ASSAY

ASSAY. assay, on the contrary, contained 11 oz. and 6 dwts. pure silver, it would be reported better than standard by 4 dwts. in the pound Troy; because 18 dwts. being the standard proportion of alloy, it was found that it only contained 14 dwts. alloy. When bullion thus assayed and reported is for sale, its value is calculated by reducing the bar or ingot of silver into standard metal.

In the first example which we have given, lbs. Troy. oz.
if the ingot of silver assayed weighed..... 50 0
there would be deducted from the weight 2
dwts. per lb. or..... 0 5
which 5 oz. is the excess of alloy above the
proportion of 18 dwts. to the 11 oz. 2 dwts.
of fine metal, and the bar of silver would be,
in standard weight,..... 49 7

On the contrary, if the ingot weighed.....
and the alloy were deficient, which is the
case when the metal is reported better than
standard by 4 dwts. in the pound Troy, there
would be added to the 50 lbs. 4 dwts. per
pound, which would be equal to..... 0 10

Making the standard weight of the ingot.... 50 10

The gold assay pound, which is from 10 to 20 grains Troy, is subdivided into 24 carats, and each carat into 4 assay grains, and each grain into quarters; so that there are 384 separate reports for gold, each equal to 15 Troy grains, or what is termed a quarter carat grain. An accurate assayer, however, can ascertain, in an assay of gold, to 3 grains Troy; but it is the custom of the trade not to report less than a quarter carat, or 15 grains Troy. A substantial reason is given for this rule to justify the practice of it. An ingot of gold generally weighs a journey weight, which is 15 lbs. Troy; from a sample cut from the two opposite ends, weighing from 10 to 20 grains, the value of the mass of 15 lbs. is to be determined: if this ingot had been imperfectly melted, the mass would not be homogeneous, and a difference might exist in it of several Troy grains; and the allowance between the quarters given in the assay report is an indemnity to the purchaser. Indeed, so particular are many in the bullion trade, that they will not purchase any foreign gold bullion until it has been remelted by refiners or melters in whose integrity they repose confidence. This, we believe, is generally the case with the Bank of England in all her purchases of foreign gold bullion.

The assay report of gold is made according as it is better or worse than standard. The standard of our gold coin is 22 carats fine and 2 carats alloy. If, by assay, an ingot of gold was found to contain 21 carats fine gold, it would be reported worse 1 carat, the mass containing a carat of alloy more than the proportion of 2 carats to 22 carats fine. If the ingot weighed 15 lbs. Troy, there would be deducted from the gross weight 1 carat, or 240 grains Troy, reducing the standard of the mass to 14 lbs. 11 oz. 10 dwts. If, on the contrary, the mass was found to contain 23 carats fine gold, it would be reported 1 carat better than standard; and this carat would be added to the gross weight of the ingot, which we have supposed to weigh 15 lbs. Troy, and would be called 15 lbs. 0 oz. 10 dwts. of standard gold. When the gold assay pound or integer is only 12 grains, the quarter assay grain weighs only $\backslash frac\{1\}\{3\}$ part of a Troy grain. This will show how delicate the scales must be by which the assayer works in order to obtain accuracy. In the Royal Mint the scales of the assayers will be sensibly affected even with the $\backslash frac\{1\}\{1000\}$th part of a Troy grain.

