This section is from the book "Leaching Gold and Silver Ores. The Plattner And Kiss Processes: A Practical Treatise", by Charles Howard Aaron. Also available from Amazon: Leaching Gold And Silver Ores.
202. Large bullion scales, and troy weights, are not always at hand; yet it is often desirable to know the value of a bar before sending it away from the works, and as good counter scales, with advoirdupois weights, are generally accessible, the following facts may be found useful.
The assay value of one advoirdupois pound of pure silver is $18.85; that of one advoirdupois ounce is $1.17 The pound of pure gold is worth $301.44, and the ounce $18.84. Hence it is easy to calculate the value of a bar of which the fineness is known, and which has been weighed on common counter scales to the nearest quarter ounce. For example: a silver bar is .969 fine, and weighs 82 lbs 3 1/4 ounces.
Then: $18.85 x .969 = $18.26 x 82 = $1,497.32 and 1.17 x.969 = 1.13 x 3 1/4 = 3.67
Making the value of the bar $1,500.99
Each 1/1000th of silver in a bar is worth 1.885 cents, and each 1/1000th of gold 30.14 cents per advoirdupois pound; therefore, if in the above example the bar contained also a little gold, say .003, and perfect accuracy is not required, it would be sufficient to add to the value 3 times 30.14 cents for each pound weight, or in all $74.14. Again, a gold bar is .969 fine and weighs 7 lbs. 13 1/2 oz., or 125 1/2 oz.; then 18.84 x .969 = 18.26 x 125 1/2 = $2,291.63. Or, the value of a gold bar may be calculated as though it were silver, and the result multiplied by 16.
The commercial or market value is different from the assay value. The discount on silver varies from 10 to 15 per cent of the assay value, to which is added one-half of one per cent for mintage, or assayer's fee. Gold bars free from base metal are subject to a discount or a premium, equal to one-tenth of one per cent for every ten "points," or thousandths, above or below the quoted par fineness; discount if above, premium if below. The apparent anomaly is due to the fact that, if there is no base metal in the bar, all that is not gold is silver, whence at par fineness, there is just enough of silver in the. bar to pay parting charges, etc., while in a finer bar those charges must be met by a discount on the value of the gold. Conversely, a bar which is below par fineness commands a premium for the extra silver it contains. On account of this arrangement, bars which are more than half gold, are stamped with the gold value only, unless they contain much base metal, when both gold and silver value is stamped on them.
Mixed bars in which the gold forms a large part of the value, but less than half the weight, are called "dore" bars, and are stamped with the value of both the gold and the silver.