Nine tables of comparative results, taken from the regular work of the service, are given. These tables begin with very fine gold, produced in an electrolytic refinery, showing close agreement in the assay results, and follow through decreasing gold fineness and increasing amounts and complexity of base metals to very impure and complex bars produced at cyanide mills, some of which give widely-varying results. Next is given a series of results on samples, were prepared and sent out to various laboratories in the service, to test the influence of different metals and various combinations upon the agreement of the assay results; 11 samples were sent out and each one was assayed from 44 to 71 times, making a total of 623 assays. To these are added 107 assays of identical samples of coin-gold.
On a previous occasion, We have endeavoured to show the degree of accuracy that may be expected in the ordinary everyday analysis of various materials, and on another occasion, have called attention to the accuracy of the commercial assay for silver.
The methods of assaying followed in the various institutions are substantially the same, and have grown up as the result of many years of experience so that with careful work on pure bullion the results obtained at different institutions ought to agree very closely; but with impure bullion, that is, bullion containing other constituents besides gold and silver, the chances for variations in the results increase. The action of different impurities varies widely. Only small amounts of some impurities induce excessive variations in the results, while comparatively large amounts of others have but little effect, and, on the other hand, a combination of several impurities in bullion may be most disastrous to any agreement of the assay- results.
The bullion is handled in the same manner at all institutions. It is weighed as received and then melted. Generally, a simple melting with soda or borax, or both, is sufficient, but sometimes it is more or less refined in the pot. In the case of large melts, 1,000 oz. or over, or of very impure bullion, a small sample may be dipped or poured out from the well-stirred pot and granulated in water. The granulations are used for the assay sample. In general, however, the metal is cast into bars, and these bars are chipped, top and bottom, to obtain the assay samples. The bars are again weighed and the assays are made when the value of the deposit is calculated from these data. If, however, the various assays made on a deposit do not agree well enough to satisfy the assayer, the bar is remelted, with or without refining in the pot.
The determination of gold in ores by the fire assay, when properly executed, is justly regarded as one of the most accurate analytical methods. With ordinary care and an excellent bead balance, 1 part of the gold in more than 20,000,000 parts of ore can be readily and accurately determined. The determination of 1 part of the gold in 5,000,000 parts of ore is very easily done. Until recently, however, it was rare for commercial ore-assaying to attain the accuracy of 1 part in 5,000,000.
The ability to determine gold in ores with such great accuracy is due to the fact that very large amounts of ore, up to 0.25 kg., are taken for the assay, and on a high-grade button-balance the resulting bead can be weighed to 1/200 mg. In assaying bullion, however, such extreme accuracy is out of the question, for the simple reason that there is a limit to the amount of bullion that can be taken for the assay. To obtain the most accurate results the assay sample must be weighed on the same high-grade balance on which is weighed the resulting cornet, and the sample also must be weighed with the same degree of care and accuracy as the cornet. Now, the load that a very sensitive bead balance will safely carry is generally limited to 1 g., and the amount of metal generally taken for a gold-bullion assay is 0.5 g. or one-half of the maximum load of the balance.
Another point in bullion assaying which militates against extreme accuracy in the results lies in the fact that the cornet which is weighed is itself gold, and, in high-grade billions, it is a very large part of the sample taken for the assay, so that even slight errors in the handling of the cornet, resulting in slight losses or gains in its weight, count heavily against the highest accuracy of the results.
Table I. shows a series of results obtained by three assayers working in the same laboratory on fine gold from an electrolytic refinery. Each assayer worked upon the same sample in each set of assays as averaged, the samples being cut from both the top and the bottom of the bars. While there is a possibility that there may be some difference in composition between the top and bottom of the bars, yet in such high-grade material as this any such difference must be slight, and eight tests of the subject showed a maximum difference between the top and bottom of only 0.0001, which is considerably less than many of the differences between individual assays. On the whole, then, the figures may be taken as fairly representing the ordinary run of commercial work upon such high-grade bullion. It will be noted that in several cases the figures exceed 1,000, which is due, in part at least, to the high grade of the material. It may also be due in part to the presence in this electrolytic gold of unusual impurities in very small amounts. These data emphasize the necessity of averaging a large number of assays to get a satisfactory determination of the fineness in such a very high-grade material.
Table II. shows results obtained by various assayers in a single laboratory in assaying granulation samples from a wide variety of bullion.
The figures given in Table III. are all taken from a single shipment and show the accuracy that can be obtained upon the material of fairly uniform composition, being mostly gold and silver, with but little base metal present. This table shows, first, the results obtained at the assay office where the bullion was originally purchased; and, second, the results obtained upon the same material when shipped to a mint. In some of these samples, there is undoubtedly a difference between the tops and bottoms of the bars, but the figures show the agreement that may be expected between the two institutions in arriving at the value of such deposits.
