CHAPTER III.

The Assay of Silver Ores.

A large proportion of the silver of commerce is extracted from ores (which are too poor to allow of their being smelted or fused) by a process called amalgamation. Founded on the ready solubility of silver, &c., in metallic mercury, the ore is first crushed to powder, then mixed with common salt, and afterwards roasted. By the adoption of this plan the silver is reduced to a state of chloride. The roasting is done in a reverberatory furnace, in which the heat is very gradually raised, the ore being constantly stirred; the heat is then increased sufficiently to raise the ore to a good red heat. It is then put into wooden barrels, revolving on iron axles attached to the ends, and scraps of iron are then added to it; both are then agitated together by rotary motion, the effect of which is to reduce the chloride of silver to a metallic state. When this is effected, it is again agitated with mercury, and a fluid amalgam is formed with the metal, together with any other metallic ingredient that may happen to be present in the roasted ore. Subsequently, to recover the silver, the mercury is driven off by heat, and the silver is thus left behind in an impure state.

There are three ways of assaying silver ores; they are in the _test_ assay as follows:--

1. Melting in a crucible. 2. Scorification. 3. Cupellation.

In the crucible assay the ore is commonly run down with a suitable flux, those most frequently employed being litharge, carbonate of soda, borax, and charcoal. These four substances are all that are required by the practical assayer in the treatment of the regular ores of silver.

The assaying of the genuine ores is performed in the following manner; that is, if they contain but little earthy matter. They may then be conveniently treated by fusing with carbonate of soda, on account of its cheapness, and borax, in a fire-clay crucible (Fig. 2). The dimensions of the crucible should be as follows: 4-1/2 inches in height, and 2-1/2 inches in its greatest diameter, which should be at the top. A quantity of litharge (a semi-vitrious substance, oxide of lead), more than is actually necessary to take up the whole of the silver in the ore, should be added, so as to promote fusion, and collect the ingredients into one mass at the bottom of the crucible. In preparing the ore for the crucible, it must be well pounded, and intimately mixed with the undermentioned chemicals:--

Pounded silver ore 240 grains. Litharge 800 " Carbonate of soda 700 " Borax 300 " Charcoal 50 "

Place two crucibles to warm during the time occupied in the preparation of the mixture, then put it into the warm crucible; take 100 grains more of litharge, and powder it over the contents in the vessel. Prepare in this manner a second mixture for the other crucible, place them both in the furnace, and put plenty of coke round them. The mixtures may be melted in an ordinary wind or melting furnace, such as is used by jewellers in the preparation of their material for art working. The fusion should take place very gradually at first, as silver in combination with lead is sensibly volatile at a high temperature: it may then be continued at a low heat for twenty-five minutes, and finally the operation may be completed with a full red heat for five minutes longer.

During the process of fusing the contents of the crucible may be watched by removing one of the bricks from the top of the furnace, and when the whole mass has become quite liquid the crucible must be seized with a pair of suitable tongs, tapped once or twice very lightly against the side of the furnace to procure the settlement of the contents, and immediately poured into an iron mould, previously warmed and greased to prevent adhesion and spitting. Allow the mould to remain for some time, in order to partially cool, and then plunge it into a vessel of cold water. On cooling, the metallic elements will be found incorporated into a button, the slag can then easily be removed by tapping with a hammer on the edge, and the plunging into cold water greatly facilitates this separation. The whole mass has then to be cupelled, in order to separate the silver from the lead and other metals.

Silver ores, containing a large proportion of the sulphides (chemical combinations of sulphur with metallic substances) of other metals, may be easily assayed by the scorification process, which is, without exception, applicable to the assay of all kinds of argentiferous ores; and is one of the best, most simple, and most exact methods that can possibly be employed in the extraction of silver from its ores. This process, like that of fusion with litharge, already described, has the effect of producing an alloy, and subsequently requires cupellation. The ore is first well pounded, and then put into a small shallow vessel made of close-grained refractory fire-clay (Fig. 3), with an excess of finely granulated lead and some borax. The fusing cup or scorifier employed in this process should be about 1-1/2 in. high and 2-1/2 ins. in its greatest diameter; some assayers, however, use them deeper in proportion to their width, and representing in form the end of an egg. The object of this shape is to preserve the bath of molten metal at the bottom, and that it may always be well covered and protected by the slag on the top during the process of fusing. In the scorification method the principles are exactly the reverse of those of the crucible assay; for in the latter the object is to reduce the oxide of lead to a metallic state, whereas in the former the metallic lead added to the pounded ore in the scorifier is oxidized by being fused in contact with the air. The charge for this assay may be as follows:--

Well pounded ore 60 grains. Finely granulated lead 600 " Borax anhydrous 100 " Powdered anthracite 5 "

The cups or scorifiers should be charged in the following manner: well mix the silver ore with 300 grains of granulated lead; place this mixture in a scorifier, and add 300 grs. more of granulated lead, and over the top of the whole put the burnt borax. The vessel may then be placed in an ordinary assay furnace or muffle, as many being introduced at one time as there is room for in the furnace, and submitted to the strongest heat for about thirty minutes; during the greater portion of this time the door should be kept closed, especially for the first fifteen minutes. On opening the muffle-door a current of air passes through the furnace, converting a portion of the lead into litharge; this enters into combination with the earthy portions of the ore, the other metallic sulphides, and also the borax, producing a fusible slag on the surface of the metallic bath, extending over the whole surface of the scorifier. The excess of lead is thus protected by this film or flux from the oxidizing effects of the currents of air admitted into the furnace, and remains united with whatever silver there may be in the ore, in a metallic state.

