| 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 assay masters.
We begin this assay process 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 glass or a capsule of copper, and there is added to it the proportion of lead suited to the quality of alloy to be assayed; there being less lead, the finer the silver is presumed to be. Those who are much in the habit of cupellation can make good guesses in this way; though it is still guess 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, &c. is not carried off, and the button of fine silver remains more or less impure. The most expert and experienced assayer 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 assays.
The best cupels weigh 121⁄2 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 pass readily down into their substance while the latter remain upon their surface; a phenomenon owing to the circumstance of the glassy 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 glass, 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 oxidizement, 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 disseminated, attached to the portion of oxide spread over the cupel, and incapable of being collected.
The furnace and implements used for assaying in the Royal Mint and the Goldsmiths’ Hall, in the city of London, are the following:—
A A A A, [fig. 58.], is a front elevation of an assay 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 assays 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 process are ignited, and heat the air that is allowed to pass 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 61⁄2 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.], 211⁄2 inches; the funnel e, 6 inches. The square of the furnace which receives the muffle and fuel is 113⁄4 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.