80/5 = 16; 16+20 = 36; 36/11 = 3.3 nearly.
The following are some results obtained which will illustrate the rule:—
| Red Lead used. | Flour used. | Lead got. | |||
| 40 | grams | 3 | grams | 25.0 | grams |
| 100 | " | 3 | " | 13.5 | " |
| 80 | " | 4 | " | 30.0 | " |
| 80 | " | 5 | " | 40.0 | " |
On the Reducing Effect of Metallic Sulphides, and the Counteracting Effect of Nitre.—The sulphides found in ores will reduce a button of lead from oxide of lead just as flour does; and, as charcoal, flour and tartar differ in their reducing power, so equal weights of the different mineral sulphides throw down different weights of lead.
One gram of iron pyrites yields about 11 grams of lead. One gram of copper pyrites, blende, fahlerz, or mispickel, yields 7 or 8 grams of lead, whilst 1 gram of antimonite will give 6, and 1 gram of galena only a little over 3 grams. It is evident that if an ore carries much of these sulphides, the quantity of lead reduced will be very much larger than that required for an assay. To counteract this effect nitre is added; 1 gram is added for each 4 grams of lead in excess of that required. For example: with a 20-gram charge of an ore containing 50 per cent. of pyrites, if no nitre were added, 110 grams of lead would be got; or, if there was not sufficient oxide of lead to yield this quantity of metal, the button would be sulphury. To reduce the weight of the button by 80 grammes, we should add 20 grams of nitre, if litharge were used; or if red lead were used, we should add 16 grams of nitre, since the oxidizing effect of 20 grams of red lead is equivalent to that of 1 of nitre, and since 80 grams of red lead are generally used in a charge. Two assays of an ore of this kind with these quantities of nitre gave 26.0 grams of lead with litharge, and 22.5 grams with red lead.
It is best to use in these assays 80 grams of red lead, 20 of soda, and 5 of borax, with 20 grams of the ore. If the lead got by the preliminary fusion in a small crucible with litharge (described under "ores containing much sulphides") is known, the following table will indicate the quantity of nitre, or flour, to be added with this charge:—
| Lead got in Preliminary Fusion with 3 grams of Ore. | Flour to be added to the Assay. | Nitre to be added to the Assay. |
| 0.0 gram | 3.3 grams | none |
| 3.0 grams | 1.3 gram | — |
| 6.0 " | none | 4.0 grams |
| 9.0 " | — | 9.0 " |
| 12.0 " | — | 14.0 " |
| 15.0 " | — | 19.0 " |
| 18.0 " | — | 24.0 " |
| 21.0 " | — | 29.0 " |
If litharge is used in the assay instead of red lead 4 grams more nitre, or 1.5 gram less flour must be used. When more than a few grams of nitre are added to a charge the proportion of "soda" and borax should be increased, because one of the products of the reaction of nitre upon sulphides in the presence of soda is sulphate of soda, and because the "soda" thus used up no longer serves as a flux; more borax should be added, as it is the best flux for the metallic oxides which are formed in the process. If in an assay too large a button of lead is got, even after this calculation has been made, and the assay is repeated, add 1 gram more nitre for each 4 grams of lead in excess. Sometimes the assay appears tranquil before the nitre has produced its full effect; in such cases it is well to seize the crucible with the tongs and mix its fused contents by rotating them; if this causes an effervescence, the crucible should be replaced in the fire and the fusion continued. The following experiments will illustrate the extent to which the above rules may be relied on. In all of them the standard flux was used, viz.:—80 grams of red lead, 20 of soda, and 5 of borax.
| Pyrites | 5 | 5 | 5 | 5 | 2.5 | 5 | 10 | 15 | 20 |
| Quartz | — | 20 | — | 20 | 17.5 | 15 | 10 | 5 | |
| Nitre | — | — | 5 | 5 | — | 4 | 16 | 28.5 | 41 |
| Lead got | 42.5 | 36.0 | 16.0 | 19.0 | 11.5 | 22.5 | 22.5 | 26.5 | 27.5 |
| Copper Pyrites | 8 | 8 | 8 | 8 |
| Quartz | — | 12 | — | 12 |
| Nitre | — | — | 4 | 4 |
| Lead got | 47.5 | 34.0 | 33.0 | 26.0 |