Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/776

of platinum, which are introduced into a suitable vessel of platinum, an arrangement by which it will be evident much time may be saved. The boiling is then continued for fifteen or twenty minutes, when the cornets are washed with distilled water, and treated with nitric acid of specific gravity 1'3, and in this the cornets remain for about the same period, after which they are again washed in distilled water and dried.

Fig. 11.

(5.) The cornets are annealed, separately, in little clay crucibles, or in the platinum cups in which they have been boiled, by heating them to bright redness. They then diminish considerably in bulk as e (fig 11), and are of a pure yellow colour.

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(6.) The cornets are then weighed in comparison with “check assays ” made on pure gold. These “ checks” are necessary, as the accuracy of the result of an assay is liable to be affected either by retention of silver or copper, or by loss of gold by volatilization in the muffle, solution in the acid, or retention in the cupel. The weight of gold, therefore, indicated by the balance, may be either less or greater than the amount originally present in the alloy. The correction to be applied to a gold assay will be evident from the following formula:^[1]—

Let 1000 be the weight of alloy originally taken ;

p the weight of the piece of gold finally obtained ; ac the actual amount of gold in the alloy expressed in thousandths ; a the weight of gold (supposed to be absolutely pure) taken as a check, which approximately equals :v ; b the loss or gain in Weight experienced by a during the process of assay, expressed in thousandths ; k the variation of “ check gold" from absolute purity, ex- pressed in thousandths ;

then the actual amount of fine gold in the check-piece = a 1 — 153-0), and ac the corrected weight of the assay will = P — {3?}; :l: b; I: being added or subtracted according as it is a loss or gain.

If a be assumed to be equal to ac this equation becomes __ p :h b I" = I" I: 1 + 1000.

Example.—Let p = 901'1 thousandths.

a = 920'0 ,, b = 0'3 ,, gain in Weight. I.- = 0-1 ,,

Then by the first formula—

$x=901\cdot 1-{\frac {920+0\cdot 1}{1000}}-0\cdot 3$;

For, as b is a. gain in weight, it must be deducted, hence

${\begin{aligned}x&=901\cdot 1-0\cdot 092-0\cdot 3\\&=900\cdot 708\end{aligned}}$

And by the second formnla—

${\begin{aligned}x&={\frac {901\cdot 1-0\cdot 3}{1+{\frac {0\cdot 1}{1000}}}}\\&=900\cdot 708\end{aligned}}$

Assay of Gold Ores.—500 grains of the finely powdered sample, which must be taken with the greatest care and accuracy, is passed through a sieve of fine wire gauze with at least 80 meshes to the linear inch. Any residue there may be of flattened particles of gold is set aside for subsequent treatment, usually by direct cupel- lation. Assay of the ore by fusion with litharqe is best suited to ores which do not contain much iron pyrites. For auriferous quartz 500 grains of the orc are fused with 500 grains of red lead, 300 grains of sodic carbonate, 20 grains of powdered charcoal, and 250 grains of borax. The mixture is introduced into a clay crucible, which it should half fill, and is fused in an air furnace. The button of reduced lead may be removed, either by pouring the contents of the crucible into a mould, or by breaking the crucible when cold. If the. ore contains much iron pyritcs, or is of the nature of “ sweep," the name given to carbonaceous residues which accumu- late in mints and goldsmiths' shops, it will be necessary to roast it in a shallow fire-clay dish placed in a mutlle. In the case of pyrites containing about 7 dth. to the ton, the operation would be con- ducted on about 1000 grains. The roasted ore is then fused with about the same mixture of fluxes as has been given for quartz.

Assay by Scorification.—Scoriiication resembles eupcllation, but the oxide of lead produced in the operation, instead of sinking into a porous cup, is held in a flat. saucer ot' fire-clay, and dissolves the earthy constituents of the ore, leaving the precious metal to pass into another portion of lead which remains in the metallic state. About 200 grains of the roasted ore are placed in the scorilier. intimately mixed with 500 grains of granulated and 50 grains of borax lead ; 500 grains of lead are then distributed over the surface of the mixture ; the contents of the scorifier are fused in a muflle ; air is admitted to oxidize the greater portion of the lead; and, at the conclusion of the operation, the litharge should be perfectly fluid and cover the molten lead. The slag may be freed from par- ticles of precious metal by the addition at the conclusion of the operation of a small quantity of powdered anthracite, “hich re- duces a portion of the litharge to metallic globules, which fall through the slag and unite with the lead button. The gold is then separated by cuponation’ and the Silver With “mph it is "Tully always associated is removed by parting in nitric acid.

Assay by means of the Spectroscope.—Lockycr and Roberts^[2] state, as the result of a careful spectroscopic investigation of the alloys of gold and copper, that it is possible to distinguish between alloys of these metals which only differ in proportion by “th11 part. Their experiments have been repeated in America by A. 15. Outerbridgc.^[3]

(w. c. r.—h. b.)

——————

It will be convenient to give here, in connexion with the article Gold, rather than in their proper alphabetical place, the articles Goldbeating and Gold Lace.

GOLDBEATING. The art of goldbcating is of great

antiquity, being referred to by Homer,- and Pliny states that one ounce of gold was extended to 750 leaves, each leaf being four fingers square, which is three times the thickness of the ordinary leaf gold of the present time. In all probability the art originated among Oriental communi- ties, where the working of gold and the use of gold orna- ments have been distinguishing characteristics from the most remote periods; and in India goldbeating is still carried on as a craft involving many mysteries and great difficulties. On the coffins of the Theban mummies speci- mens of original leaf-gilding are met with, where the gold is in so thin a state that it resembles modern gilding. The Incas of Peru do not appear to have been able to reduce gold further than to plates which could be nailed for orna- mentation 0n the walls of their temples. In England goldbeating was confined to London until within the present century. It was introduced into Scotland and the United States within that period, and it is now practised in most towns of any considerable size ; but so far as concerns Great Britain it is principally centred in London. ()ne grain of gold has been beaten out to the extent of 75 square inches, and the same weight of silver to 98 Square inches. Taking a cubic inch of gold at 4900 grains, this gold-leaf is the 367,650th part of an inch in thickness, or about 1200 times thinner than ordinary printing paper. The silver, though spread over a larger surface, was thicker, owing to the difference in its specific gravity; but, calcu- lated by weight, silver is the most malleable metal with which we are acquainted, in that respect considerably exceeding gold. This experiment does not, however, determine the extent of the malleability of either metal, as the means employed to test it were found to fail before

there was any appearance of the malleability of the metals

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1. Fourth Annual Report qftltc Deputy-.11 aster of the J! in t, 1M3, p. 42.
2. Phil. Trans., 1874, vol. clxiv. p. 495.
3. Journal of the Franklin Institute, 1874.