Assaying (old Fr. asaier, mod. Fr. essayer, to try), the chemical examination of an ore, a metal, or an alloy, to determine the proportions of its ingredients. The assay of a gold ore, to obtain the amount of gold present, consists of several operations. Fifty grammes of the ore are mixed with 80 grms. of oxide of lead, 20 of carbonate of soda, 4 of charcoal dust, and 12 of powdered glass. If the ore contains muce silica, the glass may be left out; if much sul phur, 2 grms. of nails should be added. Th mixture is placed in a Hessian sand crucible, covered by a layer of salt, and heated in a furnace for half an hour at a gentle heat, and then for half an hour at a white heat. When this crucible is taken out of the furnace and allowed to cool and then broken open, a button or globule of lead will be found at the bottom, covered by a dark glassy slag and a layer of salt. This button contains the gold and most of the silver of the original 50 grms. of ore. The oxide of lead, the quartz, and carbonate of potash form a fusible glassy slag which absorbs earthy impurities. The oxide of lead and nitre unite to drive off the sulphur as sulphurous acid.
The coal dust reduces a portion of oxide of lead to a fine spray of metallic lead, which in settling alloys the gold and silver, carrying them to the bottom of the crucible. The button usually contains, besides lead, gold, and silver, some copper, nickel, antimony, and sulphur, if these substances were present in the ore. The process of separating gold and silver from the other metals with which they are alloyed depends on the principle that they cannot be converted into oxides when heated in the air, while the other metals with which they are generally alloyed can be oxidized at a high temperature, especially when a large quantity of lead is present. The lead button is placed in an earthenware dish made of fire clay, called a scorifier (scoria, slag). A wind furnace containing a muffle is used for heating the assay in this and in the succeeding operation. The fuel generally employed is coke or anthracite; charcoal is sometimes used when the other cannot be obtained. The muffle is a flat-bottomed earthen vessel, 8 or 10 in. long, 3 or 4 in. wide, and 2 1/2 or 3 in. high, its top arched over, one end open, the other closed; in fact it is half a cylinder open only at one end. In its roof and sides are little apertures through which the air drawn in at the open end can pass.
It is set in the furnace, in the front of which is an opening corresponding to the open end of the muffle. Coals are heaped around and upon it to expose it to the full heat of the furnace. In the scorifier, when heated to a bright red heat, the so-called baser metals are oxidized and form a slag, leaving a small quantity of pure lead alloyed with silver and gold. This alloy while in the molten state is poured into a cooling mould, hammered to free it from slag, and is then ready for the next operation, which is called cupellation, and is performed in a little cup called a cupel. The cupels should be prepared of bone ashes well burnt, ground, and washed, and then shaped into cylindrical forms an inch or so high and 2 in. in diameter, their tops having a shallow depression to hold the metal. These cupels have the property of absorbing the oxides of metals and of holding those that will not oxidize; but as they cannot absorb a greater weight than their own of oxide of lead or litharge, not quite so much of this metal should be put into any one cupel as its own weight. - At the mints the assayer is mostly called upon to practise his art upon coin and bullion, alloys of copper, lead, gold, and silver, or containing two or more of these metals.
In this case the previous operations of fusion in the crucible and slagging in the scorifier are omitted, and the assay begins at this point. The alloy to be assayed is carefully weighed in a delicate balance. It may be from 2 or 3 grammes, or even less, if already considerably alloyed. A proper' quantity of lead, known to contain no silver, is put with it, and the two are placed by means of small tongs in the cupel, which with the muffle has been brought to a full red heat in the furnace. It is convenient to carry on several of these operations at once, and therefore a number of the cupels are usually introduced together on the floor of the muffle. The metals when placed in the hot cupel immediately melt and form a bright globule, which spins around and keeps in continual motion. The air drawing in through the muffle oxidizes its surface, and fumes of the oxide of lead are carried off by the draft. At the same time a floating scum of the oxide is constantly flowing down the sides of the globule and sinking into the cupel, while freshly formed oxide replaces it. Any copper that is present is oxidized with the lead and absorbed into the cupel.
