ASSAYING, taken generally, implies an examination or analysis of any ore or substance whose constituent parts are to be chemically determined. The term, however, more particularly relates to the ascertaining of the qualities of gold and silver in relation to their state of purity; and in the following observations we mean to confine ourselves entirely to this object.

In whatever point of view we consider the art of assaying these metals, it cannot fail to appear of great importance to the welfare and prosperity of the civilized world. Every one must be aware of the importance of a metallic currency agreeing in its standard fineness with the decree which establishes its circulation, and that it is an object of the greatest consequence to a nation to have the means of ascertaining with accuracy the value of the coins issued by the authority of the monarch. Since the reign of Henry VIII. we have had no capricious and unjustifiable changes in the standard fineness of our coins. That monarch, as Dr Henry remarks, “after he had squandered all his father’s treasures, the grants he had received from parliament, and the great sums he had derived from the dissolution of the religious houses, began to diminish his coins both in weight and fineness. This diminution at first was small, in hopes, perhaps, that it would not be perceived: but after he had got into this fatal career, he proceeded by rapid steps to the most pernicious lengths. In the 36th year of his reign silver money of all the different kinds was coined, which had only one half silver and the other half alloy. He did not even stop here: in the last year of his reign he coined money that had only 4 oz. of silver and 8 oz. of alloy in the pound weight; and the nominal pound of this base money was worth only 9s. 3∣d. of our present money. He began to debase his gold coins at the same time, and proceeded by the same degrees. But it would be tedious to follow him in every step. In this degraded and debased condition Henry VIII. left the money of his kingdom to his son and successor Edward VI. This shameful debasement of the money of his kingdom was one of the most imprudent, dishonourable, and pernicious measures of his reign: it was productive of innumerable inconvenciences and great perplexity in business of all kinds, and the restoration of it to its standard purity was found to be a work of great difficulty.” (Henry’s History of Great Britain, vol. xii. p. 336 and 337.) [7:3:710]To possess the art, therefore, by which such dishonourable proceedings as are just detailed maybe speedily detected, is evidently an object of the greatest utility; inasmuch as the debasement of the coin would require an adjustment of the relative value of commodities to the degraded standard; and the more facility that can be given to this adjustment, the less perplexity and injury will be sustained by the public.

The importance of the art of assaying will further appear when we consider the extent of the manufacture of plate, and ornamental articles of gold and silver, the standard value of which is determined by an assay of a few Troy grains only. The nicety and delicacy of the operation must be great, and much practical experience requisite, to obtain uniformly a satisfactory result.

The principle of assaying gold and silver is very simple: It consists of two operations—the separation of the alloy from the precious metals, and the parting of these latter from each other.

Before proceeding to the detailed description of these processes, we shall describe k the furnaces and implements used in the art of assaying.

Plate LXXVIII. AAA A, fig. 1, is a front elevation of an assay furnace; aa a view of one of the two iron rollers on which the furnace rests, and by means of which it is moved forward or backward; b the ash-pit; c c are the ash-pit dampers, which are moved in a horizontal direction towards each other for regulating the draught of the furnace; d the door or opening by which the cupels and assays are introduced into the muffle; e a movable funnel or chimney, by which the draught of the furnace is increased.

BBBB, fig. 2, is a perpendicular section of fig. 1; aa end view of the rollers; b the ash-pit; c one of the ashpit dampers; d the grate; e the plate upon which the muffle rests, and which is covered with loam nearly one inch thick; f the muffle in section representing the situation of the cupels; g the mouth-plate, and upon it are laid pieces of charcoal, which during the process are ignited, and heat the air that is allowed to pass over the cupels, and which will be more fully explained in the sequel; h the interior of the furnace, exhibiting the fuel.

