ASPHALTUM

Native bitumen, so called from the lake Asphaltites. ASSAY and ASSAYING. ( Coup ell at i on , Fr.; Abtreiben auf der capelle , Germ.) This is the proce by which the quality of gold and silver bullion, coin, plate, or trinkets is ascertained with precision, or by which the quantity of either or both these precious metals is determined in any given alloy. It is, therefore, a case of chemical analysis, in which peculiar methods are employed to attain the object in view with accuracy and dispatch. A aying has been also extended of late years, to determine the quantity of palladium and platina in certain bullion and gold dust brought from Brazil. The art of a aying gold and silver by the cupel , is founded upon the feeble affinity which these metals have for oxygen, in comparison with copper, tin, and the other cheaper metals; and on the tendency which the latter metals have to oxidize rapidly in contact with lead at a high temperature, and sink with it into any porous earthy ve el in a thin gla y or vitriform state. The porous ve el may be made either of wood-ashes, freed from their soluble matter by washing with water; or, preferably, of burned bones reduced to a fine powder. The lead added to the silver or gold to be a ayed, serves chiefly to di olve the oxidized copper, whence it appears that the quantity of lead requisite for silver a ays, ought to be directly proportional to the quantity which the silver and copper would separately require. It has been found by experiment, that 16 parts of lead are quite sufficient to pa 1 of copper through the cupel; and that 3 ⁄ 10 of lead presents the most suitable proportion for pa ing one of silver. From these principles, however, if we should always regard the dose of lead to be employed for any alloy as being equal to (16 × C) + ( 3 ⁄ 30  × S) we should certainly commit an error. The phenomena of cupellation is of a more complex nature. Long practice and delicate trials alone can guide to the proper quantity of lead to be employed for every various state of the alloy. The following Table contains the results of M. D’Arcet’s elaborate experiments upon this subject :— | Alloy. | Lead for 1 of Alloy. | Ratio of the Copper to the Lead. | | Silver. | Copper. | | 1000 | 0 | 3⁄10 | 0 | | 950 | 50 | 3 | 1: 60 | | 900 | 100 | 7 | 1: 70 | | 800 | 200 | 10 | 1: 50 | | 700 | 300 | 12 | 1: 40 | | 600 | 400 | 14 | 1: 35 | | 500 | 500 | 16 or 17 | 1: 32 | | 400 | 600 | 16 — 17 | 1: 26·7 | | 300 | 700 | 16 — 17 | 1: 22·9 | | 200 | 800 | 16 — 17 | 1: 20 | | 100 | 900 | 16 — 17 | 1: 17·8 | | 0 | 1000 | 16 — 17 | 1: 16 | Bismuth may be used as a substitute for lead in cupellation; two parts of it being nearly equivalent to three of lead. But its higher prices will prevent its general introduction among a ay masters. We begin this a ay proce by weighing, in a delicate balance, a certain weight of the metallic alloy; a gramme (= 15·444 gr.) is usually taken in France, and 12 grains in this country. This weight is wrapped up in a slip of lead foil or paper, should it consist of several fragments. This small parcel, thus enveloped, is then laid in a watch gla or a capsule of copper, and there is added to it the proportion of lead suited to the quality of alloy to be a ayed; there being le lead, the finer the silver is presumed to be. Those who are much in the habit of cupellation can make good gue es in this way; though it is still gue work, and often leads to considerable error, for if too much lead be used for the proportion of baser metal present, a portion of the silver is wasted; but if too little, then the whole of the copper, . is not carried off, and the button of fine silver remains more or le impure. The most expert and experienced a ayer by the cupel, produces merely a series of approximate conjectural results, which fall short of chemical demonstration and certainty in every instance. The lead must be, in all cases, entirely free from silver, being such as has been revived from pure litharge; otherwise errors of the most serious kind would be occasioned in the a ays. The best cupels weigh 12 1 ⁄ 2 grammes, or 193 grains. The cupels allow the fused oxides to flow through them as through a fine sieve, but are impermeable to the particles of metals; and thus the former pa readily down into their substance while the latter remain upon their surface; a phenomenon owing to the circumstance of the gla y oxides moistening, as it were, the bone-ash powder, whereas the metals can contract no adherence with it. Hence also the liquid metals preserve a hemispherical shape in the cupels, as quicksilver does in a cup of gla , while the fused oxide spreads over, and penetrates their substance, like water. A cupel may be regarded, in some measure, as a filter permeable only to certain liquids. If we put into a cupel, therefore, two metals, of which the one is unalterable in the air, the other susceptible