Thus the operation goes on till it terminates by all the lead being oxidized, which is indicated by a sudden brightening up and subsequent darkening of the little globule, and the cessation of the appearance of the fumes and scum of oxide. This little globule, which is pure silver, pure gold, or an alloy of the two metals, shows by its weight the quantity that was in the sample. Care should be taken to avoid too intense heat, as this may volatilize a portion of the silver; and the globule should not be cooled suddenly, as the pure metal absorbs oxygen when melted, and gives it out in cooling. If the change is sudden, some silver is apt to be ejected with the gas. By a little experience and care this operation is made so perfect that no sensible difference should be detected in the weight of two buttons obtained from two assays of equal weights, when tested by a balance that turns with. 1/20 of a milligramme. The quantity of lead that should be added is a matter that can only be determined by experience. Too little lead for the oxidation of impurities prevents the formation of a clean button of silver, free of oxide, and too much lead is apt to carry down with it into the cupel a small quantity of silver.
This operation is often performed with the blowpipe, and small cupels adapted to its uses. The weight of the little button is ascertained by the size of the round hole, of a graduated series of such holes in a brass plate, which it fits, the weight of a button of silver or one of gold for each hole having been previously ascertained. In skilful hands this is conducted very expeditiously, and with considerable accuracy. It is especially adapted to the testing of argentiferous lead ores, to determine approximately their percentage in silver. The lead also may be quantitatively determined by the reducing process with the blowpipe, that must precede the cupelling. If the button when taken from the cupel proves to be pure silver, it shows at once the value of the sample of ore or bullion; but if it contains gold, as in the gold assay, the amount of gold must be found out and subtracted from the weight of the button, and the amount of each metal will then be known. To this end the alloy of these metals is separated by the process called parting, or quartation, as it is usually conducted upon an alloy made to contain at least three parts of silver to one of gold.
If the silver is in larger proportion, the gold cornet will crumble; but when of small amount compared with the gold, it is shielded by the gold from the action of the dilute nitric acid which is used to dissolve out the silver. To insure a perfect union of the gold and silver added to it, it is well to melt them with lead, and then separate the lead by cupelling. More heat may be safely applied than when silver is cupelled without gold, as the alloy of these cannot waste by volatilization. The button is hammered out, heated red-hot, and annealed, and then rolled into a thin plate, which is coiled up of the size of a quill, and called a cornet. This is put into a parting glass, and two or three times its weight of pure nitric acid is poured upon it. Some heat is applied, when red fumes of hyponitric acid are given off, and in a short time the silver is dissolved, and the gold is left, still retaining the form of the coil, but forming a brittle, spongy, brown mass. The solution of silver is poured off, and a strong acid is added to the gold, and heated to dissolve out the last traces of silver. This is poured off, and the gold is washed with hot distilled water.
It is carefully taken out, put in a crucible, and heated, when it shrinks together and regains its metallic lustre and the fine color of gold, with its softness and flexibility. Being now weighed, the process is finished by the calculation of the quantity lost. The silver is recovered by precipitating it from the solution by the introduction of bright sheets of copper, for which metal the acid has a greater affinity than for the silver. It is ascertained that in this process the silver is never entirely taken up by the nitric acid, and that some gold is dissolved by the strong acid, as is found by preserving for years the same acid to extract the last traces of silver. The inside of the bottle containing it becomes at last coated with fine gold. This has been noticed in the British mint, and full 30 grains of gold have been collected from bottles thus used. Very small errors are thus involved in estimating the quantities of silver and gold by this process. - Assayers and metallurgists at the present time prefer what is termed the wet method, performed by the aid of acids and solutions, and called wet in contradistinction to the dry or furnace assay, for the determination of the amount of iron, zinc, copper, and antimony in the ores of these metals.
The estimation of the amount of iron in an ore is performed by the aid of a solution of permanganate of potassium. When a solution of this salt, which is of a beautiful violet color, is added to a solution of protoxide of iron, the protoxide is immediately converted into the peroxide, and the solution loses its color. If, however, the permanganate of potassium is added with constant stirring until all the pro-toxide is converted into peroxide, and one drop too much added, that one drop will color the whole iron solution very distinctly. It is found that the same amount of iron always requires the same amount of permanganate of potassium to give the first color. The permanganate of potassium is termed a standard solution. If then 0.2 grm. of iron is dissolved in acid (muriatic), and the standard solution added from a measuring tube, we can determine the amount of solution needed for 0.2 grm. iron; and when an ore is dissolved, and changed to protoxide by dissolving zinc in it, and the standard solution added, we obtain the amount of the solution needed for the amount of iron in the ore. And the problem is solved by this proportion: as first amount of standard is to second amount of standard, so is 0.2 grm. of iron to the amount of iron in the ore.