When the assay pound is subdivided, as for silver, in the same manner as the Troy pound, it is obvious that all the lower denominations bear the same relation to each other, which is some little advantage in transferring the assay reports to real mixtures for use. On the contrary, the carat subdivision for gold is confined to assaying; but its fractions being aliquot parts of the pound Troy, the calculation for real use is very easy. As the pound Troy contains 5760 grains, the carat corresponds with 240 grains or 10 dwts.; the assay grain, or fourth part of a carat, with 60 Troy grains; and the assay quarter-grain with 15 Troy grains; to which report, when the assayer has separated the gold (4 oz. for example), he adds $\backslash frac\{1\}\{4\}$ oz. gold in a pound Troy. Whereas in gold-parting he takes two equal pieces, treats one as a silver assay and the other as a gold assay, to find the absolute quantity of each metal, after which the report is made on gold singly, to which is added the report of the silver separately. Thus, if he finds 4 oz. of gold, and 3 oz. of silver, he reports worse 14 carats (2 carats being equivalent to an assay ounce, and consequently the 4 oz. of gold equal to 8 carats, which subtracted from 22 carats, the gold standard, leaves 14), to which report he adds, fine silver 3 oz. But when the mixed metal contains more than half alloy, it is called metal for gold and silver, and the absolute quantity of each is reported separately.

Having made the reader acquainted with these details, we shall now proceed to explain the process of assaying silver, commonly known by the name of cupellation.

When we have an assay of silver to make, we flatten the Assay portion of metal upon a polished anvil; the face of the of silver. flattening hammer is also highly polished, that the metal may receive no extraneous matter whatever. The piece of metal is flattened to about the thinness of a sixpence, and an assay pound is cut from it, and most accurately weighed in such scales as we have already noticed. This assay pound is then enveloped in a sheet of lead, which is flattened from a lead bullet, and circular, but made into a funnel shape, in order to contain the silver; and the more nicely to prevent any portion of the silver from being lost, the corners of this leaden funnel are closely and firmly folded down. If the assay master has 45, or, indeed, any number short of 45, they are ranged according to their number upon the table, fig. 13. When the furnace and cupels have been prepared according to the number of assays to be made, and when the proper degree of heat has been attained, the assays are charged into the cupels; and the following method is followed in this part of the process:—In the first instance, a ball of lead is charged into each cupel, with the charging tongs (fig. 12), and its weight is according to the quality of the silver to be assayed, the assayer keeping a stock of leaden bullets of different weights for the purpose. As soon as this lead is melted, which is instantaneous, the assays of silver enveloped in lead are also charged into the cupels. The mass is very soon in complete fusion. The mouth of the muffle, which had before been partially closed with cylinders of charcoal about six or seven inches long, and of different diameters suitable to the convenience of the assayers, as represented in fig. 8, is now nearly closed by smaller cylinders of charcoal. The object of this precaution is, that the stream of air admitted to pass over the surface of the cupels, and which is indispensably necessary for the oxidation of the lead in the process of cupellation, may not chill the muffle and retard the progress of the assay. The oxidation of the metal will proceed with more or less rapidity, according as the stream of air admitted is great or small, and which the assayer has it always in his power to regulate at pleasure. The work already referred to in this article has so beautifully and accurately described

Assaying. the progressive appearance of the assay process of silver, that we cannot do better than quote the description:—
“The melted metal begins to send off dense fumes, and a minute stream of red fused matter is seen perpetually flowing from the top of the globule down its sides to the surface of the cupel, through which it sinks and is lost to view. This fume and the stream of melted matter consists of the lead oxidated by the heat and air, in one case volatilized, in the other vitrified; and in sinking through the cupel it carries down with it the copper or other alloy of the silver. In proportion to the violence of the heat is the density of the fume, the violence with which it is given off, the convexity of the surface of the globule of melted matter, and the rapidity with which the vitrified oxide circulates (as it is termed) or falls down the sides of the metal. As the cupellation advances, the melted button becomes rounder, its surface becomes streaky with large bright points of the fused oxide, which moves with increased rapidity, till at last the globule, being now freed from all the lead and other alloy, suddenly lightens; the last portions of litharge on the surface disappear with great rapidity, showing the melted metal bright with iridescent colours, which directly after becomes opaque, and suddenly appears brilliant, clean, and white, as if a curtain had been withdrawn from it. The operation being now finished, and the silver left pure, the cupel is allowed to cool gradually, till the globule of silver is fixed, after which it is taken out of the cupel while still hot, and when cold weighed with as much accuracy as at first. The difference between the globule and the silver at first put in shows the quantity of alloy, the globule being now perfectly pure silver if the operation has been well performed. The reason of cooling the globule or button gradually is, that pure silver, when congealing, assumes a crystalline texture; and if the outer surface is too suddenly fixed, it forcibly contracts on the still fluid part in the centre, causing it to spurt out in aborescent shoots, by which some minute portions are often thrown out of the cupel, and the assay spoiled.” (Aikin's Dictionary.)

