prove this statement, let the student add to one of his charges either antimony or copper, and then examine the resultant button. From this the student will gather that before assaying a strange lead ore, he must examine it qualitatively for these or similar substances. If these be present to any

extent, fire methods must be replaced by wet processes.

(b) Oxidised Ores containing Carbonates and Sulphates of Lead. Method. For carbonates the following charge serves:

But where sulphates are present the Cyanide method is preferable; in fact, for both classes of ore, when fairly pure, this latter method is both simple and accurate.

Details of the operations need not be repeated. Let the student weigh out four charges,-two of the charge just given, and two of the cyanide charge as previously given. Fire carefully, and compare the results.

In conclusion, the student is reminded that practice and careful observation are necessary before success can be obtained. Where unsatisfactory results are obtained, the student should see that (1) The ore is ground sufficiently fine; (2) That the fluxes are well mixed with the ore, intimate mixture being necessary; (3) That with every assay he note the state of the fire, the time that elapses before the charge fuses, the time of 'frothy' fusion, the time of tranquil fusion; (4) That he examine qualitatively the lead button and determine what foreign ingredients are present; (5) If results are low, examine the slags for lead.

By these and other tests the source of error may be located, and from the nature of the error the student will see whether his work is at fault, or, as may be the case, that the Fire method is unsuitable to the ore in question. It is only by taking pains that the student can arrive at any certainty as to the value of his work, and without certainty an assay is of little value. The student has not the time necessary to fully check all his operations, but wherever reasonably practicable he should make some attempt. in this direction, and also should consult reference works on such points.

CHAPTER IX.

THE ASSAY OF GOLD IN ITS ORES AND PRODUCTS.

THE student is required to estimate gold in

I. Ores:

(a) Clean (Siliceous) Ores.
(b) Basic Oxidised Ores.

(c) Sulphide Ores.
(1) Iron Pyrites.

(2) Cupriferous.

(3) Antimonial.

(4) Arsenical.

(d) Telluride Ores.

II. Products:

(a) Mattes.

(b) Blister Copper.

(c) Gold Bullion.

Introductory. In order that the subsequent descriptions of methods may not be unduly lengthened by explanations regarding fluxing and other details, some general principles and notes have been collected, and are here laid down for the guidance of the student, who before going further should again read the chapter (II.) on Reagents and Fluxes.

The two most important fluxes used in the assay of gold ores are borax and soda; borax being used as a flux for CaO, FeO, and other like bases; soda for SiO, and silicates; 1 Na,CO, or 23 NaHCO3, fluxing 1 SiO, by weight. The compounds thus formed (borates and silicates) are fusible at a lower temperature than the metallic oxides, silica, or silicates alone.

Next in importance are the lead oxides litharge and red lead. Besides acting as collectors of precious metals, the monoxide is a powerful base, and in certain cases-for instance, when oxides of copper are present-it assists in the formation of readily fusible double silicates. Litharge is best suited for quartz ores and red lead for pyritic ores when an oxidising action is required.

Of the remaining fluxes fluor spar is of importance, being of service with ores containing considerable proportions of sulphates of barium and calcium or calcium phosphate The action here seems to be partly that of a change of solvent, the fluidity of the charge being increased. Fluor spar also removes SiO2 as SiH4.

As a general rule, 2 to 3 parts by weight of flux to 1 part of ore give a fusible slag, and where this is not so the remedy lies in the nature and proportion of the fluxes used, not in the quantity. It should be noticed that too fusible a mixture may be obtained, with the result that the lead has not the opportunity of thoroughly collecting the precious metals, as the lead sinks too rapidly to the bottom of the crucible. With ores containing much FeO the addition of alkaline carbonates forms double silicates of

1

such a nature that the slag formed is very fluid if a large excess of carbonates is used; therefore, as the metallic oxides in an ore increase, diminish the proportion of soda and increase the proportion of borax, litharge, and charcoal (see following notes on Reducers), but for ores containing much lime alumina or zinc oxide, the proportion of alkaline carbonates can be considerably increased.

