in metallurgy, are chemical operations made in small, to determine the quantity of metal or other matter which is contained in minerals, or to discover the value or purity of any mass of gold or silver.

We shall treat here of the latter kind; referring for the former, to Metallurgy.

1. Essay of the Value of Silver, to examine its purity, or the quantity of alloy mixed with it. The common method of examining the purity of silver, is by mixing it with a quantity of lead proportionable to the quantity of imperfect metals with which it is supposed to be alloyed; by testing this mixture; and afterwards by weighing the remaining button of silver. The loss of weight which the silver suffers by cupellation shews the quantity of imperfect metals which it contained.

We may hence perceive, that the essay of silver is nothing else than the refining of it by cupellation. The only difference between these two operations is, That when silver is tested merely for the purpose of refining it, its value is generally known; and it is therefore mixed with the due proportion of lead, and tested without any necessity of attending to the loss of weight it sustains during the operation: whereas, in the essay, all possible methods ought to be employed to ascertain precisely this loss of weight. The first of these operations, or the mere refining of silver, is made in the great, in the smelting of silver ores, and in mints for making money. The second operation is never made but in small; because the expenses of small operations are less than of great, and in the requisite accuracy is more easily attended to. The last operation is our present subject, and is to be performed in the following manner.

We suppose, first, that the mass or ingot of silver, of which an essay is to be made, consists of 12 parts perfectly equal; and these 12 parts are called penny-weights. Thus, if the ingot of silver be an ounce weight, each of these 12 parts will be $\frac{1}{12}$ of an ounce; or if it be a mark, each of these will be $\frac{1}{12}$ of a mark, &c. Hence if the mass of silver be free from all alloy, it is called silver of 12 penny-weights; if it contains $\frac{1}{12}$ of its weight of alloy, it is called silver of 11 penny-weights; if $\frac{1}{12}$ of its weight be alloy, it is called silver of 10 penny-weights; and these 10 penny-weights or parts of pure silver are called fine penny-weights.

We ought to observe here concerning these penny-weights, that essayers give also the name penny-weight to a weight equal to 24 real grains: which latter real penny-weight must not be confounded with the former, which is only ideal and proportional; and such a confusion is the more likely to take place, as this ideal penny-weight is also, like the former, divided into 24 ideal grains, which are called fine grains.

An ingot of fine silver, or silver of 12 penny-weights, contains then 288 fine grains; if this ingot contains $\frac{1}{12}$ part of alloy, it is said to be silver of 11 penny-weight and 23 grains; if it contains $\frac{1}{12}$ of alloy, it is called silver of 11 penny-weight and 22 grains; if it contains $\frac{1}{12}$, it is called silver of 11 penny-weight and 10 grains; and so on. Lastly, the fine grain has also its fractions, as $\frac{1}{2}$, $\frac{1}{4}$ of a grain, &c.

As essays to discover the value of silver are always made in small, essayers only take a small portion of an ingot for the trial; and the custom in France is to take 36 real grains for this purpose, which is consequently the largest weight they employ, and represents 12 fine penny-weights. This weight is subdivided into a sufficient number of other smaller weights, which also represent fractions of fine penny-weights and grains. Thus 18 real grains, which is half of the quantity employed, represent six fine penny-weights; three real grains represent one fine penny-weight, or 24 fine grains; a real grain and a half represent 12 fine grains; and $\frac{1}{2}$ part of a real grain represents $\frac{1}{2}$ part of a fine grain, which is only $\frac{1}{12}$ part of a mass of 12 penny-weights.

We may easily perceive, that weights so small, and essay-balances, ought to be exceedingly accurate. These balances are very small, suspended and inclosed in a box the sides of which are panes of glass, that they may be preserved from dust, and that their motion may not be affected by agitated air, so as to disorder their action.

When an essay of a mass or ingot of silver is to be made, the custom is to make a double essay. For this purpose, two fictitious semi-marks, each of which may be equal to 36 real grains, are to be cut from the ingot. These two portions of silver ought to be weighed very exactly; and they ought also to have been taken from opposite sides of the ingot.

Persons accustomed to these operations know pretty nearly the value of silver merely by the look of the ingot, and still better by rubbing it on a touchstone. By the judgment they form of the purity of the ingot, they regulate the quantity of lead which is to be added to it, as this quantity must be always proportionable to the quantity of imperfect metal mixed with the silver.

Nevertheless, this proportion of lead to the alloy has not been precisely determined. Authors who treat of this subject differ much. They who direct the largest quantity of lead say, that thereby the alloy is more certainly destroyed; and others who direct a small quantity of lead, pretend, that no more of that metal ought to be used than is absolutely necessary, because it carries off with it always some portion of silver. Every assayer uses his own particular method of proceeding, to which he is attached.