The total height of the furnace is 2 feet 6⅛ inches; from the bottom to the grate, 6 inches; the grate, muffle, plate, and bed of loam with which it is covered, 3 inches; from the upper surface of the grate to the commencement of the funnel, e fig. 1, 21⅜ inches; the funnel e 6 inches. The square of the furnace which receives the muffle and fuel is lli inches by 15 inches. The external sides of the furnace are made of plates of wrought iron, and are lined with a 2-inch fire-brick.

CCCC, fig. 3, is a horizontal section of the furnace over the grate, showing the width of the mouth-piece or plate of wrought iron, which is 6 inches, and the opening which receives the muffle-plate.

Fig. 4 represents the muffle or pot, which is 12 inches long, 6 inches broad inside; in the clear 6į; in height 41∙ inside measure, and nearly 5⅛ in the clear.

Fig 5, the muffle-plate, and which is of the same size as the bottom of the muffle.

Fig 6 is a representation of the sliding door of the mouth-plate, as shown at d in fig. 1.

Fig. 7, a front view of the mouth-plate or piece, d fig. 1.

Fig. 8, a representation of the mode of making, or shutting up with pieces of charcoal, the mouth of the furnace.

Fig. 9, a view of the cupel, which is generally one inch by ⅝ths of an inch.

Fig. 10, the teaser for cleaning the grate.

Fig. 11, a larger teaser, which is introduced at the top of the furnace, for keeping a complete supply of charcoal around the muffle.

Fig. 12, the tongs used for charging the assays into the cupels.

Fig. 13 represents a board of wood used as a register, and is divided into 45 equal compartments, upon which the assays are placed previous to their being introduced into the furnace. When the operation is performed, the cupels are placed in the furnace in situations corresponding to these assays on the board. By these means all confusion is avoided, and without this regularity it would be impossible to preserve the accuracy which the delicate operations of the assayer require.

The furnace and implements which we have just detailed are such as are used in the Royal Mint and Goldsmiths’ Hall in the city of London.

We shall now proceed to a description of a small assay furnace invented by Messrs Anfrye and D’Arect of Paris. They term it Le Petit Fourneau a Coupelle. Fig. 14 represents this furnace, and it is composed of a chimney or pipe of wrought iron a, and of the furnace B. It is 17∣ inches high and 7£ inches wide. The furnace is formed of three pieces; of a dome A; the body of the furnace B; and the ash-pit C, which is used as the base of the furnace, fig. 14 and 15. The principal piece or body of the furnace B has the form of a hollow tower, or of a hollow cylinder, flattened equally at the two opposite sides, parallel to the axis, in such a manner that the horizontal section is elliptical. The foot which supports it is a hollow truncated cone, flattened in like manner upon the two opposite sides, and having consequently for its basis two ellipses of different diameters; the smallest ought to be equal to that of the furnace, so that the bottom of the latter may exactly fit it. The dome, which forms an arch above the furnace, has also its base elliptical, whilst that of the superior orifice by which the smoke goes out preserves the cylindrical form. The tube of wrought iron is 18 inches long and 2⅜ inches diameter, having one of its ends a little enlarged and slightly conical, that it may be exactly fitted or jointed upon the upper part of the furnace dome d, fig. 14. At the union of the conical and cylindrical parts of the tube there is placed a small gallery of iron e, fig. 14, 15, and 16. See also the plan of it, fig. 17. This gallery is both ingenious, useful, and necessary. Upon it are placed the cupels, which are properly heated during the ordinary work of the furnace, that they may be introduced into the muffle when it is brought into its proper degree of heat. A little above this gallery is a door/ by which, if thought proper, the charcoal could be introduced into the furnace; and above that there is placed at g a key or valve, which is used for regulating the draught of the furnace at pleasure. Messrs Anfrye and D’Arect say, that, to give the furnace the necessary degree of heat so as to work the assays of gold, the tube must be about 18 inches high above the gallery for annealing or heating the cupels. The circular opening h, in the dome, fig. 14, and as seen in the section, fig. 15, is used to introduce the charcoal into the furnace: it is also used to inspect the interior of the furnace, and to arrange the charcoal round the muffle. This opening is kept shut during the working of the furnace, with the mouth-piece, of which the face is seen at n, fig. 15.