The dry method of assaying iron ores is still used to assist the masters of iron furnaces in planning the proportions of ingredients to be used in the blast furnace for the production of iron. It is based upon the same principles as-the reducing them in the blast furnace. The oxygen with which the metal is combined must be taken up by presenting to it some substance for which it has stronger attractions than for iron, and the earthy impurities must have such substances added to them that the product of their union will be a glassy fluid, through which the globules of metallic iron can easily sink and collect together in a button. Charcoal is the substance for deoxidizing the ore in the blast furnace and in the crucible. The matters for aiding the fusion, called the flux, vary according to the earthy ingredients of the ore. The desired glassy fluid is a silicate of lime and alumina, and it may be of magnesia. If the ores already contain much silica, carbonate of lime, with the addition of some alumina or common clay, constitutes the proper flux. Ores deficient in silica require an addition of it. Some ores contain such a mixture of proper fluxing ingredients, that they melt easily without any addition of these matters.
In the crucible, a little borax increases very much the fusibility of the mixture. The ore and fluxes should be thoroughly ground and mixed together, and placed in a brasqued crucible, that is, one carefully filled and rammed with fine charcoal, moistened with water to a paste, and out of the top of which a cavity is excavated for holding the assay sample. The crucible is to be placed in a wind furnace, and gradually heated for half an hour, when the whole force of the blast is to be applied for half an hour longer. A button of cast iron will be found in the bottom of the crucible when it has cooled. - The wet assay of copper is performed by dissolving a weighed amount of ore in nitric acid, and removing sulphur if present by an addition of chlorate of potassium. Muriatic acid is added, and the nitric acid removed by evaporation. The residue is dissolved in water and muriatic acid and filtered; the copper is precipitated from this solution by pure zinc or iron, and the resulting copper sponge is washed by decanting the liquid and replacing it by distilled water, and then quickly dried and weighed as metallic copper; from this weight the value of the ore is easily calculated. The dry assay of copper is still in use in Cornwall, at Swansea, and at some other places.
It is, as conducted by metallurgists, often an empirical process, the fluxes being added with very vague ideas as to their true effect. The ores are properly classified into those which contain no sulphur, arsenic, or any foreign metals but iron; those which contain sulphur, iron, arsenic, antimony, etc. Ores of the first class, containing over 3 per cent, of copper, are reduced in a crucible by the addition of three parts of black flux. Poorer ores may be assayed in the wet way. The second class are sulphates or sul-phurets. The former are easily decomposed by heat in a platinum crucible, when they may be treated as substances of the first class. The sulphurets, under which general head are included most of the workable ores of commerce, are treated in a great variety of ways. The first operation, after reducing them to fine powder, is to roast or calcine them, to expel the sulphur. This process requires care and experience, and is most thoroughly effected, according to Mitchell, by adding one tenth of their weight of carbonate of ammonia to the roasting mass in the crucible, constantly stirring it in as the calcining goes on.
Sulphate of copper is produced by the roasting; and on addition of carbonate of ammonia, by double decomposition, sulphate of ammonia forms, which being volatile can be expelled by heat. The ore is then thoroughly mixed in a mortar with 25 per cent, of its original weight of lime, and 10 to 20 per cent. of fine charcoal, and 1 1/2 time its weight of dry carbonate of soda. The whole is to be placed in the same crucible in which the roasting was done, and covered with its weight of glass of borax. It is then subjected to a moderate heat for a quarter of an hour, and to a bright red heat as much longer. On cooling, and breaking the crucible, the button of copper will be found in the bottom. It is well to make two parallel assays of these ores, that one may confirm or disprove the other. - The varieties of lead ores which are most commonly subjected to assay are the sulphurets (galena) and the carbonates. The former is treated by taking 400 or 500 grains, coarsely powdered, and mixing with it one fourth its weight of black flux, one fourth of iron nails, and one eighth of cream of tartar. The crucible should be large enough to contain double the quantity, and the charge should be covered with common salt halt an inch deep.
After being exposed to a high heat for ten minutes, the lead may be poured out, or suffered to cool in the crucible. If the ore contain much earthy or pyritous matter, a less proportion of iron filings should be used, and a little fluor spar and borax be added. Galena is conveniently assayed in an iron crucible, the crucible itself furnishing the material for desulphurizing the ore. The usual quantity, say 400 or 500 grains, is mixed with 2 1/2 times its weight of carbonate of soda, and put in an iron crucible, which is covered. The galena is decomposed, and sulphuret of iron formed. The lead is poured out into an ingot mould, and the crucible well tapped to obtain all the lead. Another sample is immediately put in while the crucible is hot, and the operation repeated as long as the crucible lasts. The carbonates are assayed with half their weight of black flux, and a little cream of tartar, with a superficial covering of salt.