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PREPARATION OF ORES.

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are machines in which the work is ground under hard stones, which have a rotatory motion, and weigh upwards of seven or eight cwt. They are not yet acquainted with washing with the tub (setz wäsche), nor washing on sleeping tables (tables dormantes) (liegende-heerde), or percussion (stossheerde). The preparation under the stumps (mazos) or in the tahonas, to which I shall give the name of edge mills, on account of their resemblance to some oil and snuff mills, differs according as the ore is destined to be smelted or amalgamated. The mills properly belong only to this last process; however, very rich metallic grains called polvillos, which have passed through the trituration of the tahona, are also smelted.

The quantity of silver extracted from the ores by means of mercury, is in the proportion of 31⁄2 to 1 of that produced by smelting. This proportion is taken from the general table formed by the provincial treasuries, from the different districts of mines of New Spain. There are, however, some of those districts, for example those of Sombrerete and Zimapan, in which the produce from smelting exceeds that of analgamation.

Silver (plata quintada) extracted from the Mines of New Spain, from the 1st January 1785 to the 31st December 1789.

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In times of peace, amalgamation gains a gradual ascendancy over smelting, which is generally badly managed. As wood is becoming yearly more scarce on the ridge of the Cordilleras, which is the most populous part, the diminution of the produce of smelting is very advantageous to the manufactories which require a great consumption of combustibles. In time of war the want of mercury arrests the progress of amalgamation, and compels the miner to endeavour to improve the process of smelting. M. Velasquez, the director general of the mines, supposed even in 1797, before the dis

AMALGAMATION.

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covery of the rich mines of Catorce, where there is scarcely any smelting, that of all the ores of New Spain two-fifths were smelted, and the other threefifths amalgamated.

The limits prescribed by us in the execution of this work, do not permit us to enter into any detail of the processes of amalgamation used in Mexico. It is sufficient to give a general idea of them, to examine the chemical phenomena which are exhibited in the greatest part of these processes, and to show the difficulties which in the New Continent oppose the introduction of the method invented in Germany in 1786, by Born, Ruprecht, and Gellert.

The ancients knew the property which mercury possesses of combining with gold; and they made use of amalgamation in gilding copper, and collecting the gold contained in their worn out dresses, by reducing them to ashes in clay vessels. It appears certain, that before the discovery of America the German miners used mercury, not only in washing auriferous earths, but also in extracting the gold disseminated in veins, both in its native state, and mixed with iron pyrites, and with the gray copper ore. But the amalgamation of silver ores, and the ingenious process now used in the New World, to which we owe the greater part of the valuable metals existing in Europe, or which have flowed from Europe to Asia, go no further back than the year 1557. It was invented in

Mexico by a miner of Pachuca of the name of Bartholome de Medina.

Cold amalgamation was found so profitable in Mexico, that in 1562, five years after the first discovery of the process of Medina, there were already 35 works at Zacatecas in which minerals were treated with mercury, notwithstanding Zacatecas is three times further from Pachuca than the old mines of Tasco, Zultipeque, and Tlapujahua.

The Mexican miners do not appear to follow any very fixed principle, in the selection of the ores submitted to smelting or amalgamation; for we see them smelt in one district of mines, the same mineral substances which in another they believe can only be managed with mercury. The ores which contain muriate of silver, for example, are sometimes smelted with carbonate of soda (tequesquite), and sometimes destined to the processes of hot and cold amalgamation; and it is frequently only the abundance of mercury, and the facility in procuring it, which determine the miner in the choice of his method. In general they find it necessary to smelt the very rich meagre ores, those which contain from 6lbs. to 7lbs, troy of silver per cwt., argentiferous sulphuret of lead, and the ores mixed of blende and vitreous copper. On the other hand, they find it profitable to amalgamate the pacos or colorados, destitute of metallic lustre; native, vitreous, red, black, and horn-silver; fah lore rich in silver; and all the meagre ores

BREAKING OF ORES.

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which are disseminated in very small particles in the gangue.

The ores destined for amalgamation must be triturated, or reduced to a very fine powder, to present the greatest possible contact with the mercury. This trituration under the arastras or mills, of which we have already spoken, is of all the metallurgical operations that which is executed in the greatest perfection in most of the Mexican works. In no part of Europe have I ever seen pulverized ores or powdered schlich so fine, and of so equal a grain, as in the great haciendas de plata of Guanaxuato, belonging to Count de la Valenciana, Colonel Rul, and Count Perez Galvez. When the ores are very pyritous, they are burnt (quemar) in the open air in heaps, on beds of wood, as at Sombrerete, or in schlich in reverberating furnaces (comalillos). The latter I found at Tehuilotepec: they are 38 feet in length, without chimneys, but managed by two fires, the flames of which traverse the laboratory. This chemical preparation of the ores is however very rare in general; the size of the fragments of substances to be amalgamated, and the want of combustibles on the table-land of New Spain, render the process equally difficult and expensive.

The dry braying is done by mazos, eight of which work together, kept in motion by hydraulic wheels or by mules. The brayed ore (granza) passes through a hide pierced with holes; and it is re

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