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Assaying (Volumetric Assay of Copper)

Volumetric Assay of Copper Ores. (Percy.) Dissolve 10 grs. of the copper ore finely pulverized and moistened with strong sulphuric acid, in strong nitric acid, adding the acid gradually; and when the fumes of hyponitric acid cease to be evolved, add a small amount of water and boil for a few minutes. Dilute to 10 oz. and treat with ammonia in excess, and it will become of a deep blue color. Set aside to cool, and prepare the following solution: Dissolve 500 grs. of granulated cyanide of potassium in 20 oz. of distilled water, and keep in a tight-stoppered bottle in the dark. Mark "Standard Solution Cyanide of Potassium." To ascertain the standard of this solution, dissolve 10 grs. of electrotype copper in diin dilute nitric acid and boil to expel hyponitric acid fumes, and dilute to 10 oz. with distilled water.

Take of this solution 1 oz. and dilute to 5 oz. with distilled water, and allow the standard cyanide solution to flow very slowly into it at intervals, from a graduated pipette, and note the amount used to render it nearly colorless. This process takes from to of an hour. Proceed in the same manner to test the solution of ore first obtained, noting the amount required to reduce the color to a faint lilac. Divide this amount by the amount found required for 1 gr. of metallic copper, and the result is the number of grains of metallic copper in the ore tested.

Bismuth Ores.

If the ore be mineralized by sulphur, or sulphur and iron, a previous roasting will be necessary.

The strong ores require no roasting, but only to be reduced to a fine powder. Take the assay weight and mix it with half the quantity of calcined borax, and the same of pounded glass; line the crucible with charcoal, melt it as quickly as possible, and when well done, take out the crucible and let it cool gradually. The regulus will be found at the bottom.

In the Humid Way.

Bismuth is easily soluble in nitric acid or aqua regia. Its solution is colorless and is precipitable by the addition of pure water; 118 grs. of the precipitate from nitric acid, well washed and dried, are equal to 100 of bismuth in its metallic form.

Antimonial ores.

Take a common crucible, bore a number of small holes in the bottom, and place it in another crucible a size smaller, luting them well together; then put the proper quantity of ore in small lumps into the upper crucible, and lute thereon a cover; a cover; place these vessels on a hearth and surround them with stones about 6 in. distant from them; the intermediate space must be filled with ashes, so that the undermost crucible may be covered with them; but upon the upper charcoal must be laid, and the whole made red hot by the assistance of hand bellows. The antimony being of easy fusion with charcoal-dust, covering the crucible with another inverted and luted on, and exposing it to the strongest heat of a forge for an hour or more. The ore is very difficult to reduce.

Arsenical Ores.

This assay is made by sublimation in close ves sels. Beat the ore into small pieces and put them into a matrass, which place in a sand-pot with a proper degree of heat. The arsenic sublimes in this operation and adheres to the upper part of the vessel; when it must be carefully collected, with a view to ascertain its weight. A single sublimation will not be sufficient. It is better to perform the first sublimation with a moderate heat, and afterwards bruise the remainder and expose it to a stronger heat. The addition of charcoal is useful.

In the Humid Way.

Digest the ore in muriatic acid, adding nitric by degrees, to help the solution. The sulphur will be found on the filter; the arsenic will remain in the solution, and may be precipitated in its metallic form by boiling with a strip of copper.

Nickel Ore.

The ores must be well roasted to expel the sulphur and arsenic; the greener the calx proves during this torrefaction, the more it abounds in the nickel; but the redder it is, the more iron it contains. The proper quantity of this roasted ore is fused in an open crucible, with twice or thrice its weight of black flux, and the whole covered with common salt. By exposing the crucible to the strongest heat of a forge fire, and making the fusion complete, a regulus will be produced. This regulus is not pure, but contains a portion of arsenic, cobalt, and iron. Of the first it may be deprived by a fresh calcination, with the addition of powdered charcoal; and of the second by scorification. But it is with difficulty that it is entirely freed from the iron.

In the Humid Way.