Regarding reducers, the student will find that flour or argol-preferably the latter is preferable to charcoal, for the following reasons: The larger bulk of argol (4 times) permits of a much more intimate mixing of the reducer and charge, the whole charge, on fusion, thus becoming evenly permeated with very minute globules of metallic lead. Also in routine work less care need be exercised in weighing the argol. One grain error in weighing or measuring the reducer produces a difference of 24 grains lead with charcoal and only 6 grains with argol.

Besides reducing litharge or red lead to metallic lead, it is frequently necessary to reduce ferric oxide (Fe,O) to ferrous oxide (FeO). With hematite ores or roasted pyritic ores, as the ferric oxide is infusible and renders the slag pasty, and as ferrous oxide readily forms fusible compounds, it is necessary to reduce the higher to the lower oxide. If charcoal be used for this purpose, about 1 grain of charcoal is required for every 10 grains of ferric oxide present.

Salt is used largely as a cover to protect the charge from oxidation, the layer of fused salt floating on the surface of the charge. Further, it is applied in 'pinches' to 'settle' a charge which is 'boiling' too violently, the action resembling that of oil on water.

The student is advised in every case to bring his chemical knowledge to bear, and to proportion his ore and fluxes rationally, and not by the socalled universal formulæ given in some text-books. The analogy between the blast furnace and charge and the crucible and charge is very close from the chemical standpoint, and the varied nature of the ores treated demands in both cases a thorough grasp of the underlying principles, and not a slavish following of some universal formula supposed to suit all classes of ore.

(a) Clean (Siliceous) Ores.-These ores contain gold, a little silver, over 95% silica, and a small quantity of metallic oxides.

Method No. 1-Crucible.

The silica is fluxed partly by soda and partly by litharge, forming double silicates. According to the equations

43
27

=

1 gm. Na, CO unites with 57 gm. SiO, and 1 gm. PbO unites with 27 gm. SiO2. Generally, for every 1 gm. ore, about 1 gm. Na,CO, is added; this leaves about 43 gm. SiO2 (157) to be fluxed by PbO. This requires gms. 1.6 gms. PbO. Further, a button of about 12 to 15 gms. lead is required, and this brings the total PbO up to about 2 gms. for every 1 gm. of ore. A little more soda and less litharge, or vice versa, may be used. A small quantity of borax is added to flux the metallic oxides present. Sufficient reducer is used to obtain a button of the required weight. Salt is added as a cover.

The lead button thus obtained is cupelled, the silver and gold being obtained as a bead.

The silver in the bead is dissolved (after inquartation) by nitric acid, leaving the gold, which is then weighed.

Apparatus.-Wind and muffle furnaces and tools, pulp scales, assay balance, weights, porcelain cups, cupels, crucibles, and the usual accessories.

Reagents. Soda, litharge, argol, borax or borax glass, salt, test silver, parting acids, etc.

Details of the Assay.-Reduce and sample the ore, passing the final sample through an 80 sieve. (In this and all assays the student should make a point of attending to his furnace before weighing out his charge.)

Weigh out in duplicate the following charge

Note.-If the bicarbonate of sodium be used, a greater weight must be taken to flux the same amount of SiO2 (Na,CO, is equivalent to 2NaHCO3), 106 parts by weight of Na,CO, being equal to 144 parts by weight of NaHCO.

Thoroughly mix the two charges; transfer them to their crucibles (G); place the salt covers on the charges and transfer the charged crucibles to a red fire. Cover with lids, placing the lid so that it projects a little over one side to allow the insertion of a poker to wedge up the lid if it happens to stick fast. If the charge after fusion shows signs of boiling over, add a little salt now and then till the violent effervescence ceases. Fuse at a red heat for 5 minutes, and at a bright red for 20 to 25 minutes. By this time the charge should be in a state of quiet fusion.

Remove the lids; withdraw and pour the charges into clean hot moulds, gently tapping each crucible on the furnace top before pouring, and at the same time giving a slight circular swirl to the crucible to wash the sides with the slag. When the mould has cooled sufficiently to solidify the lead, the slag and lead are removed and the slag broken off the button, which is then hammered into a cube with flattened corners. It is important that all the slag be removed by the hammering. The student will learn by experience that many assays are lost by undue haste in tipping the charge out of the mould before the lead cools, and that a cupellation may be spoiled by carelessness in cleaning the button.