. The section of the furnace, fig. 15, presents several openings: the principal, which is that of the muffle, is placed in i; it is shut with the semicircular door m, fig- 14, and as seen in the section m, fig. 15. In front of this opening is the table or shelf upon which the door of the muffle is made to advance or recede; the letter q, fig. 1ð> shows the face, side, and cross section of the shelf, which makes part of the furnace. Immediately under the she« is a horizontal slit I, which is pierced at the level of the upper part of the grate, and used for the introduction of [7:3:711] the rod of iron, fig. 31, that the grate may be easily kept clean. This opening is shut at pleasure by the wedge represented at h, fig. 14 and 15.

Upon the back of the furnace is a horizontal slit p, fig. 15, which supports the fire-brick S, fig. 15, and upon which the end of the muffle, if necessary, may rest: w, fig. 15, is the opening in the furnace where the muffle is placed.

Fig. 19 is a plan of the grate of the furnace, and fig. 20 a horizontal view of it. These two figures show us the dimensions of the ellipses, and determine the general form of the furnace, and thickness of the grate. To give strength and solidity to the grate, it is encircled by a bar or hoop of iron. We see at z the groove in which the hoop of iron is fixed. The holes of the grate are truncated cones, having the greatest base below, that the ashes may more easily fall into the ash-pit. The letter v, fig. 15, shows the form of these holes. The grate is supported by a small bank or shelf, making part of the furnace, as seen at a, fig. 15.

The ash-pit C has an opening y in front, fig. 15, and is shut when necessary by the mouth-piece r, fig. 14 and 15.

To give strength and solidity to the furnace, it is bound with hoops of iron at b, b, b, b, fig. 14.

Figs. 21, 22, and 23, are views of the muffle.

Fig. 24 is a view of a crucible for annealing gold.

Figs. 25, 26, and 27, are cupels of various sizes to be used in the furnace. They are the same as those used by assayers in their ordinary furnaces.

Figs. 28 and 29 are views of the hand-shovels used for filling the furnace with charcoal; they should be made of such size and form as to fit the opening h in figs. 14 and 15.

Fig. 30, the smaller pincers or tongs by which the assays are charged into the cupels, and by which the latter are withdrawn from the furnace.

Fig. 18, the teaser for cleaning the grate of the furnace.

Fig. 16 is a representation of the furnace first constructed by Messrs Anfrye and d’Arect, and which was worked by means of a pair of bellows, which forced a current of air through the brass tube b, entering the ash-pit under the grate at the circular hole c, fig. 15. The strength of the blast or current of air can be regulated at pleasure by the stop-cock d, fig. 16.

We shall now proceed to a description of the process of assaying, as performed by the assayers of the Royal Mint and Goldsmiths’ Hall, and shall then state the facility afforded by the furnace of Messrs Anfrye and d’Arect in conducting this operation upon a smaller scale and reduced expense.

Some preliminary observations may be requisite in regard to the muffle and cupels, to the proportioning of lead in assaying, &c. before the operation of the assay commences.

In the furnace above described, the number of assays that can be made at one time is 45. The same number of cupels are put into the muffle. The furnace is then filled with charcoal to the top, and upon this are laid a few pieces already ignited. In the course of three hours, a little more or less, according to circumstances, the whole is ignited, during which period the muffle, which is made of fire-clay, is gradually heated to redness, and is prevented from cracking, which a less regular or more sudden increase of temperature would not fail to do: the cupels also become properly annealed. All moisture being dispelled, they are in a fit state to receive the piece of silver or gold to be assayed.

The greater care that is exercised in this operation, the less liable is the assayer to accidents from the breaking of the muffle, which it is both expensive and troublesome to fit properly into the furnace.

