THE QUAETERLY JOURNAL OF THE CHENIGAL SOCIETY IX.-Om certain sources sf loss of Precious Hetal in some operations of Assaying. BY G. H. MAKINS. INthe assay of specimens of alloyed gold the degree of heat used in the first operation viz. cupellation (when this is carefully performed) is usually somew hat varied according to presupposed proportions in the alloy,-for example a much lower temperature being suBicient where the gold is only associated with silver than where it is also mixed with oxidisable metals which latter have to be separated entirely by this first operation About five years since I chanced to have before me some 20 assays of gold wherein the silver also had to be estimated and in which for other reasons an extraordinary degree of heat was required to be employed ; and as I made simultaneously with these several synthetieal prods I was milch struck by the great extent of loss of gold and silvep; and satisfied that it could riot be eutirely due to what is known amongst assayers as cupel absorption I determined upon examinirrg scme af the contents of an iron flue of a onpel furnace which passed from its hood into a chimney iu order to see if gold had been volatilized along with other metals and if so to what extent.VOL. XIIX. H WAKTNS ON PRECIOUS METALS IN ASSAYING. A quantity of apparently carbonacernxs matter mas therefore taken out of it; and aq this flue mas at the time recently ptit up afresh I had the means of knowing that it had been used for the cupeliation of gold assays only.Upon examiiration of tliis material by the microscope under an inch power it exlitbited very distrzlctly portions of oxide of lead in yellow masses little nodules of suboxide of copper minute grains of silt er together u ith portion8 resembling silver reduced from chloride,-the whole mixed with a quantity of' carbonized matter containing small grains of unburnt fuel From pressure of bnsiness at tEe time this material was laid aside (after cursory examination sufficient to prove the presence of the metals) until it cotild be completely examined. This mas not done horn ever until after the appearance of some collateral matter comniunicated to the Society by my late pupil Mr. Kapier. On further examination the metallic matters present mere found to consist of oxide of lead mixed with small portions of gold silver and oxide of copper; arid as my object was only to estimate the gold and silver present I took separate portions viz.two of 50 and two of 100 grains each. They were each treated with 400 grains of pure lead that is to say with eight times the weight of the former two and four times the latter each having also its own weight of borax added. The mixtures were then scorified and the operation carried to its full extent so as to diminish as much as possible the quantity of lead tor the subsequent cupel process. The process completed the specimens were poured in a god fluid conditiou; the slag (which was quite free from metallic beads) separated ; and the resulting mehi next subjected to cupel-lation.The little buttons 80 obtrtined consistiog of pure silver and gold were then weighed. Ti'r~~n the first two assays (the material operated upon being 3 00 grains) the resulting buttons weighed (after compensating for silver contained in &he lead employed) just .089 grain. From the second two upon 900 grains the buttons weighed *166. If pie average these ahich are very nearly alike and calm-late the quantity for 10 grains we should have a result of -850 grain. They were next all parted for gold and the result gas -026; equal to *087in every 1000grains MAKINS OX PRECIOUS METALS IX ASSAYING. Deducting then the weight of this gold from the weight of the compound metal we fiud the quantity of the precious metals in 1000 grains to be Gold ..-087 SJver . . -763 Of coiirse I do not attempt to establish any ratio between these quantities arid an individual assay; for although the flue whence this material wa3 taken had only been reriewed a few months yet during that time the fumes of many thousand assays had passed through it. But on the other haud I would rerriark that considering that this flue is removed daily when the furnace is at work the proportion which the precious metals bear to the whole is very large indeed. I will now call attention to a second cause of loss of gold occurring in parting operations and possibly also in refining upon the large scale viz. the solution of a portion of gold iu the nitric acid used although the latter is pure and quite free from hydro- chloric acid Berselius in speaking of this fact says <‘Itmust not be forgotten that if nitric acid contains hydrochloric or nitrous acid it will dissolve gold; in truth the quantity dissolved is incon- siderable in the latter case but sufficient to lead to an incorrect result.” NOW,in parting gold by nitric acid we are generating during the operation quantities of nitrous acid and consequently have the conditions present for this loss to take place.Some time since Mr Field (the Queen’s Assay Master) had in his office a pear-shaped bottle irrto which nitric acid was con- stantly being potired after parting operations it was thickly coated with gold. He showed this to Sir J. Herschell vho was then Master of the Mint otsserting that it was deposited from an actual solution of the gold.The latter gentleman supposed rather that it was a deposit from mechanically suspended particles; but this cpuld have hardly been the case as. the bottle was a large