To ascertain these doubtful points, three chemists of the Academy of Sciences at Paris, Messrs Hellot, Tillet, and Macquer, were appointed by the French government. They were directed to ascertain everything concerning the assay of gold and silver by authenticated experiments, made under the inspection of a minister whose superior knowledge is equal to his desire of public good, and in presence of the officers of the mint.

The experiments made by these chemists, and the consequent regulation, have determined that four parts of lead are requisite for one part of silver of 11 penny-weight and 12 grains, that six parts of lead are requisite for silver of 11 penny-weight, eight parts of lead for silver of 10 penny-weight, ten parts of lead for silver of nine penny-weight, and so on in the same progression.

Two cupels of equal size and weight are to be chosen. The custom is to use cupels of such a size that their weight shall be equal to that of one half of the lead employed in the assay; because such cupels have been found capable of imbibing all the litharge formed during the operation. These cupels are to be placed together under a muffle in an assay-furnace. The fire is to be kindled, and the cupels are to be made red-hot, and to be kept so during half an hour at least before any metal be put into them. This precaution is necessary to dry and calcine them perfectly; because if they contained any moisture or inflammable matter, an ebullition and effervescence would be occasioned in the assay. When the cupels are heated so as to become almost white, the lead is to be put into them; the fire is to be increased, which is done by opening the door of the ash-hole so as to admit air, till the lead becomes red, smoking, and is agitated by a motion of its parts called its circulation, and till its surface becomes smooth and clear.

Then the silver, previously beat into small plates for its easier fusion, is to be put into the cupels; the fire is to be continued, and even increased, by putting hot coals at the mouth of the muffle, till the silver shall have entered the lead, that is, till it have melted and mixed with the lead. When the melted matter circulates well, the heat is to be diminished by taking away, partly or entirely, the coals put at the mouth of the muffle, and by closing more or less the doors of the furnace.

The heat ought to be regulated so, that the assays in the cupels shall have surfaces feebly convex, and shall appear ardent, while the cupels are left red; that the smoke shall rise almost to the roof of the muffle; that undulations shall be made in all directions upon the surfaces of the assays, which are called circulations; that their middles shall be smooth, and surrounded with a small circle of litharge, which is continually imbibed by the cupels.

The assays are to be kept in this state till the operation is finished, that is, till the lead and alloy have soaked into the cupel; and the surfaces of the buttons of silver being no longer covered with a pellicle of litharge, become suddenly bright and shining, and are then said to lighten. If the operation has been well conducted, the two assays ought to become bright nearly at the same time. When the silver has been by this operation well refined, we may see, immediately after it has brightened, the surface of the silver covered with rainbow colours, which quickly undulate and cross each other, and then the buttons become fixed or solid.

The management of the fire is an important article in assays. For if the heat be too great, the lead is scorified and imbibed by the cupel so quickly, that it has not sufficient time to scorify and carry along with it all the alloy; and if the heat be too little, the litharge is gathered upon the surface, and does not penetrate the cupel. The assayers say then, that the assay is choked or drowned. In this case the assay does not advance; because the litharge covering the surface of the metal defends it from the contact of air, which is absolutely necessary for the calcination of metals.

We have above related the marks of a successful assay. The heat may be known to be too great, from the convexity of the surface of the melted metal; from a too strong circulation; from the too vivid appearance of the cupel, so that the colours given to it by the litharge cannot be distinguished; and, lastly, by the smoke rising up to the roof of the muffle, or not being at all visible from its being so ardent and red-hot as not to be distinguishable. In this case, the heat must be diminished by shutting the door of the ash-hole. Some assayers, for this purpose, put round the cupels, small, oblong, cold pieces of baked clay, which they call instruments.

If, on the contrary, the melted metal have a surface not very spherical, relatively to its extent; if the cupel appear dark-coloured, and the smoke of the assay do only creep upon the surface; if the circulation be too weak, and the scoria, which appears like bright drops, have but a dull motion, and be not soaked into the cupel; we may be assured that the heat is too weak; much more may we be assured of it when the metal fixes, as the assayers call it. In this case, the fire ought to be increased by opening the door of the ash-hole, and by placing large burning coals at the mouth of the muffle, or even by laying them across upon the cupels.

As soon as the lead is put into the cupels, the fire is to be increased, because they are then cooled by the cold metal; and the lead ought to be quickly melted, to prevent its calx from collecting upon its surface in too great quantity before it be formed into litharge; which it would do, and be difficulty fused, if the heat were too weak.

When the silver is added to the lead, the heat must be still increased; not only because the silver cools the mass, but because it is less fusible than lead. And as all these effects ought to be produced as quickly as possible, more heat is at length given than ought to be continued; and therefore, when the silver has entered the lead, the heat is to be diminished till it becomes of a due intensity for the operation.