The fusing should be continued longer than the thirty minutes--in fact until the slag or flux is reduced into a perfectly liquid state; stirring it well with a slender iron rod will facilitate the operation, as it will tend to mix with the mass any hard portions remaining undissolved and attached to the sides or other parts of the vessels. This condition of the flux is absolutely indispensable; when the slags are quite liquid, which with a strong fire will take place in from thirty to forty minutes, wrap up in a piece of paper the powdered anthracite, and drop it into the scorifier while still in the furnace or muffle. The object of adding the anthracite at the last moment is to reduce any minute portions of the metal that may exist in the slags, and remain separated from the bulk. When the anthracite has burnt off, which process usually takes about five minutes, this point is considered to have been attained, and the operation is then complete. The scorifier may be immediately withdrawn from the fire, and the contents poured into a suitable casting-mould, of the form represented in Fig. 4, a button of silver lead being the result. When cold, the metallic mass is readily separated from the slag or flux by slightly tapping with a hammer; the former may then be passed on to the next operation, viz. to be purified of its lead by the process of cupellation, which will be presently described.

When there is not enough borax present the assayer will observe an infusible skin floating upon the surface; should this be the case more borax must at once be employed, in order to dissolve such impurity. When a chloride of silver ore is to be assayed, carbonate of soda must be added to the mixture to prevent sublimation.

The following method of assaying is adopted in several large Continental establishments, where the ores have, beside the usual earthy matter and the sulphides of lead, an admixture of zinc, iron, and copper. The process is precisely similar to the crucible assay, in the case of genuine silver ores, as already described--with this exception, that no more lead is added than the ores then contain--that is, if we are treating _galena_ or _silver lead_; other ores require different treatment according to their known composition. In this process wrought-iron crucibles are employed having the form and shape as shown in Fig. 5. They are made of thick iron plate, and are rendered secure by welding the edges firmly together. Their dimensions are as follows: a depth of 4-1/2 ins., with a thickness of iron at the bottom of 1-1/2 in., and a 1/4 of an inch in the sides; the diameter at the top of the crucible should be about 2-1/2 ins., and at the bottom between 2 and 2-1/4 ins. A mechanical mixture or flux is prepared to use with the ores to which we have referred, consisting of the following chemicals, all of which should be finely powdered and well mixed with the ore to be assayed:--

Carbonate of soda 6 parts. Tartar 3 " Saltpetre 2 " Borax 1 part.

The furnace used for this assay is the ordinary one, having rather a high chimney, to insure a perfect draught. In effecting the reduction of the silver, the crucible is first placed as before on the fire, and allowed to become hot; when this is accomplished, take

Well powdered ore 480 grains. Prepared flux 500 "

These ingredients should be thoroughly mixed together, and put into the red hot crucible. Fuse at a low heat for about twenty minutes, when the whole will be in a perfect state of fusion; then give about five minutes strong heat, and at the end of that time the crucible may be withdrawn, and its contents poured into an iron mould, as represented in Fig. 4, having one or two conical holes for the reception of the fused mass. The silver and lead collect at the bottom of the mould by reason of its high specific gravity. It may be removed by reversing the position of the latter, when a gentle tap or two will deprive it of that slag or flux which is usually attached to it. A large quantity of silver can be readily collected from its ores by an alternate use of crucibles, in which case it is possible to make a regular number of fusions per hour. Wrought-iron crucibles, when strongly prepared and carefully made, will stand about thirty of these fusions, giving way in the end on account of the action of the sulphur contained in the ores.

Another kind of crucible, in addition to those already mentioned, is used by the trade, and is recommended by many assayers as superior to all others. Fig. 6 represents the form of it. It is about 4-1/2 ins. high, and 2 ins. in its greatest interior diameter, being in the form of a skittle. The charge consists of the following in this assay:

Finely powdered ore 60 grains. Small pieces of iron 12 " Black flux 180 " Common salt 50 "

Put the powdered ore into the crucible, and place upon it the iron, which should not be in the form of filings or dust, but in small pieces; upon the ore and iron should be put the black flux, and lastly the common salt must be placed above all these substances as a protection against the air. The crucibles, as many as convenient, may now be introduced into the furnace, and slowly raised to a strong red heat, at which temperature they should be kept for about half an hour; at the end of that period they should be removed from the fire, slightly tapped to settle the contents, and then placed aside to cool. When this has taken place, a few blows with a hammer near the base of the crucibles, each in turn, will soon expose the button of silver attached to the undecomposed iron; the latter substance may, however, be easily detached by a few well-directed blows with the hammer.