The buttons obtained have now to be cupelled, but the student will find it advisable to repeat the work and obtain two more buttons before firing the muffle, which is to be heated between a red and a bright red.

Place four good cupels in the muffle, and in about five minutes remove them with the tongs and examine them for cracks, rejecting any showing faults. Replace the good cupels. Place the lead buttons carefully on the cupels, one on each. Close the doors and keep them so till the layer on top of the molten lead clears, that is, becomes bright and glowing. Now open the doors and continue the cupellation so that feathers' just form. Towards the end of the cupellation the alloy becomes nearly spherical in shape, and bright rainbow-coloured bands or rings rapidly move from the centre outwards, till finally they disappear, and a round bead remains and appears to rotate rapidly. The doors may now be closed or the cupel shifted to a hotter spot to remove the last traces of litharge.

As there is generally little silver in the ore that is now being assayed, the cupel may be removed at once and placed on a sheet-iron cupel tray. The time

of cupellation depends on the size of button and the temperature and quantity of air present, and on the average is from 30 to 45 minutes for a button of about 16 gms.

The four cupels and beads now rest in the cupel tray. Take four small porcelain crucibles about 25 c.m. in diameter, and taking each cupel in turn remove the bead carefully and steadily with a pair of suitable forceps and place it in a cup, and place the cup on the cupel. Remove the tray and contents to a small anvil with polished face. Examine each bead, and if any bone ash is attached fold the bead in a small slip of paper (to prevent it rolling away) and flatten with a few taps on the anvil and rub between the paper.

Weigh each bead on the assay balance, the weights giving the gold and silver present. The gold has now to be separated from the silver. The student will find in bullion assaying that great care is taken in proportioning the silver to the gold, in the proportion generally of 2 silver to 1 gold, but here it is sufficient for all practical purposes to add, if necessary, sufficient silver to obtain from 3 to 5 or 6 parts of silver to 1 part of gold. Therefore, if the bead appears silvery white, add about its own weight of silver, but if yellow, add about three times its own weight of silver. This added silver is roughly weighed out (with practice it is judged by sight by cutting given areas from a ribbon of silver of uniform thickness), preferably as foil, in which the bead is then wrapped and transferred to a small cavity in a piece of charcoal, and fused up for a minute or so with the mouth blowpipe. A hot pointed flame gives the best results. To secure an even alloy of the gold and silver the bead must be heated till it appears as a white spinning globule. (Large beads must be cupelled.)

Each bead is thus treated and replaced in its cup. The beads are then, each in its turn, flattened on the anvil till about the thickness of an ordinary visiting card, and then replaced in their respective cups.

Fill each cup about two-thirds full of parting acid No. 1. Place the four cups in order on a hot iron plate or sand bath and heat till the solvent action appears to cease-generally three to five minutes-then remove the cups and decant the liquid down a glass rod into a 'waste silver' bottle. No. 2 acid is now used in the same manner for about three to five minutes' heating, but avoiding boiling. (As before mentioned, the use of only one acid much facilitates the operation and gives accurate results.)

Remove the cups, decant and wash twice by filling up with distilled water from the wash bottle. By a little care and dexterity the washing and decantation can be carried out quickly and accurately, the black gold residue being retained in the cup.

At the last decantation drain the cup well, and turn it so that the last drop of water lies away from the gold. In this position dry it on the plate or sand bath. When dry, remove with the forceps and direct a strong O.F. by the mouth blowpipe on the outside of the crucible nearest to where the gold lies. In ten or twenty seconds the black residue becomes yellow. Now place the cup back on its cupel to cool, and treat the other beads in the same manner, returning each cup and bead to its respective cupel. When cool, take the tray into the balance room and weigh the gold obtained. This is done by removing the little pan to the front of the balance case and then bringing up the cup to the pan and carefully brushing in the gold with a fine brush on to the pan, which is then replaced and the gold weighed as usual. The student must see that his cups are clean, and that he transfers no bone ash from the cupels to the balance, also that he checks the balance frequently when weighing a number of beads, also that his 'rider' agrees with his weights.