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Page:Popular Science Monthly Volume 24.djvu/639

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center of which is a sieve through which the finer pieces are shoveled into a vault below. These pieces are then carried to the arrastras, flat stones of hard rock kept revolving in a large tub half-filled with water, where they are in twenty-four hours ground to a fine powder.

PSM V24 D639 Silver mill at pachuca.jpg
Fig. 2.—Silver Mill at Pachuca.

The pulverized ore, called lama, is next carried to the patio, a court-yard paved with large flat stones, where it is allowed to accumulate to a depth of about two feet. The muddy mass is then mixed with magistral, or blue vitriol, salt, and quicksilver, and the whole, now called torta, is thoroughly stirred together by the trampling of mules. This process is kept up for seven hours daily, for from two to four weeks, according to the quality of the ore. The torta is then carried to the lavaderos, or large cisterns, where it is washed and stirred by means of