Preparation to Sample

Samples should be accurately referred to some permanent object, such as a cross-cut, winze, or survey station. The intervals between samples should be measured along the center of the drift, as they otherwise will differ widely according as they are measured on one side of the drift or the other, and will therefore fail to plot correctly on the map. It is poor judgment to mark the points at which the samples are to be taken in advance of the actual sampling; this amounts to an advertisement that a sample is to be taken along a certain line, and permits the evilly disposed to assist nature in the distribution of values.

A face that is to be sampled should be thoroughly cleaned. If the groimd is soft, a strip a few inches wider than the sample cut should be cleaned off with a pick; if the ore is hard, a brush or broom should be used either dry or with water. In driving any working, the fine material, often the richest, is powdered and thrown against the roof and walls, where a portion of it adheres; it is, therefore, of the greatest importance that the face to be sampled should be thoroughly cleaned. Irregular projections and loose pieces should be knocked off, to give, in so far as possible, a flat surface from which to cut the sample.

The face to be sampled should be examined carefully for soluble salts. In copper mines in dry climates an eflflorescence of chalcanthite and other salts is usual, and the sampling of old workings is attended with considerable risk of salting from this cause.

Samples containing these efflorescences, even after boiling in water, show blue crystals under the microscope. These efflorescences are due to the evaporation of migrating solutions on the walls of the workings, and represent an enrichment not found throughout the mass of rock. The writer had occasion to resample a mine where there was much efflorescence. His samples were boiled in water with a little caustic soda and averaged about 3/10 per cent, copper; the sampling thus discredited averaged in excess of 2 per cent.

Where it is necessary to take samples from the floors of drifts it is best to cut large samples and to wash from them and disregard all fine material; fine particles of heavy minerals work into cracks in the floor and give deceptively high results. This method may give results somewhat too low, but is not so liable to serious error as would result from the inclusion of the fine material.

Samples should always be taken as nearly as possible at right angles to the lines of distribution of the minerals through the ore.