pear-shaped one a4d the gold coated the upper part as thickly as the lower and yery uniformly which certainly could not have been the case had it been simply a deposit of mechanically SUE. pended particles. It was no doubt as Mr. Field observed a good example of deposit from an actual solution. I tried the following experiment in order to arrive at gome idea of $he quantity so taken up in ordiaary assaying operations H2 100 MAKINS ON PREIOUS METALS IN ASSAYING. Four assay-quantities of pure gold were accurately weighed then added to the usual proportion of fine silver aud of lead and cupelled.The buttons then rolled and coiled were parted with nitric acid. For the hoilings acids mere prepared of specific gravity 1.25 and 1.35 respectively. KO.1 assay was then boiled 20 minutes in the weaker and aftervrards for 13 minutes in the stronger acid between which operations the cornet was washed with hot distilled water. No. 2 assay was boiled once in the first and twice in the second acid. No. 3. Once in the first and three times in the second. And No. 4. Once in the first and four times in the second; the boilings as I have said being continued for 20minutes in the weak and for 13 miniites in the second acid.The several cornets were then washed off with hot distilled water annealed and weighed and the following is the result :-Calling the weighi:tgs before the operations 1000 the first cornet weighed 999.6 the second 992 the third 998.7 and the fourth 997.9; the loss thus increasing in proportion as we multiply the number of boilings. Now the qucstion occurs as to whence the loss arises. I may first say that it has been suggested that it is due to silver being more completely separated by the repeated boilings. But inreply to this I can state that I have examined these cornets over and over again; and that after the first boiling as above described the amount of silver left in them is very uniformly 1.16 in the I000 parts. But the difference between 999.6 the weight of the first cornet and 997.9 that of the fourth is 1.70 ; here then supposing the latter to be entirely free from silver (which is not really the case) we have 0.54 abstracted a quantity just equal to 3 graius in the troy pound which loss can only be traced to the removal of gold.Next it has been urged that; the appearance of gold in the used pasting acid is due to mechanical causes dependent upon the friction of an exceedingly soft and spongy body by its being thrown violently about in a dense acid fluid and by the rapid evolution of bubbles of acid or acid vapour rushing from its surface all this being of course increased as the density of the acid is increased by evaporation. I believe that this is not the case and that in the fluid medium no such abrasion takes place but that it is a true case of solution MAKINS ON PRECIOUS BlETALS IN ASSAYING.101 and by the agency of the nitrous acid evolved. In proof of this I have taken a portion of this acid which had been so used and after dilution separated the silver then evaporated arid obtained small quantities of gold which were nest redissolved in nitro-hydrochloric acid and proved to be such by the usual tests. Again I do not think that the deposit upon Mr. Field’s store bottles (to which I have alluded) could have taken place in so regular a manner upon their upper portions without actual solution; for had it been mere mechanical suspension the metal upon the lower part would of course have been much in excess.Now as to the evolution of this nitrous acid. At the first boiling when the silver is present in such large quantity nitrous acid is given off most copiously; but probably its solvent action upon the gold is then controlled and checked by electrical action that is by the gold becoming the negative and the silver the positive element in a voltaic circuit; but when the silver is wholly (or nearly) removed this check no longer exists and the solution of the gold progresses more rapidly. Then with respect to the cause of the evolution of nitrous acid,-of course as I have already said as long as there is any silver to oxidise it will be generated; and when the silver is nearly abstracted the method of boiling adopted by many assayers induces its continuance; for it is scarcely possible to boil an assay in strong acid without some means of disengaging the vapour formed if some help be not afforded the.acid will “bump” so much as almost if not quite to throw it out of the flask. Hence it is the custom with some to introduce a small portion of char-cod which from its porosity tends to the steady evolution of this vapour. Now if the charcoal be entirely carbonized it does not materially affect the acid; but if it coritairi much woody matter not thoroughly carbonized it is then sure to decompose it and set nitrous acid free. Indeed I have for some years ceased to use this body from its injurious effects upon the acid. I am aware that the matters mentioned in this paper are some- what trivial in a scientific point of view; but their commercial importance will be at once admitted when we remember the enormous values dealt with in this country and consider more-over that they are at times turned over and over again and that 10.2 PERHIN AND DUPPA ON BIBROMOSUCCINIC ACID the question of profit and loss in such commercial operations are much if not entirely in the hands of the assayer.Lastly this publication may serve to account for some of the discrepancies which are sometimes found between assayers.