During the operation, the heat ought gradually to be augmented to the end of it, both because the metallic mixture becomes less fusible as the quantity of lead diminishes; and also because the lead is more difficultly scorifiable, as it is united with a larger proportion of silver. Hence the effays must be rendered very hot before they brighten.

When the operation is finished, the cupels are left in the same heat during some seconds, to give time to the last portions of litharge to be entirely absorbed; because if any of it remained under the buttons of silver, it would stick to them. The fire is then allowed to extinguish, and the cupels to cool gradually, till the buttons have entirely fixed, particularly if they be pretty large; because if they cool too quickly, their surfaces fix and contract before the internal mass, which is thereby so strongly compressed as to burst through the external solid coat and form vegetations, or even to be entirely detached from the rest of the mass, and dissipated. This is called the vegetation of the button. It ought to be carefully prevented, because small bits of silver are sometimes thrown out of the cupel.

Lastly, when the buttons are thoroughly fixed, they are to be disengaged from the cupels by a small iron utensil while they are yet hot; otherwise they could not be disengaged clean and free from part of the cupels which strongly adhere to them when the heat is much diminished.

Nothing then remains to complete the effay, but to weigh the buttons. The diminution of weight which they have sustained by cupellation will show the purity or value of the ingot of silver.

We ought to observe, that as almost all lead naturally contains silver, and that after cupellation this silver is mixed with the silver of the ingot in the button of the effay; before we employ any lead in this operation, we ought to know how much silver it contains, that we may subtract this quantity from the weight of the button, when we compute the fineness of the silver of the ingot effayed. For this purpose effayers generally cupel a certain quantity of their lead separately, and weigh accurately the button of silver it yields: or, at the same time when they effay silver, they put into a third cupel, in the muffle, a quantity of lead equal to that employed in both their effays; and when the operation is finished, and the buttons are to be weighed, they throw the small button produced from the lead alone into the scale which contains the weights; and as this exactly counterpoises the small portion of silver which the effay buttons have received from the lead employed in the cupellation, the weights will show precisely the quantity of silver contained in the ingot, and thus the trouble of calculating is prevented. The small button of silver procured from the cupellation of lead alone is called the witness. But to prevent this trouble, effayers generally employ lead which contains no silver, such as that from Willach in Carinthia, which is therefore procured by effayers.

In the second place, we shall observe, that a certain quantity of silver always passes into the cupel, as refiners in the great have long observed, and which happens also in effaying small quantities. The quantity of silver thus absorbed, varies according to the quantity of the lead employed, and the matter and form of the cupels; all which objects will undoubtedly be determined by the above-mentioned chemists.

The cupellation which we have now described is exactly the same for effays by which the produce of a silver ore, or of an ore of another metal containing silver, is determined. But as these ores contain frequently gold, and sometimes in considerable quantity, when these effays are made, the buttons of silver obtained by the effays ought to be subjected to the operation called parting. See Silver, Refining, Furnace, Muffle, and Cupel.

M. Tillet has published a memoir, shewing that effays of silver made in the common method are uncertain and not to be depended upon, and that this uncertainty proceeds from the different quantities of silver absorbed by the cupel in different effays, according as the heat and other circumstances happened to vary. He therefore proposes, in order to render effays accurate, to extract from the cupel the quantity of silver it has absorbed during the operation, and to add this particle of silver to the button, as these two contain the whole quantity of silver in the matter effayed.

The variations in the different results of different effayers, or of the same effayer at different times, upon the same mass of silver, are sufficient proofs of the uncertainty mentioned by M. Tillet. These variations are occasioned, according to that author, principally from the following causes: 1. From the inaccuracy of the balances and weights employed. 2. From the faulty fusion of the mass to be effayed; by which means, the contained alloy may be unequally diffused. 3. From the impurity of the lead, especially from its containing silver, which is not always equally diffused through its mass. 4. From the different proportions of lead used by different effayers. 5. From the difference of the intensity of heat: for, if the heat be not sufficiently intense, the silver will still contain a portion of alloy; and if the heat be too intense, too much of the silver will be imbibed by the cupel. 6. From the want of care in picking the small particles of silver, which frequently adhere to the sides of the cupel separately, from the principal button. 7. From the spouting which sometimes happens unobserved by the effayer; and which may further falsify the effays of other pieces included under the same muffle, by the falling of the particles thrown out of one cupel into others adjacent. But, with all the attentions to avoid these causes of error, the author obtained different results from different effays of the same mass of silver. Nor could he, by any method, make his different effays consistent with each other, but by adding to each button the particle extracted from the cupel; and this method he found by accurate experiments to be perfectly exact.

