GOLD ASSAYING ?2$ 



portion is first removed, and the washing repeated with successive portions of water, 

 and with a shaking motion, which can only be acquired by practice, so as to finally 

 collect the gold in the lower part of the vessel. When the earthy matters have been 

 removed, the gold is carefully dried, collected, and weighed, or in some cases mercury 

 is added to collect the gold, and the amalgam afterwards heated to drive off the 

 mercury, and the residual gold weighed. When pyrites is present, it remains 

 for the most part with the gold. To separate it, the residue is roasted or calcined, 

 and the resulting oxides of iron and copper removed by submitting it to a washing 

 process, with or without the addition of mercury. 



2. The Assaying of Gold Alloys. a. Gold-parting. Bar-gold, and alloys used 

 for monetary and other purposes, are assayed by this method. 1 grs., or other convenient 

 quantity, of the sample is weighed out on a balance turning with ^oo tn of a grain. 

 To the weighed metal is added 2 times its weight, or less of silver, and both are 

 rolled up in 60 grs. or more of lead-foil, according to the quantity of copper present, 

 and cupelled, in order to alloy the gold with the silver, and remove the copper or 

 other metallic impurities. When cold the resulting button is removed from the cupel, 

 cleansed, and flattened out under the hammer, or rolled out into a riband or fillet of 

 from 2 to 3 inches in length. The fillet is annealed by heating it to dull redness, and 

 then rolled up into a loose coil. The coil is transferred to a parting flask, and boiled 

 for 15 minutes in nitric acid, of specific gravity 1-2 (about 1 part by measure of acid 

 to 2 of water). The acid is carefully poured off, the cornet washed with warm distilled 

 water, and then boiled for 20 minutes in nitric acid, of specific gravity 1*3 (about 1 

 part by measure of acid to 1 of water), a fire-clay pea, or a small piece of well -burnt 

 charcoal, being placed in the flask to prevent the ebullition taking place irregularly 

 and with sudden bursts, which would be liable to break the cornet, or eject a portion 

 of the liquid from the flask. The acid is poured off, the cornet washed with 

 water, and the clay pea removed. The cornet is transferred to a small clay crucible, 

 by dexterously inverting the flask previously filled with water. The water is drained 

 off from the crucible, and the cornet dried, annealed, or heated to dull redness, and 

 weighed. A very small portion of silver is retained by the gold. In order to 

 ascertain the amount of error due to the surcharge or the correction for this silver 

 when several assays are made, a number of proofs or checks are passed through the 

 process along with the alloys. These proofs consist of weighed portions of pure gold, 

 to which is added a proportion of copper equal to that estimated to exist in the alloy 

 to be assayed. The excess of weight finally obtained in these proofs gives the amount 

 to be deducted. It generally varies from O'l to 0'5 in 1,000. The amount of correction 

 will depend on the weight of alloy submitted to assay, the proportion of silver added, 

 the care in firing, the strength of the nitric acid used, and the time employed in the 

 first and second boilings. The cornets are sometimes boiled a third time in nitric 

 acid. In accurate assaying of gold bullion and other alloys of gold, it is absolutely 

 necessary that the acids should be pure, and that the silver used is most carefully 

 freed from the traces of gold which it usually contains. 



The last traces of silver may be removed from the cornet by treating it before the 

 final annealing with fusing bisulphate of potash in a porcelain crucible. When suffi- 

 ciently cool, the whole is heated with hot water containing a little sulphuric acid, and 

 the cornet dried and ignited. By this means gold may be obtained of almost absolute 

 purity, or i, as it is termed. The following results obtained by the late Mr. T. H. 

 Henry will serve for illustration : Ten grains of pure gold, alloyed with three times 

 its weight of silver, cupelled, and boiled with acid at 22 B., and 32 B., once, weighed 

 10'016. Ten grains of a half-sovereign, with silver, &c., and acid at 22, and twice 

 at 32 B., gave 915'4 ; again, 915'6. With acid, as before, and bisulphate of potash, 

 915-2; again, 915-2. 



Pure gold alloyed with copper, to bring it to standard, cupelled with silver and 

 lead, and treated with acids and bisulphate, gave in one case precisely the same as 

 was taken originally, or igg, and in another 999 '98. 



The assays are reported on 1,000 parts to rd of a thousandth. The assays are also 

 returned on the 24 parts or carats ; pure gold is said to define. Thus if 18 parts by 

 weight of gold are obtained from 24 parts of the alloy submitted to assay, it is said to 

 be 18-carats fine. For jewellers and others the assays are also reported in relation 

 to British Standard. Standard gold contains 22 parts of gold in 24. If the assay re- 

 sult of gold is less than this amount it is said to be worse, or if more is said to be better, 

 than standard. For example, 23 parts in 24 would be reported 1 carat better, or 21 

 parts would be reported 1 carat worse than standard. 



Where a number of assays have to be made, a series of flasks are heated simul- 

 taneously over a number of gas-burners arranged in rows. In order to save time and 

 to diminish the amount of manipulation, several methods have more recently been de- 

 vised. At the Sydney Mint each fillet coil is impressed with a number, and 16 of them 