By solution in nitric acid it is freed from its sulphur; and by adding water to the solution, bismuth, if any, may be precipitated; as may silver, if contained it, by muriatic acid; and copper, when any, by iron. To separate cobalt from nickel, the two oxides are dissolved in muriatic acid; the solution diluted with distilled water. The liquor is saturated with chlorine, and when cold, an excess of precipitated carbonate of baryta added. It is then set aside for 18 hours, when the cobalt will be precipitated as sesquioxide, while the nickel will remain in solution.

Cobalt Ores.

Free them as much as possible from earthy matters by well washing, and from sulphur and arsenic by roasting. The ore thus prepared is to be mixed with 3 parts of black flux, and a little deis separated, and runs through the holes of the crepitated sea-salt; put the mixture in a lined crucible, cover it, and place it in a forge-fire, or upper vessel into the inferior one, where it is collected. Humid Assay of Arseniated Antimony.

Dissolve the ore in aqua regia; the sulphur is separated by filtration. Evaporate the solution to dryness and heat below redness until all the nitric | acid is expelled. The resulting antimonic acid contains 76.33 per cent. of metallic antimony.

Manganese Ores.

The regulus is obtained by mixing the calx or ore of manganese with oil, making it into a ball, and putting it into a crucible lined with powdered charcoal 1-10th of an inch on the sides, and 4 of an inch at bottom; then filling the empty space crucible, cover it, and place it in a forge-fire, or in a hot furnace, for this ore is very difficult of fusion.

When well fused, a metallic regulus will be found at the bottom, covered with a scoria of a deep blue color; as almost all cobalt ores contain bismuth, this is reduced by the same operation as the regulus of cobalt; but as they are incapable of chemically uniting together, they are always found distinct from each other in the crucible.

The regulus of bismuth, having a greater specific gravity, is always at the bottom, and may be separated by a blow with a hammer.

In the Humid Way.

Make a solution of the ore in nitric acid, or aqua regia, and evaporate to dryness; the residuum, treated with the acetic acid, will yield to it the cobalt; the arsenic should be first precipitated by the addition of water.

Mercurial Ores.

The calciform ores of mercury are easily reduced without any addition. A quintal of the ore is put into a retort, and a receiver luted on, containing some water; the retort is placed in a sand-bath, and a sufficient degree of heat given it, to force over the mercury which is condensed in the water of the receiver.

Sulphuretted Mercurial Ores.

The sulphurous ores are assayed by distillation in the manner above, only these ores require an equal weight of clean iron-filings to be mixed with them, to disengage the sulphur, while the heat volatilizes the mercury, and forces it into the receiver. These ores should likewise be tried for cinnabar, to know whether it will answer the purpose of extracting it from them; for this a determinate quantity of the ore is finely powdered and put into a glass vessel, which is exposed to a gentle heat at first, and gradually increased till nothing more is sublimed. By the quantity thus acquired, a judgment may be formed whether the process will answer. Sometimes this cinnabar is not of so lively a color as that which is used in trade; in this case it may be refined by a second sublimation, and if it be still of too dark a color, it may be brightened by the addition of a quantity of mercury, and subliming it again.

Humid Assay of Cinnabar.

The stony matrix should be dissolved in nitric acid, and the cinnabar being disengaged, should be boiled in 8 or 10 times its weight of aqua regia, composed of 3 parts of nitric, and 1 of muriatic acid. The mercury may be precipitated in the metallic form by zinc.

Silver Ores.

