The Mechanical Assay of Quartz.

Cromwell Argus, Volume III, Issue 156, 5 November 1872, Page 7

The Mechanical Assay of Quartz.

At a meeting of the Royal Society of Victoria held recently, Mr Foord read a paper 1 on the subject of the mechanical assay of | quartz. He commenced by explaining the > immense value of assay from, the practical [ point of view ; that it was the means of preventing the outlay of capital in developing 1 quartz which might not contain a particle of ' gold ; and then stated that assay, as a test of 1 whether stone was auriferous or not, was 1 comparatively ignored in Victoria. He then | explained the process of assay by fire, point- ' ing out that however valuable the method 1 might be to the analytic chemist, it would be useless in the hands of the unskilled, or of the miner who at a moment's notice was de- | i sirous of discovering whether a newly-found 1 rreef contained any of the precious metal. The method followed by him in the mechanical assay was as follows:—The stone was first pulverised in a large cast-iron mortar. A sieve of brass wire of fine mesh—about the thirty-fifth of an inch square—was then used, , and after the sample had passed through the wire, the powder was ground still finer in a i shallow cast-iron mortar about eleven inches in diameter and five inches deep. A weighted quantity of this very finely-ground sample was submitted to careful washing in a porcelain dish. A residue was obtained almost , entirely of the metallic sulphides of the ore. These were transferred to a large agate mortar, were again ground and washed until a small grey residue showed that no gold remained. The grey residue consisted almost entirely of particles of cast-iron or wroughtiron abraded during the operations. Such particles may be easily separated. A darningneedle or steel-pen is magnetised by drawing it once or twice, always in the same direction, over the surface of an ordinary horse-shoe magnet, and to the little steel magnet thus obtained the particles of iron will attach themselves, and may be freed from any entangled particles of gold by drawing them backwards and forwards through the water with which the agate mortar is filled. When the cast- | iron particles have been removed by the magnet, the gold in the sample, if any, will be revealed, and can be examined, collected, and weighed. A little chemical treatment, such as can be done by the aid of the mo3t \ portable instruments, will often prove advantageous at the close of the purely mechanical I treatment. The residue can be cupelled, and I a spherical button of gold will be obtained, j and the return per ton can be ascertained by the diameter of the bead, measuring it instead of weighing it. The tools necessary for the ■ mechanical treatment just mentioned will cost a few pounds, but the outlay will in many cases soon repay itself, and would in ' most cases be warranted as a preliminary expense, rather than going blindly to work with a newly-discovered reef, —rather than pro-' j ceeding to sink shafts and erect machinery j j without acquaintance with the contents of the I j stone. By this treatment it cannot be said i j that no gold will escape, but at least as high j a return will be obtained by the samples as 1 j the yield on the large scale. Of course, even | for these trials a certain amount of skill and some delicacy of manipulation were required ; but such, he believed, as a person of fair intelligence could quickly master. The acqui- : sition of the little details of manipulation \

would amply repay the miner's trouble in mastering them ; and as there was now a movement towards the establishment of mining schools, it was desirable that the mechanical assay of quartz should be taught, among other matters, in such institutions. The old method of simply burning the quartz was a pernicious one, because when the burnt pyrites was brought to a fusion point, it locked up the gold, which could be separated afterwards only with great difficulty. He thought it would be well if the quartz were properly roasted in a kiln before being crushed, but pointed out that this operation" taking into consideration the general average of the quartz, would not repay the outlay.