Table IV. gives the assays of 14 bars which were referred to the Bureau laboratory for adjustment, although the average differences between the mint and the assay office were only slight.
The handling of bullion produced at mills using the cyanide process of gold extraction has given a great deal of trouble.
Even when properly prepared such bars are likely to be troublesome, but when, as not infrequently happens, the precipitates are not properly purified before being cast into bars they may give no end to trouble.
A very mild case of variation in cyanide bars is shown in Table V. As received, these bars were chipped and the chips assayed. Since the figures thus obtained were considerably higher than the shipper’s figures, the bars were then carefully bored and the borings assayed. Finally, the bars were remelted, with small losses in each case, and granulations are taken. The granulations were then assayed.
Table VI. exhibits the results obtained by sampling three cyanide-bars, high in gold and very low in silver, in three different ways. The assays show a wide variation on the chip- samples. While the drill-sample assays are fairly concordant for this class of material, the dip-sample assays agree much better and are to be preferred.
An assay-office had received a cyanide-bar which showed 546, 545.5, 546.2, 546 fine in gold. This was considered satisfactory, and it was shipped to a mint, but the chip-samples there yielded most varying results, as follows: 544.6, 535.2, 543, 535, 542.4, 555.6. The bar was then remelted, and granulations showed 550.2 and 551.2. Another cyanide-bar received at the same assay-office from the same mill showed 592, 593.9, 592.9, 593.3 fine in gold, and was accepted. It was shipped to the same mint, where chips showed 603.6 and 590, while borings showed 588 and 588.6. The bar, which weighed 559.65
- Troy, was remelted, with a loss of 1.78 oz., and granulations from the melt showed 601.8 and 601.8 fine in gold.
Having had a great deal of trouble with some bars from this mill, while others gave but little trouble, the assay office gave one of the bad bars a very thorough treatment by melting and refining in the pot. As received, the bar weighed 643.30 oz. Troy, and was probably about 847 fine in gold. It was melted seven times, when it weighed 502.01 oz., showing a loss of 141.29 oz. in weight. The final bar was 933.4 fine in gold and 21 fine in silver. The gold loss from this excessive course of meltings was only approximately 3.75 oz., most of which could undoubtedly be recovered from the slags.
The details of the meetings are shown in Table VII. It should be noted that the fourth melt shows practically no refining, and the weight was only slightly reduced so that no practical change is shown in the assays.
Thirteen miscellaneous deposits were united in a mass melt and cast into 17 bars weighing 2841.77 oz. Each bar was chipped twice, and each chip was assayed in duplicate for gold. Table VIII. shows the number of times the stated fineness was obtained :
The average of the 68 assays showed the mass to be 407.16 fine in gold.
When made from the highest grade of metals our coin-gold, 900 gold, and 100 copper, does not segregate. The gold used may contain a very small amount of silver, but should be as free as possible from all other impurities, and the copper should be of the highest purity possible. Occasionally, in practice, however, there will be slight segregation due to some impurities present in minute amounts. On one occasion an inside strip cut from a double eagle was assayed six times and yielded the following gradually decreasing figures: 900.2, 900.1, 899.9, 899.85, 899.5, 899.45. On another occasion a double eagle was cut as indicated in Fig. 1 and the following results were obtained:
In making ingots from this metal an unusual number of melts had to be rejected and remelted for want of uniformity. It was expected that the coins made from this bullion would run low, but none of those regularly tested did. In fact, two from one delivery were most unusually high, viz.: 900.6 and 900.7. Thereupon 12 coins were selected from the same delivery and 46 assays were made upon them, with the following results:
In order to get a much wider range of comparison, and to test the influence of the different metals and of various combinations upon the gold assay, a series of samples was prepared in the laboratory and sent out to various laboratories in the service for assay. In preparing the samples the metal was thoroughly mixed by stirring when molten and remelted as often as appeared necessary. They were finally cast into small bars, and when sufficiently ductile were rolled out thin.
The strips were cut into small squares, and these were mixed up and the samples for each institution were taken out of the mixed pile of pieces. In the case of the brittle bars, they were hammered out and rolled until they crumbled to pieces. The larger pieces were then cut up, and the whole was mixed before the samples were taken out.
All through the preparation of the samples very great care was exercised so that in each set every sample sent for assay should be identical, and thus eliminate from the assay results in all chances of differences being due to differences in the samples operated upon, and to confine the differences shown to the actual assay-work. In one very base sample, which will be further noted, it was not possible to adhere to this rule because the metal was too hard.
In making such small melts it is practically impossible to adhere to any predetermined composition with any degree of closeness.