of oxidize ment, and of producing a very fusible oxide, it is obvious that, by exposing both to a proper degree of heat, we shall succeed in separating them. We should also succeed, though the oxide were infusible, by placing it in contact with another one, which may render it fusible. In both cases, however, the metal from which we wish to part the oxides must not be volatile; it should also melt, and form a button at the heat of cupellation; for otherwise it would continue di eminated, attached to the portion of oxide spread over the cupel, and incapable of being collected. The furnace and implements used for a aying in the Royal Mint and the Goldsmiths’ Hall, in the city of London, are the following :— IMG:4147767755307473660_illo0062.png:A aying furnace A A A A , fig. 58. , is a front elevation of an a ay furnace; a a , 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 a ays are introduced into the muffle; e , a moveable funnel or chimney by which the draught of the furnace is increased. B B B B , fig. 59. , is a perpendicular section of fig. 58. ; a a , end view of the rollers; b the ash-pit; c one of the ash-pit dampers; d the grate, over which is 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 proce are ignited, and heat the air that is allowed to pa over the cupels, as 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 6 1 ⁄ 2 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. 58. , 21 1 ⁄ 2 inches; the funnel e , 6 inches. The square of the furnace which receives the muffle and fuel is 11 3 ⁄ 4 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. IMG:4147767755307473660_illo0063a.png:Section over grate C C C C , fig. 60. , 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. IMG:4147767755307473660_illo0063b.png:Muffle Fig. 61. represents the muffle or pot, which is 12 inches long, 6 inches broad inside; in the clear 6 3 ⁄ 4 : in height 4 1 ⁄ 2 inside measure, and nearly 5 1 ⁄ 2 in the clear. IMG:4147767755307473660_illo0063c.png:Muffle plate Fig. 62. , the muffle-plate, which is of the same size as the bottom of the muffle. IMG:4147767755307473660_illo0063d.png:Sliding door Fig. 63. is a representation of the sliding-door of the mouth-plate, as shewn at d , in fig. 58. IMG:4147767755307473660_illo0063e.png:Mouth plate Fig. 64. , a front view of the mouth-plate or piece, d , fig. 58. IMG:4147767755307473660_illo0063f.png:Furnace mouth Fig. 65. , a representation of the mode of making, or shutting up with pieces of charcoal, the mouth of the furnace. Fig. 66. , the teaser for cleaning the grate. IMG:4147767755307473660_illo0063g.png:Teasers and tongs Fig. 67. , a larger teaser, which is introduced at the top of the furnace, for keeping a complete supply of charcoal around the muffle. Fig. 68. , the tongs used for charging the a ays into the cups. IMG:4147767755307473660_illo0063h.png:Register board Fig. 69. represents a board of wood used as a register, and is divided into 45 equal compartments, upon which the a ays are placed previously to their being introduced into the furnace. When the operation is performed, the cupels are placed in the furnace in situations corresponding to these a ays on the board. By these means all confusion is avoided, and without this regularity it would be impo ible to preserve the accuracy which the delicate operations of the a ayer require. IMG:4147767755307473660_illo0064a.png:A ay furnace I shall now proceed to a description of a small a ay furnace, invented by Me rs. Anfrye and d’Arcet, of Paris. They term it, Le Petit Fourneau à Coupelle . Fig. 70. represents this furnace, and it is composed of a chimney or pipe of wrought iron a , and of the furnace B . It is 17 1 ⁄ 2 inches high, and 7 1 ⁄ 4 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. 70. and 71. 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 2 1 ⁄ 2 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. 70. At the union of the conical and cylindrical parts of the tube, there is placed a small gallery of iron, e , fig. 70 , 71. See also a plan of it, fig. 72. This gallery is both ingenious and useful. Upon it are placed the cupels, which are thus annealed 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; above that there is placed at g a throttle valve, which is used for regulating the draught of the furnace at pleasure. Me rs. Anfrye and d’Arcet say, that, to give the furnace the nece ary degree of heat so as to work the a ays of gold, the tube must be about 18 inches above the gallery, for annealing or heating the cupels. The circular opening h , in the dome, fig. 70. , and as seen in the section, fig. 71. , 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. 71. The section of the furnace, fig. 71. , presents several openings, the principal of which is that of the muffle; it is placed at i ; it is shut with the semicircular door m , fig. 70. , and seen in the section m , fig. 71. 