The cupels used in the assay process are made of the ashes of burnt bones (phosphate of lime). In the Royal Mint the cores of oxhorn are selected for this purpose, and the ashes produced are about four times the expense of the bone-ash used in the process of cupellation upon the large scale. So much depends upon the accuracy of an assay of gold or silver, where a mass of 15 lbs. Troy in the first and 60 lbs. Troy in the second instance is determined by the analysis of a portion not exceeding 20 Troy grains, that every precaution which the longest experience has suggested is used to obtain an accurate result: hence the attention paid to the selection of the most proper materials for making the cupels.

The cupels are formed in a circular mould made of cast steel, very nicely turned, and by which means they are easily freed from the mould when struck. The bone-ash is used moistened with a quantity of water sufficient to make the particles adhere firmly together. The circular, mould is filled and pressed level with its surface, after which a pestle or rammer, having its end nicely turned, of a globular or convex shape, and its size equal to the degree of concavity wished to be made in the cupel for the reception of the assay, is placed upon the ashes in the mould, and struck with a hammer until the cupel is properly formed. These cupels are allowed to dry in the air for some time before they are used. If the weather is dry, a fortnight will be sufficient.

The greatest possible attention should be paid to the quality of the lead used in assaying. If it contain silver, it will be easy to perceive a source of material error in the delicate operations of the assayer. Lead revived from litharge contains only about half a grain in the pound weight, and is preferred on that account to lead immediately revived from the ore, which usually contains a larger quantity.

The proportion of lead used in an assay of silver varies according as the external character of the silver to be assayed indicates a comparative state of fineness or coarseness to standard metal; which an expert assayer ,may pretty accurately determine by the eye: but his opinion will also in some measure be regulated by the comparative ease or difficulty of flattening upon an anvil the piece of silver to be assayed ;—if coarse, the metal is harder than standard, and of a brilliant glossy appearance; but if soft and easily flattened, and of a dead white colour, it will indicate a state approaching to purity. The quantity of lead is then proportioned by the opinion of the assayer, and varies from 10 to 20 times the weight of the silver used. It should be observed, that a cupel is capable of absorbing only its own weight of litharge; and attention should accordingly be paid to the size of the cupel, when any silver is to be assayed which requires a great quantity of lead.

As it is always requisite to proportion the lead to the estimated quantity of alloy in the silver before cupellation, the ancient assayers made use of touch-needles, which were bars or slips of metal made with pure silver, alloyed with definite proportions of copper in a regularly increasing series, from the least to the greatest proportion which may ever be required. The silver to be assayed was examined in comparison with the touch-needles in colour, tenacity, and other external characters; and its alloy was estimated by that of the needle to which it showed the closest resemblance. These needles are seldom or never used now; and the external character of the metal is sufficient to guide an experienced assayer in the proportioning of the lead to the estimated alloy in the silver which he has to assay.

In Aikin’s Dictionary of Chemistry and Mineralogy, under the article Assaying, there is a table of the pro [7:3:712] portions of lead to the estimated alloy in fine silver, founded upon the experiments of Messrs Tillet, Hellot, and Macquer, which were the basis of a regulation subsequently adopted by an edict of the late French government. The great uncertainty of the use of the touch-needles probably suggested these experiments to the French chemists; and as this table may be extremely useful to inexperienced assayers, we shall insert it here, together with the observations accompanying it in the above work.

“Copper, the usual alloy of the fine metals, when taken singly, is found to require from ten to fourteen times its weight of lead for complete scorification on the cupel. Now, all admixtures of fine metal tend to protect the copper from the action of the litharge, and the more obstinately, the greater the proportion of fine metal; so that copper with three times its weight of silver (or 9 oz. fine), requires forty times as much lead as copper; with eleven parts of silver it requires seventy-two parts of lead, and the like in an increasing ratio. The following is the table of the proportions of lead required to different alloys of copper, of which a few points are founded on the above-mentioned experiments, and the rest filled up according to the estimated ratio of increase (being multiples of the assay integer 24 in arithmetical progression). In the three first columns is shown the absolute increase of the quantity of lead in alloys of decreasing fineness; in the three last columns will be seen the gradual diminution of the protecting power of fine metal against scorification in proportion to the increase of alloy, shown by the decreasing quantity of lead required for the same weight of copper under different mixtures.”
[table]

In the article just referred to, it is remarked that many assayers of good authority use proportions of lead to alloy considerably different from the above table, and that the whole of the numbers here given may be considered as rather high in regard to the quantity of lead. The German assayers, it is added, observe the following rule:

Copper. Silver. Lead.

