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The arastra is a slow instrument, but in some important respects it is superior to any other method of working auriferous quartz. It grinds the quartz well, is unsurpassable as an amalgamator, is very cheap and simple, requires no chemical knowledge or peculiar mechanical skill in the work, requires but little power, and very little water all of them important considerations.
The arastra is the simplest instrument for grinding auriferous quartz. It is a circular bed of stone, from eight to twenty feet in diameter, on which the quartz is ground by a large stone dragged round and round by horse or mule-power. There are two kinds of arastras, the rude or improved. The rude arastra is made with a pavement of unhewn flat stones, which are usually laid down in clay.
The paste is thus dissolved in the water, and the gold, quicksilver and amalgam have an opportunity to fall to the bottom. At the end of half an hour, or sooner, the thin mud of the arastra is allowed to run off, leaving the precious material at the bottom. Another charge of broken quartz is now put in and the process is repeated, and so on.
In the improved arastra the paving fits so closely together, that the quicksilver and amalgam do not get down between them, but remain on the surface, and can readily be brushed up into a little pan, and therefore cleaning up is much less troublesome and is more frequently repeated than in the rude arastras; besides there is a greater need of frequent cleaning up in the improved arastras, because the amount of work done within a given time is usually greater.
To build an arastra, a circular depression ten or twelve feet wide and a foot or more deep is made in the ground. This depression is lined with stone, which forms a hard bottom or floor. Four bars extend outward from an upright post placed in the middle of the floor, and a large flat stone is fastened to the end of each bar by means of a rope.
The length of a "run," or the period from one cleaning up to another, varies much in different places. In the rude arastra a run is seldom less than a week, and sometimes three or four. The amalgam having settled down between the paving stones, the bed must be dug up and all the dirt between them carefully washed.
The pavement of the improved arastra is made of hewn stone, cut very accurately and laid down in cement. In the centre of the bed of the arastra is an upright post which turns on a pivot, and running through the post is a horizontal bar, projecting on each side to the outer edge of the pavement.
Some day the inexhaustible placer would be found; or, on a mountainside where the porphyry was stained, he would carelessly chip off a fragment of rock, turn it up to the sun, and behold it rich in ruby silver; or, some day, the vein instead of pinching out would widen; there would be pay ore almost from the grass-roots rich, yellow, free-milling gold, so that he could put up a little arastra, beat out enough in a week to buy a small stamp-mill, and then, in six months ten years more of this fruitless but nourishing certainty were his, ten years of the awful solitudes, shared sometimes by his hardy and equally confident wife, and, at the last, by his boy, who had become old enough to endure with his father the snow and ice of the mountain tops and the withering heat of the alkali wastes.
A horse is hitched to one of the bars, which is purposely left longer than the others. The ore is thrown into the arastra, and water is admitted, a little at a time. As the horse is driven around the stones are dragged over the circular depression, crushing the ore and setting free the gold. This way of separating the gold was too slow, and in a short time the stamp-mill was invented.
In many places, the scarcity of water alone is enough to enable the arastra to pay a larger profit than any other method. Again, if a miner finds a rich spot in a lode, he may be doubtful as to the amount of paying rock which he can obtain.
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