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. 71. , shows the face, side, and cro 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 a slender rod of iron, that the grate may be easily kept clean. This opening is shut at pleasure, by the wedge represented at k , fig. 70. and 71. Upon the back of the furnace is a horizontal slit p , fig. 71 , which supports the fire-brick, s , and upon which the end of the muffle, if nece ary, may rest; u , fig. 71. , is the opening in the furnace where the muffle is placed. IMG:4147767755307473660_illo0064b.png:Horizontal view of grate The plan of the grate of the furnace is an ellipse: fig. 73. is a horizontal view of it. The dimensions of that ellipsis determine the general form of the furnace, and thickne of the grate. To give strength and solidity to the grate, it is encircled by a bar or hoop of iron. There is a groove in which the hoop of iron is fixed. The holes of the grate are truncated cones, having the greater base below, that the ashes may more easily fall into the ash-pit. The letter v , fig. 71. , 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. 71. The ash-pit, C , has an opening y in front, fig. 71. ; and is shut when nece ary by the mouth-piece r , fig. 70. and 71. To give strength and solidity to the furnace, it is bound with hoops of iron, at b , b , b , b , fig. 70. IMG:4147767755307473660_illo0064c.png:Muffles Figs. 74. 75. 76. are views of the muffle. Fig. 77. is a view of a crucible for annealing gold. IMG:4147767755307473660_illo0064d.png:Crucible and cupels Figs. 78. 79. 80. are cupels of various sizes, to be used in the furnace. They are the same as those used by a ayers in their ordinary furnaces. IMG:4147767755307473660_illo0064e.png:Hand-shovels Figs. 81. and 82. 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. 70. and 71. The smaller pincers or tongs, by which the a ays are charged into the cupels, and by which the latter are withdrawn from the furnace, as well as the teaser for cleaning the grate of the furnace, are similar to those used in the British Mint. In the furnace of the Mint above described, the number of a ays 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 le , according to circumstances, the whole is ignited; during which period, the muffle, which is made of fire-clay, is gradually heated to redne , and is prevented from cracking; which a le 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 a ayed. The greater care that is exercised in this operation, the le liable is the a ayer 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 a ay proce , are made of the ashes of burnt bones (phosphate of lime). In the Royal Mint, the cores of ox-horn are selected for this purpose; and the ashes produced are about four times the expense of the bone-ash, used in the proce of cupellation upon the large scale. So much depends upon the accuracy of an a ay of gold or silver, where a ma of 15lbs. troy in the first, and 60lbs. 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, 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 pre ed level with its surface; after which, a pestle or rammer, having its end nicely turned, of a globular or convex shape, and of a size equal to the degree of concavity wished to be made in the cupel for the reception of the a ay, 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 fine, a fortnight will be sufficient. An a ay may prove defective for several reasons. Sometimes the button or bead sends forth crystalline vegetations on its surface with such force, as to make one suppose a portion of the silver may be thrown out of the cupel. When the surface of the bead is dull and flat, the a ay is considered to have been too hot, and it indicates a lo of silver in fumes. When the tint of the bead is not uniform, when its inferior surface is bubbly, when yellow scales of oxide of lead remain on the bottom of the cupel, and the bead adheres strongly to it, by these signs it is judged that the a ay has been too cold, and that the silver retains some lead. Lastly, the a ay is thought to be good if the bead is of a round form, if its upper surface is brilliant, if its lower surface is granular and of a dead white, and if it separates readily from the cupel. After the lead is put into the cupel, it gets immediately covered with a coat of oxide, which resists the admi ion of the silver to be a ayed into the melted metal; so that the alloy cannot form. When a bit of silver is laid on a lead bath in this predicament, we see it swim about for a long time without di olving. In order to avoid this result, the silver is wrapped up in a bit of paper; and the carburetted hydrogen generated by its combustion, reduces the film of the lead oxide, gives the bath immediately a bright metallic lustre, and enables the two metals readily to combine. As the heat rises, the oxide of lead flows round about over the surface, till