The assaying of gold, preparatory to the parting process, which we are about to describe, is exactly the same as in the case of silver; the object in the process of cupellation being to destroy the base metal or alloy contained in the gold. If gold contained only copper as alloy, the assaying of gold would be as simple and expeditious as that of silver; but all gold contains a portion of silver, which, though reckoned as alloy, cannot, as we have already seen, be destroyed by cupellation. Recourse is had to the process commonly called the parting process, to get rid of the silver contained in the gold. This is done by means of nitric acid, which entirely dissolves the silver, and leaves the gold perfectly pure. The quantity of silver which gold generally contains is too small to allow the nitric acid to act upon it without addition; and the general allowance by assayers is two or three parts of silver to one of gold. If the quantity of silver greatly exceeded these proportions, the operation would not succeed so well; the fine gold would be obtained in the state of brown powder, the particles having been too minutely divided by the excess of silver.

When assays of gold have passed the test, by which all the alloy, excepting silver, has been destroyed, it is in this process that the additional quantity of silver is added. Suppose, for example, that a gold assay is made from the integer, or pound, weighing 12 grains Troy, an addition of from 24 to 36 grains of pure silver is made in addition to the small portion already supposed to exist in the mass. This becomes thoroughly incorporated with the gold in the process of cupellation. The globule or button, as soon as it is taken from the furnace, is passed

between a pair of polished steel rollers, and drawn out Assaying. into a thin lamina or plate of the thickness of a sixpence, and returned into the furnace to be annealed. After being kept in a red heat for some time, it is taken out and suffered to cool. It is then wound up into a cornet. This is put into a glass matrass, of the shape of an inverted cone, and with about twice or thrice its weight of very pure nitric acid. M. Vauquelin recommends it to be 1.25 specific gravity. But the true test of its strength is in the working of the process. The assayer's attention being directed to the point of strength that will maintain the gold when the silver is extracted in the spiral form, if the acid were too strong, or the silver in too great excess, the gold, as we have already mentioned, would be reduced to powder; and considerable danger exists that it would not be accurately collected, by which an imperfect result would be obtained. The glass matrass is placed upon a sand heat or bath, which is generally a square or oblong pan of copper, with from one to two inches of dry sand in the bottom. The pan is placed over a small square furnace, in which is burning charcoal or coke. As soon as the acid is warm it begins to act upon the silver, and a dense stream of nitrous gas is disengaged. As long as the acid continues to act, the metal appears everywhere to be studded with very minute bubbles, which issue in jets. The disappearance of these, or their uniting into a few large ones, is a sign or mark that the acid has ceased to act. The disappearance also of the nitrous fumes is an indication that the acid has no silver to act upon. In the course of fifteen or twenty minutes the process is finished. But, in order to extend the last portions of silver which the mixture may contain, a small quantity of highly concentrated acid is poured upon the cornet, and boiled, by which the last portions of the silver are extracted. The cornets of gold are thoroughly corroded, but retain the same form, having lost all the silver to two thirds or three fourths of their weight; they are slender and brittle, as we observed before. It is an object of considerable importance to prevent the cornets from being broken, the result being more likely to be accurate than having the gold in fragments; and to prevent this, the quantity of silver used is no more than is absolutely necessary, it being obvious that the less the quantity of gold compared to the silver used in the assay, the more likely is the gold to be broken into pieces.

