The calculation becomes more complicated when, as is frequently the case, the ore contains metallic particles. These show themselves by refusing to pass through the sieve when the ore is powdered. When they are present, a large portion, or if feasible the whole, of the sample is powdered and sifted. The weights of the sifted portion and of the "metallics," or prills, are taken; the sum of these weights gives that of the whole of the sample taken. It is very important that nothing be lost during the operation of powdering.
Each portion has to be assayed separately. It is usual to assay a portion of the sifted sample, say, 20 or 50 grams, and to add to the produce of this its share of the "metallics." This way of calculating, which is more convenient than correct, is illustrated by the following example:—
| Weight of | whole sample | 400 | grams |
| Made up of | sifted portions | 399 | " |
| " | "Metallics" | 1 | " |
| ———— | |||
| 400 | " |
Twenty grams of the sifted portion, when assayed, gave 0.1050 gram of silver. The whole of the "metallics" scorified and cupelled gave 0.842 gram of silver. Since the 20 grams assayed was 1-20th of the whole, 1-20th part of the 0.842 gram of silver (from the metallics) must be added to its produce. We thus get 0.1471 gram (0.1050 + 0.0421).
Referring to the 20 gram column, we get—
0.1 = 163.33
0.04 = 65.33
0.007 = 11.43
0.0001 = 0.16
—————————
0.1471 = 240.25 ounces per ton.
A more legitimate method of calculation is as follows:—Calculate separately the produce of each fraction as if they were from different ores. Multiply each produce (best stated in per cents.) by the weight of the corresponding fraction. Add together the products, and divide by the weight of the whole sample. Taking the same example for illustration, we have:—
Metallics.—Weight 1 gram.
1 gram of it yielded 0.842 grams of silver.
∴ Produce = 84.2 per cent.
Produce multiplied by the weight is still 84.2.
Sifted Portion.—Weight 399 grams.
20 grams of it yielded 0.105 gram of silver.
∴ Produce = 0.525 per cent.
Produce multiplied by weight (0.525 × 399) is 209.475.
Add together; and divide by 400, the weight of the whole sample—