BBBB

BBBB, fig. 2, is a perpendicular section of fig. 1; aa end view of the rollers; b the ash-pit; c one of the ash-pit 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 6½ 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, 21½ inches; the funnel e 6 inches. The square of the furnace which receives the muffle and fuel is 11½ 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 6½; in height 4½ inside measure, and nearly 5½ 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 ¾ths 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'Arcet of Paris. They term it Le Petit Fourneau à 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 17½ inches high and 7½ 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 2½ 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 f, 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'Arcet 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. 15, shows the face, side, and cross section of the shelf, which makes part of the furnace. Immediately under the shelf is a horizontal slit l, which is pierced at the level of the upper part of the grate, and used for the introduction of

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 k, 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: u, 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 k 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'Arcet, 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'Arcet 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-

Assaying. 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."

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:

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 retent 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 $\backslash frac\{1\}\{128\}$ of the mass, when the natural admixture of the silver is only about $\backslash frac\{1\}\{152\}$. 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 $\backslash frac\{1\}\{480\}$ 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