1 with 30 requires 128

1 15 96

1 7 64

1 4 56

1 3 40

1 1 30

1 ⅜ 2θ

. 1 ⅛ 17

In proportioning the lead to the alloy supposed to exist in the silver to be assayed, care must be taken not unnecessarily to increase the quantity; though it would be all oxidated or absorbed by the cupel sooner or later; which is proved by the cupellation of lead per se, in order to ascertain the portion of silver it contains; the latter being always found in a globular shape on the cupel and in a state of purity.

In the process of cupellation with lead, however, there is always a loss of silver. Mr Tillet found, by experiments which he made with pure silver and lead, whose retenţ of silver was known, that after the process of cupellation, the button of silver was never precisely of the same weight as before, but was always a portion lighter, even when the heat of the assay furnace was not sufficient to drive off any of the silver. The conclusion was obvious,—a part of the silver was carried into the cupel by the lead. This was proved by reviving the oxide of lead from the cupel, and cupelling the lead by itself, when the Quantity of silver left upon the test was found to be ten times as great as the natural proportion of this metal in the lead, and very nearly corresponded with the loss of silver in the first instance. It will be obvious, then, that the assayer’s report of the title or purity of any sample of silver, unless corrected, would make the metal somewhat less pure than it actually is, because all loss is put to the account of alloy. Mr Tillet calculates, when no more lead is used than is necessary for the entire separation of the alloy, that it carries down into the cupel as much silver as, when the whole is again reduced, would make the noble metal τ <⅛ of the mass, when the natural admixture of the silver is only about ττ 1 y i . But if an excess of lead is employed for cupellation, this loss of silver is somewhat greater, though it does not increase in the ratio of the excess of the lead; for 10 parts of lead to a given alloy will not carry down twice as much silver as 5 parts, though the difference of loss will be very sensible.

The weights used in assaying gold and silver are peculiar to the profession. In the assaying of silver a given number of grains are taken, which is called the assay pound. This assay pound varies from 14 to 24 grains Troy. This imaginary pound is subdivided into ounces and pennyweights, and the latter into half-pennyweights, which is the lowest term used in reporting assays of silver; so that there are 480 different reports for silver (this being the number of half-pennyweights in the pound); and therefore each nominal half-pennyweight weighs 2 ⅛ of a Troy grain when the entire assay pound is 24 grains.

The report of an assay of silver is made according to the proportion of pure metal which it is found to contain. The legal standard of Sterling money of silver is 11 oz. and 2 dwts. fine, and 18 dwts. alloy. If an assay of silver was found to contain 11 oz. only of pure silver, it would be reported worse 2 dwts., meaning worse than standard silver by 2 dwts. or 48 grains in the pound Troy. If an [7:3:713] 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

lbs. oz.

Oil the contrary, if the ingot weighed 50 0

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 3 ⅛ 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 ∙τ1⅛^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 240grains 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 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,yζwe 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 portion of metal upon a polished anvil; the face of the flatting 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 [7:3:714]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 irridescent 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 çool 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 differ- ence 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 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. Μ. 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 ⅛ 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 Ş [7:3:715]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 a Coupelle of Messrs Anfrye and d’Arect. 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.
[table]

So that each assay, on an average, was performed in 11∣ minutes, and the charcoal used did not much exceed ∣th 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’Arect 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 milimètres:
[table]

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 Bureaux 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 A, 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.)