Take the assay quantity of the ore finely powdered, and roast it well in a proper degree of heat, frequently stirring it with an iron rod; then add to it about double the quantity of granulated lead, put it in a covered crucible, and place it in a furnace; raise the fire gently at first, and continue to increase it gradually, till the metal begins to work; if it should appear too thick, make it thinner by the addition of a little more lead; if the metal should boil too rapidly, the fire should be diminished. The surface will be covered by degrees with a mass of scoria, at which time the metal should be carefully stirred with an iron hook heated, especially towards the border, lest any of the ore should remain undissolved; and if what is adherent to the hook when raised from the crucible melts quickly again, and the extremity of the hook, after it is grown cold is covered with a thin, shining smooth crust, the scorification is perfect; but, on the contrary, if, while stirring it, any considerable clamminess is perceived in the scoria, and when it adheres to the hook, though red hot, and appears unequally tinged, and seems dusty or rough, with grains interspersed here and there, the scorification is incomplete; in consequence of which the fire should be increased a little, and what adheres to the hook should be gently beaten off, and returned with a small ladle into the crucible again. When the scorification is perfect, the metal should be poured into a cone, previously rubbed with a little tallow, and when it becomes cold, the scoria may be separated by a few strokes of a hammer. The button is the produce of the assay. By Cupellation. it with an equal portion of litharge, divide it into Take the assay quantity of ore, roast and grind 2 or 3 parts, and wrap each up in a small piece of paper; put a cupel previously seasoned under a muffle, with about 6 times the quantity of lead upon it. When the lead begins to work, carefully put one of the papers upon it, and after this is absorbed, put on a second, and so on till the whole quantity is introduced; then raise the fire, and as the scoria is formed it will be taken up by the cupel, and at last the silver will remain alone.

This will be the produce of the assay, unless the lead contains a small portion of silver, which may be discovered by putting an equal quantity of the same lead on another cupel, and working it off at the same time; if any silver be produced it must be deducted from the assay. This is called the witness.

In the Humid Way - Gay Lussac's Method.

Dissolve the ore or coin in nitric acid. Prepare a standard solution of common salt; 542.74 of common salt will precipitate 1000 parts of silver.

It is convenient to have, also, solutions of 1-10th the standard strength for the final precipitations.

Add the solution until no precipitate appears.

From the amount of solution, and consequently of salt used, the amount of silver is at once determined without further weighing. To correct the result a standard silver solution is used at the same time, and any correction it may require is applied to the rest of the assay.

To Assay the Value of Silver.

The general method of examining the purity of silver is by mixing it with a quantity of lead proportionate to the supposed portion of alloy; by remaining button of silver. This is the same protesting this mixture, and afterwards weighing the cess as refining silver by cupellation.

It is supposed that the mass of silver to be examined consists of 12 equal parts, called pennyweights; so that if an ingot weighs 1 oz., each of the parts will be 1-12th oz. Hence, if the mass of silver be pure, it is called silver of 12 dwts.; if it contain 1-12th of its weight of alloy, it is called silver of 11 dwts.; if 2-12ths of its weight be alloy, it is called silver of 10 dwts; which parts of pure silver are called 5 dwts. It must be observed here that assayers give the name dwt. to a weight equal to 24 real grs., which must not be confounded with their ideal weight. The assayers' grs. are called fine grs. An ingot of fine silver, or silver of 12 dwts., contains, then, 288 fine grs.; if this ingot contain 1-288th of alloy, it is said to be silver of 11 dwts. and 23 grs.; if it contain 4-288th of alloy, it is said to be 11 dwts., 20 grs., etc. Now a certain real weight must be taken to represent the assay-weights; for instance, 36 real grs. represent 12 fine dwts.; this is subdivided into a sufficient number of other smaller weights, which also represent fractions of fine dwts. and grs. Thus, 18 real grs. represent 6 fine dwts; 3 real grs. represent 1 fine dwt., or 24 grs; 1½ real grs. represent 12 grs.; 1-32d of a real gr. represents of a fine gr., which is only 1-752d part of a mass of 12 dwt.

Double Assay of Silver.

It is customary to make a double assay. Tho silver for the assay should be taken from opposite sides of the ingot, and tried on a touch stone.

Assayers know pretty nearly the value of silver merely by the look of the ingot, and still better by the test of the touch-stone. The quantity of alloy, which being in general copper, will be lead to be added is regulated by the portion of nearly as follows:

Of silver dwt. gr. dwt. gr. Requires from 11 6 to 5 to 6 012 to- 8 to 9 19 18 to 90 12 to 13 8 6 to 7 12 13 to 14 6 18 to 6 0 14 to 15 30 to 1 12 0 to 16 From 1 12 to 0 18 0 to 20 its Times weight of lead.