A text-book of assaying

by C. Beringer; J. J. Beringer

The silver retained in the gold after parting is, in bullion assays, an important matter; it is roughly equal to the loss of gold due to absorption by the cupel. Mr. Lowe working on .5 oz. of gold, obtained by parting in assaying bullion, found it to contain .123 per cent. of silver. Dr. Rose in some special assay pieces found by a less direct method of assaying, from .06 to .09 per cent. of silver. The proportion of silver retained varies in a marked way with the proportion of gold to silver in the alloy before parting. It is generally stated that the retained silver is least when this proportion is 1 to 2-1/2, and more or less silver than this leads to a less pure gold after parting.

Platinum in an alloy being parted is dissolved along with the silver either altogether or in part. It imparts a straw yellow colour to the parting acid. Palladium gives an orange colour to the acid.

The loss of gold by solution in the acid during parting is small, but easily demonstrable. On a 500-milligram charge of bullion it may amount to from .05 to .15 milligram; i.e. from .01 to .03 per cent. It is due to gold actually dissolved and not merely held in suspension.

Assaying with checks. Surcharge.—It will be seen from what has been stated that the errors in gold parting are of two kinds: viz. (1) a loss of gold on the cupel and to a less extent by solution in the acid, and (2) an apparent gain of gold due to the retention of silver in the parted material. Both errors are small, and as they are of an opposite character they tend to neutralise each other. Hence they are altogether without effect on the accuracy of the assays of ores when the total gold is reckoned in milligrams. And even with the larger amounts present in bullion assays their influence is so small that an uncorrected result is still fairly accurate; the resultant error would not be more than one part in two or three thousand.

It is customary to report the purity of bullion, or its fineness as it is called, in parts per thousand of bullion. The sum of the errors of an assay, which is called the surcharge, is reported in the same way. Thus a surcharge of + .3 means that the gold as weighed was .3 part per 1000 more than the gold actually present. But a surcharge - .3 means that on the whole there was a loss of .3 part per 1000 in the assay.

Speaking roughly the retained silver will vary with the weight of gold present; if one alloy contains twice as much gold as another the retained silver will be about twice as much also. On the other hand, as already explained, the cupellation loss on the poorer alloy is as much as, or even more than, with the richer one, because of the copper, &c. present. With rich gold alloys the silver more than compensates for the loss and the surcharge is positive; but with poorer alloys the loss is greater and the surcharge is negative.

In Mints and places where bullion assays must be made with the highest attainable accuracy, the surcharge is determined by experiment, and the proper correction is made in the reports on the bullion. This is done by making assays of gold of the highest degree of purity alongside of those of the bullion whose quality has to be determined. These "checks" are so made that they do not differ from the actual assays in any material point. Thus, being of the same quality and weight and undergoing exactly the same treatment, they may reasonably be expected to have the same surcharge as the assays they imitate. Suppose the bullion being assayed varies only a little, up or down, from 900 gold and 100 copper in the thousand, and that .5 gram of it is used in each assay. A quantity of gold differing only a little from .450 gram would be very exactly weighed and placed with .050 gram of copper in the same weight of lead as is being used in the other assays. It would be cupelled, parted, &c., as nearly as possible under the same conditions as the actual assays. Suppose the pure gold weighed .45016 gram and the parted gold weighed .45025 gram, the gain in weight, .00009 gram, would be deducted from the actual assays. A surcharge correction is never applied except to bullion of the same quality as that represented by the "check assay" it was calculated from.

It is evident that unless the gold is of the highest degree of purity these check assays will introduce an error almost equal to that which it is designed to remedy. Moreover, to work the checks to the greatest advantage, a very systematic and uniform method of working must be adopted.

Parting in special apparatus.—One plan for obtaining greater uniformity is to stamp each cornet with a number for purposes of identification, and to treat several, including one or more check assays in the same acid contained in a beaker; all the assays under these conditions evidently receive precisely the same acid treatment. Such a plan can of course only be adopted where there is no risk of the gold breaking up during the parting. An improvement on this is to have a porcelain basin[29] about 8-1/2 inches in diameter and with a capacity of about 1-1/2 litres. It is provided with a porcelain cover with 30 numbered holes through which tubes dip into the acid. The cover is removable. The tubes are like test-tubes and are supported by the cover; their bottoms are perforated with holes or slits. The acid is placed in the basin and boiled over a flat burner; it enters the tubes through the slits. The cornets are placed each in its proper tube. When the boiling is finished, the cover with the tubes is lifted and at the same time the acid drains back into the basin. A dip into a basin of distilled water washes at one operation all 30 assays. The cover is then put on a basin containing the stronger parting acid which is already boiling. This boiling is continued for half an hour. The cover with the 30 cornets is then lifted out from the acid and dipped two or three times in distilled water to wash off the last traces of acid. To transfer the cornets from the tubes to the porous cups the whole of the tube must be dipped under the water; otherwise the operation is exactly as when working with test-tubes.

A still simpler method of working is to use small platinum cups[30] provided with fine slits which admit the acid but retain the gold. A number of these, say 60, are supported on a platinum tray. The parting acids are boiled in platinum dishes under a hood; and the 60 cornets (each in its proper cup) are placed in the acid all at once: the tray carrying the cups is provided with a handle suitable for this purpose. After a proper boiling the tray is lifted out of the weaker acid into the stronger one, where it undergoes the second boiling. It is next dipped several times in distilled water and lastly, after a gentle drying, it is raised to an annealing temperature which must not be too high for fear of the gold sticking to the platinum. After cooling, the cornets are transferred from the platinum cups directly to the pan of the balance. Here all 60 cornets have exactly the same treatment and the "checks" may be compared with great exactness with the other assays accompanying them. There is, too, a great saving of labour.[31]