it is absorbed by the cupel. When the lead is wasted to a certain degree, a very thin film of it only remains on the silver, which causes the iridescent appearance, like the colours of soap-bubbles; a phenomenon, called by the old chemists, fulguration. When the cupel cools in the progre of the a ay, the oxygenation of the lead ceases; and, instead of a very liquid vitreous oxide, an imperfectly melted oxide is formed, which the cupel cannot absorb. To correct a cold a ay, the temperature of the furnace ought to be raised, and pieces of paper ought to be put into the cupel, till the oxide of lead which adheres to it, be reduced. On keeping up the heat, the a ay will resume its ordinary train. Pure silver almost always vegetates. Some traces of copper destroy this property, which is obviously due to the oxygen which the silver can absorb while it is in fusion, and which is disengaged the moment it solidifies. An exce of lead, by removing all the copper at an early stage, tends to cause the vegetation. The brightening is caused by the heat evolved, when the button pa es from the liquid to the solid state. Many other substances present the same phenomenon. In the above operation it is nece ary to employ lead which is very pure, or at least free from silver. That kind is called poor lead . It has been observed at all times, that the oxide of lead carries off with it, into the cupel, a little silver in the state of an oxide. This effect becomes le , or even disappears, when there is some copper remaining; and the more copper, the le chance there is of any silver being lost. The lo of silver increases, on the other hand, with the dose of lead. Hence the reason why it is so important to proportion the lead with a precision which, at first sight, would appear to be superfluous. Hence, also, the reason of the attempts which have, of late years, been made to change the whole system of silver a ays, and to have recourse to a method exempt from the above causes of error. M. d’Arcet, charged by the Commi ion of the Mint in Paris, to examine into the justice of the reclamations made by the French silversmiths against the public a ays, ascertained that they were well founded; and that the results of cupellation gave for the alloys between 897 and 903 thousandths (the limits of their standard coin) an inferior standard, by from 4 to 5 thousandth parts, from the standard or title which should result from the absolute or actual alloy. The mode of a ay shows, in fact, that an ingot, experimentally composed of 900 thousandths of fine silver, and 100 thousandths of copper, appears, by cupellation, to be only, at the utmost, 896 or 897 thousandths; whereas fine silver, of 1000 thousandths, comes out nearly of its real standard. Consequently a director of the Mint, who should compound his alloy with fine silver, would be obliged to employ 903 or 904 thousandths, in order that, by the a ay in the laboratory of the Mint, it should appear to have the standard of 900 thousandths. These 3 or 4 thousandths would be lost to him, since they would be disguised by the mode of a ay, the definitive criterion of the quantity of silver, of which the government keeps count from the coiner of the money. From experiments subsequently made by M. d’Arcet, it appears that silver a ays always suffer a lo of the precious metal, which varies, however, with the standard of the alloy. It is 1 thousandth for fine silver, | 4·3 | thousandths | for | silver | of | 900 | thousandths, | | 4·9 | — | for | — | of | 800 | — | | 4·2 | — | for | — | of | 500 | — | and diminishes thereafter, progre ively, till the alloy contains only 100 thousandths of silver, at which point the lo is only 0·4. A ays requested by the Commi ion of the Paris Mint, from the a ayers of the principal Royal Mints in Europe, to which the same alloys, synthetically compounded, were sent, afforded the results inscribed in the following table. | Names of the A ayers. | Cities where they reside. | Standards found for the Mathematical Alloys. | | 950 mill. | 900 mill. | 800 mill. | | F. de Castenhole, Mint A ayer | Vienna | 946 | ·20 | 898 | ·40 | 795 | ·10 | | A. R. Vervaëz, Ditto | Madrid | 944 | ·40 | 893 | ·70 | 789 | ·20 | | D. M. Cabrera, A ayer in Spain | Ditto | 944 | ·40 | 893 | ·70 | 788 | ·60 | | A ayer | Amsterdam | 947 | ·00 | 895 | ·00 | 795 | ·00 | | Mr. Bingley, A ay Master | London | 946 | ·25 | 896 | ·25 | 794 | ·25 | | Mr. Johnson, A ayer | Ditto | 933 | ·33 | 883 | ·50 | 783 | ·33 | | Inspector of the Mint | Utrecht | 945 | ·00 | 896 | ·50 | 799 | ·00 | | A ayer of the Mint | Naples | 945 | ·00 | 891 | ·00 | 787 | ·00 | | A ayer of Trade | Ditto | 945 | ·00 | 891 | ·00 | 787 | ·00 | | A ayer of the Mint | Hamburgh | 946 | ·13⁄72 | 897 | ·41⁄72 | 798 | ·44⁄72 | | Ditto | Altona | 942 | ·1⁄4 | 894 | ·00 | 790 | | These results, as well as those in still greater numbers, obtained from the ablest