, in one instance of gold and silver bearing sulphides, though it gave the gold correctly, it did not yield as much of the silver as the assay by scorification.
167. Oxidized coppery ores, and especially cement copper contaminated with basic chlorides, may be conveniently assayed for gold and silver by a similar process, as follows:
Ore or cement copper.............1/2 ounce.
Sodium bicarb.................720 grains.
Some flour, or charcoal, borax, and enough sulphur to convert all the copper, the sodium, and the lead from the litharge, into sulphides. An excess of sulphur does no harm. The mixture, covered as usual with salt, is first fused; the nails are added, and a strong heat is continued for half an hour. The copper remains in the state of matte; the nails reduce the lead sulphide, and take up all excess of sulphur. Massive copper, and other metals, may be treated in the same way, if first broken, or cut into fragments.
168. 6th Assay. Scorification. Ore......................... 60 grains.
Granulated lead................480 "
Mix in a scorifier, and cover with another ounce of granulated lead. Place the scorifier in the muffle, under a low heat, and increase the temperature gradually, until the "bull's eye" appears - that is, until the surface of the melted lead is clear in the centre, and the fused, or partly fused, ore forms a ring around it. Keep up a cupelling heat, until the slag quite covers the lead, then, with the tongs, add some lumps of borax, and give a strong heat. Stir with a red hot piece of stout iron wire, to detach pasty lumps from the side of the scorifier. Pour when the slag has become so fluid as to run completely off the stirrer. If the lead button is too large, return it to the scorifier, after detaching the cooled slag, and oxidize it until of the proper weight. 169). Nearly clean silver glance is conveniently assayed by mixing it with two or three parts of litharge, melting quickly, and pouring as soon as well fused. The assay No. 5 is well adapted to heavily sulphuretted silver ores, containing antimony, arsenic, and zinc, as well as copper. It is rarely necessary to roast silver ore for assaying.
170. In assaying rich silver ores, that part of the cupel which is saturated with the lead oxide must be pulverized, dressed with litharge and flour, smelted, and the resulting lead cupelled, as a considerable quantity of silver is thus added to the result. This is quite important.
171. Tailings of concentrated and roasted gold-bearing sulphides are treated precisely as No. I, omit-ing the roasting. The reason why as much as 40 grains of flour must be used in an assay of 240 grains of the roasted ore is, that the iron peroxide which it contains consumes the carbonaceous matter, being reduced to protoxide, and if only the usual quantity of the reducing agent were used, little or no lead would be produced.