M. Tillet observed, that the quantity of lead directed tend in the regulations established in consequence of the report made by Messrs Macquer, Hellot, and Tillet, is not sufficient to purify the silver perfectly from its alloy. He nevertheless approves of the said regulation; and considers the weight of the alloy retained by the button, as some compensation for the weight of the silver absorbed by the cupel. And as it is a constant fact, that the more lead is used, the greater is the loss by the absorption of the cupel, he remarks, that a regulation, directing a larger proportion of lead for France, than is used in other countries, would be disadvantageous to that kingdom; as thereby the silver of the same denomination would be required to be finer in that than in other countries where a less proportion of lead was employed. He observes, that the above-mentioned rule, "that the more lead is used, the greater is the loss by the absorption of the cupel," does not extend to quantities of lead much above double the usual quantities. Thus 32 parts of lead to one of silver, will not occasion more absorption than 16 parts of lead. For the refining scarcely takes place till the extraordinary quantity of lead be gone, and the silver is only or chiefly carried into the cupel along with the copper. Accordingly, he found that he could render the silver finer by using four parts of lead at first, and afterwards adding two more parts when the irises began to appear, than by employing all the fix parts of the lead at once. By this method of dividing the quantity of lead, the loss of silver by absorption was greater. M. Tillet did not find, that by employing bismuth alone, or mixed with lead, his essays were more certain, than when lead alone was used. He observed, however, that the addition of bismuth made the silver purer, but occasioned a greater absorption by the cupel.

2. Essay of the Value of Gold. The fictitious weights used to determine the purity of gold, and to essay this metal, are different from those of silver. See the preceding article. A mass of gold perfectly pure, or which contains no alloy, is ideally divided into 24 parts, called carats; this pure gold is therefore called gold of 24 carats. If the mass or ingot contains $\frac{1}{2}$ part of its weight of alloy, the gold is then of 23 carats; and if it contains $\frac{1}{2}$ or $\frac{1}{4}$ of alloy, it is gold of 22 carats, &c. Hence we see, that the carat of gold is only a relative and proportional weight, so that the real weight of the carat varies according to the total weight of the mass of gold to be examined. If this mass of gold weighs a mark, the real weight of the carat will be $\frac{1}{2}$ of eight ounces, which is equal to a mark. If the mass weigh an ounce, the carat will be $\frac{1}{2}$ part of an ounce, or 24 grains. If it is only a penny-weight or 24 grains, the real weight of a carat will be one grain; and so on.

For greater accuracy, the carat of gold is divided into 32 parts, which are relative and proportional weights, as the carat itself is. Thus $\frac{1}{2}$ of a carat of gold is $\frac{1}{2}$ of $\frac{1}{2}$, or the $\frac{1}{2}$ of any mass of gold; and the gold which contains an alloy equal to the $\frac{1}{2}$ part of the whole mass is called gold of 23 carats and $\frac{1}{2}$; gold which contains $\frac{1}{2}$ of alloy is gold of 23 carats and $\frac{1}{2}$; and so on.

The real weight now generally used in the operation for determining the purity of gold, is six grains. This weight then represents 24 carats. The half of this weight, or three real grains, represents 12 carats. According to this progression, we shall find that $\frac{1}{2}$ of a real grain represents one carat, and the $\frac{1}{2}$ part of a grain represents the $\frac{1}{2}$ of a carat, or the $\frac{1}{2}$ part of a mass of gold to be essayed.

As these weights are exceedingly small, some essayers employ a weight of 12 grains, which must be very convenient.

When a mass or ingot of gold is to be essayed, six grains are to be cut off, and exactly weighed; also 18 grains of fine silver are to be weighed. These two metals are to be cupelled together with about ten times as much lead as the weight of the gold. This cupellation is conducted precisely like that of the essay to determine the purity of the silver, excepting that the heat must be raised a little more towards the end of the operation when the essay is going to brighten. Then the gold is freed from all alloy but silver. If the quantity of copper or other alloy destructible by cupellation be required to be known, the remaining button is accurately weighed. The diminution of weight from the sum of the weights of the gold and of the silver determines the quantity of this alloy.

The button containing gold and silver is then to be flattened upon a polished piece of steel, and care must be taken to anneal it from time to time, to prevent its splitting and cracking. By this method it is reduced to a thin plate, which is to be rolled up, in order to be parted by aqua fortis*. The diminution found after the parting from the original weight of the gold essayed, shows the whole quantity of alloy contained in that gold.

The essay for determining the purity of gold is then made by two operations: the first, which is cupellation, deprives it of all its imperfect metals; and the second, which is parting, separates all the silver from it. By antimony also gold may be purified, which is a kind of dry parting. By this single operation, all the imperfect metals, and silver with which gold is alloyed, are separated. See Purification, Gold, Silver, Refining.

Essay-Hatch, is the miners term for a little trench or hole, which they dig to search for shod or ore.