Paris i an a ayers, upon identical alloys of silver and copper, prove that the mode of a ay applied to them brings out the standard too low; and further, that the quantity of silver masked or disguised, is not uniform for these different eminent a ay masters. An alloy, for example, at the standard of 900 thousandths is judged at | M. | | the Mint of | Paris | to have a standard of | 895·6 | | At that of | Vienna | — | 898·4 | | — | Madrid | — | 893·7 | | — | Naples | — | 891·0 | The fact thus so clearly made out of a lo in the standard of silver bullion and coin, merits the most serious attention; and it will appear astonishing, perhaps, that a thing recurring every day, should have remained for so long a time in the dark. In reality, however, the fact is not new; as the very numerous and well-made experiments of Tillet from 1760 to 1763, which are related in the memoirs of the Academy of Sciences, show, in the silver a ays, a lo still greater than that which was experienced lately in the laboratory of the Commi ion of the French Mint. But he thought that, as the error was common to the nations in general, it was not worth while or prudent to introduce any innovation. A mode of a aying, to give, with certainty, the standard of silver bullion, should be entirely independent of the variable circumstances of temperature, and the unknown proportions of copper, so difficult to regulate by the mere judgment of the senses. The proce by the humid way, recommended by me to the Royal Mint in 1829, and exhibited as to its principles before the Right Honourable John Herries, then Master, in 1830, has all the precision and certainty we could wish. It is founded on the well-known property which silver has, when di olved in nitric acid, to be precipitated in a chloride of silver quite insoluble, by a solution of sea salt, or by muriatic acid; but, instead of determining the weight of the chloride of silver, which would be somewhat uncertain and rather tedious, on account of the difficulty of drying it, we take the quantity of the solution of sea salt which has been nece ary for the precipitation of the silver. To put the proce in execution, a liquor is prepared, composed of water and sea salt in such proportions that 1000 measures of this liquor may precipitate, completely, 12 grains of silver, perfectly pure, or of the standard 1000, previously di olved in nitric acid. The liquor thus prepared, gives, immediately, the true standard of any alloy whatever, of silver and copper, by the weight of it which may be nece ary to precipitate 12 grains of this alloy. If, for example 905 measures have been required to precipitate the 12 grains of alloy, its standard would be 905 thousandths. The proce by the humid way is, so to speak, independent of the operator. The manipulations are so easy; and the term of the operation is very distinctly announced by the absence of any sensible nebulosities on the affusion of sea salt into the silver solution, while there remains in it 1 ⁄ 2 thousandth of metal. The proce is not tedious, and in experienced hands it may rival the cupel in rapidity; it has the advantage over the cupel of being more within the reach of ordinary operators, and of not requiring a long apprenticeship. It is particularly useful to such a ayers as have only a few a ays to make daily, as it will cost them very little time and expense. By agitating briskly during two minutes, or thereby, the liquid rendered milky by the precipitation of the chloride of silver, it may be sufficiently clarified to enable us to appreciate, after a few moments of repose, the disturbance that can be produced in it by the addition of 1000 of a grain of silver. Filtration is more efficacious than agitation, especially when it is employed afterwards; it may be sometimes used; but agitation, which is much more prompt, is generally sufficient. The presence of lead and copper, or any other metal, except mercury, has no perceptible influence on the quantity of sea salt nece ary to precipitate the silver; that is to say, the same quantity of silver, pure or alloyed, requires for its precipitation a constant quantity of the solution of sea salt. Supposing that we operate upon a gramme of pure silver, the solution of sea salt ought to be such that 100 centimetres cube may precipitate exactly the whole silver. The standard of an alloy is given by the number of thousandths of solution of sea salt nece ary to precipitate the silver contained in a gramme of the alloy. When any mercury is accidentally present, which is, however, a rare occurrence, it is made obvious by the precipitated chloride remaining white when exposed to daylight, whereas when there is no mercury present, it becomes speedily first grey and then purple. Silver so contaminated must be strongly ignited in fusion before being a ayed, and its lo of weight noted. In this case, a cupel a ay must be had recourse to.