In order to ascertain the exact amount of the precious metal--that is, the silver--contained in the buttons of lead obtained as the results of the foregoing operations, they are subjected to a purifying process by the metallurgist, called cupellation. By this means the lead and other impurities are driven off by heat in contact with a current of air, and the silver is left behind in a pure state. To perform this operation it is necessary to expose the buttons on some absorbing medium or porous support, and this support is commonly known as a _cupel_. No doubt many porous substances could be made available for the formation of cupels, but bone-ash is the best for all practical purposes, such as are required by the assayer. The bone-ash, in the condition of a very fine powder, is mixed with a little water in which has been dissolved a small quantity of potash, and moulded into the desired shape. The cupels are tightly consolidated by pressure in an iron mould of the form shown in Fig. 7, which is the best in use, being well adapted for the manufacture of cupels. It consists of a slightly conical steel ring, 2 ins. in depth, and about 1-1/2 in. in diameter at the top internally; a steel die with a wooden handle (Fig. 8) is made to fit the mould. To make a cupel the space in the ring is nearly filled with the moistened bone-ash, and pressed down by the hand, and afterwards by the die, the latter being driven into the ring by the application of a wooden mallet (Fig. 9) to the handle of the die. It will be seen from the illustration that the die forms a cavity in the cupel capable of receiving the charge of metal for assay. When the bone-ash has been sufficiently compressed, the die is withdrawn, and the cupel removed from the ring. This is a delicate operation, as sometimes the edges of the cupel are liable to be injured; to prevent which and facilitate the removal a loose plate of iron, exactly fitting the bottom of the mould, should be introduced previous to putting in the bone-ash. The iron plate of course being removed with the cupel, it must be replaced before another can be made. By introducing a cylindrical piece of wood to the lower aperture of the steel ring, the cupel can be removed without difficulty.

The size of the cupel should always be regulated according to the quantity of foreign matter to be absorbed, it being generally understood that the material of which it is formed takes up double its weight of lead. The process of cupelling is conducted in the furnace of the assayer, an apparatus of peculiar construction, the most important part of which, however, is the muffle (Fig. 10), consisting of a small arched oven of fire-clay closed at one end, and furnished with perpendicular slits in the sides, in order to allow of a free access of air to the cupels inside.

The position of the muffle in the furnace is so arranged that it can be readily heated on every side; and when it has become red hot, six or eight cupels, previously well dried, are taken and placed on the floor of it, which should be covered with a thin layer of bone-ash. The form of tongs required for this purpose is shown in Fig. 11. When the cupels have been raised to the temperature of the muffle itself, the assays are put in by a very slender pair of tongs, the door of the furnace is then closed for a few minutes, when the metal will have become fused, and the litharge will begin to be taken up by the bone-ash of which the cupel is composed. The temperature of the furnace is now lowered as much as possible, although not to such an extent that it will retard the progress of oxidation and absorption. When nearly the whole of the lead has been thus absorbed, the bead remaining will have become very rich in silver, and, as the oxidation proceeds, will appear much agitated, assuming a rapid circular movement, and revolving with great rapidity. The silver gradually concentrates itself in the centre of the cupel, taking the form of a globule, and at this stage the fire should be made sharper, the operation being carefully watched. When the last particle of lead leaves the silver, the agitation will suddenly cease, and a beautiful phenomenon be witnessed, called by assayers the _brightening_. The button of silver then becomes brilliant and immovable, and the operation, when this takes place, is complete. The cupel must be cooled with very great care, in order to prevent the silver from _sprouting_; which if allowed to take place would result in considerable loss, besides destroying the accuracy of the assay. To prevent this sprouting it is a good plan immediately to cover the cupel by another, which has been heated for that purpose; the two are withdrawn together, and allowed to remain at the mouth of the muffle until the silver has become solid; the metal is then in a state of almost chemical purity, and may be detached and weighed. Previous to the latter, however, it should be carefully cleansed from all foreign matter, and flattened on a smooth-faced anvil, this process greatly assisting in the removal of any oxide of lead, which not unfrequently attaches itself to the globule of silver. The weighing is conducted with a pair of scales having an extremely delicate balance; and where any commercial transaction depends upon the accuracy of the assay, it is always imperative to make several tests of the same sample, to avoid the consequences of any accident or mistake.

The chief element in combination with silver on the large scale is lead. Formerly the plan adopted in the separation of this metal was cupellation alone. This process on the large scale is somewhat different from that just described; and as it may appear to the reader interesting and instructive, a brief explanation of it may not be considered out of place.