The hot acid is poured very carefully from the matrass, and warm water is added to wash any remain of silver from the gold, and the addition repeated until the water comes off perfectly clear. The cornets of gold, which are of a dull brown colour and unmetallic appearance, are then put according to their numbers into small clay crucibles, into which they are allowed gently to fall by inverting the matrass, with a portion of water in it, which breaks their fall, and also collects any grains of gold that may be in the matrass. The water is then poured off, and they are put into the furnace, and annealed under a bright cherry heat. When cooled, the pieces of gold have regained their beautiful metallic lustre, and possess all the softness and flexibility of this truly noble metal.

The pieces of gold, thus thoroughly purified, are carefully and accurately weighed, the absolute loss in weight indicating the purity of the metal assayed.

It is a matter of the greatest importance that the silver used in this process should contain no gold, otherwise a source of very material error would arise in the delicate operations of the assayer. Silver generally contains a small portion of gold. Spanish dollars, for example, are found to contain about 4 troy grains in the pound, and are generally preferred in the parting process upon the large scale; but assayers in general use silver revived from a

precipitation of the nitrate of silver, which they are sure contains no gold.

The nitrate of silver is precipitated by immersing in it plates of copper, which throw down the silver in the metallic state. It may also be recovered by a solution of common salt, which converts the silver into luna cornea, of which, when washed and well dried, 100 parts contain 75 of silver. The accuracy of the assay may also be proved by this process. The luna cornea, however, is more difficult to reduce to the metallic state, and the mode of recovery by plates of copper is always preferred.

It remains for us now briefly to mention the process of assaying by the Petit Fourneau à Coupelle of Messrs Anfrye and d'Arcet. This process is the same in principle, in all respects, as that which we have been detailing. The only difference consists in the greater facility of the process, and the comparative diminution in the fuel used. The furnace first used by these gentlemen had a small pair of bellows attached to it (see fig. 16); and after the furnace was brought to a proper degree of heat, which required two hours and a half, the following was the result of the experiments made.

So that each assay, on an average, was performed in 11½ minutes, and the charcoal used did not much exceed ¼th of a pound weight. The standard of the silver used in these experiments was proved by an assay in the ordinary furnace to be 949 millièmes, and the average result by the new furnace was 948.75 millièmes, a difference not more than occurs in the use of the large furnace, and an object of no importance in point of accuracy.

Experiments were also tried with this small furnace, to prove the highest degree of heat that could be produced; and two balls of Wedgwood's pyrometer were put into the

furnace, which, when cold, indicated, the one 35, the other 30 degrees, and is fully equal to the heat of the ordinary assay furnace.

When Messrs Anfrye and d'Arcet had improved their furnace by adopting a tube of iron in place of the bellows, they could raise the proper degree of heat in the furnace in half an hour, which required by the original construction two hours and a half.

The following experiments were made upon some five-franc pieces, the standard of which, according to law, should have been from 897 to 903 millimètres:

These experiments differ very little from those we have already stated, a trifling increase in the duration of the assay, and in the charcoal consumed, being the only difference; and the greater facility which this furnace has of being raised to the necessary degree of heat, before the assays are charged into the cupels, more than compensates for the increased duration of the assay and charcoal consumed.

This small furnace is particularly recommended for the service of the Bureau de Garantie de Province, where a limited number of assays are from time to time made; and, in point of economy, presents many advantages to recommend it to a place in the chemical laboratory.

This furnace may also be used for a melting furnace; and to convert it to this purpose, the muffle is taken out, and the various apertures, which are open when the assays are making, closed by their respective stoppers: a stand may then be put upon the grate, and a crucible of such size as the furnace will admit, placed upon it, and which can very readily and conveniently be done from the opening h, fig. 15. In this use of the furnace coke may be employed instead of the charcoal, the heat being greater and more steady with the former than the latter. (E. E.)