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they are without chimneys, but managed by two fires of which the flames traverse the laboratory. The chemical preparation of the minerals is however very rare in general; the greatness of the volume 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 hydraulical wheels or by mules. The brayed mineral (granza) passes through a hide pierced with holes; and it is reduced to a very fine flour under the arastras or tahonas, which are called sencillas or de marco, according as they are furnished with two or four blocks of porphyry or basalt (piedras voladoras), which revolve in a circle from 9 to 12 metres in circumference*. From 12 to 15 of these arastras or mills, are generally ranged in a row under one shed; and they are moved by water, or mules which are relieved every eight hours. One of these machines brays in the space of 24 hours, from three to four hundred kilogrammest of minerals. The humid schlich (lama) which leaves the arastras, is sometimes washed again in ditches (estanques de deslamar), the construction of which in the districtof mines of Zacatecas,


* From 29 to 38 feet. Trans.'

- † From 662 to 882 lb. avoird. Trans. VOL. III.

iv. has been recently carried to perfection by M. Garces. When the minerals are very rich, as in the mine of Rayas at Guanaxuato, they are only reduced under the stones of the mills to the size of gravel (xalsonte), and they separate, bywashing, the richest metallick grains (polvillos), which are destined for smelting. This very economical operation is called apartar polvillos.

I have been assured, that in destining for amalgamation silver minerals which are very poor in gold, they pour mercury into the vessel or trough, on the bottom of which the stones of the arastras turn; and the auriferous amalgamation goes on then in proportion as the mineral is reduced to powder, the giratory motion of the piedras voladeras being favourable to the combination of the metals. I had no opportunity of seeing this operation, which is not practised at Guanaxuato. In some great amalgamation works of New Spain, the arastras are still unknown; they are contented with the braying of the mazos; and the schlich which comes from under them is passed through sieves (cedazos and tolvas). This preparation of the flour is very imperfect; for a powder of an unequal and coarse grain amalgamates very ill; and the health of the workmen suffer greatly, in a place where a cloud of metallick dust is perpetually flying about.

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The moistened schlich is carried from the

. XI.] KINGDOM OF NEW SPAIN. 259 mills or arastras, into the court of amalgamation, (patio or galera) which is generally paved with Aags. The four is ranged in piles (montones) which contain from 15 to 35 quintals. Forty or fifty of these montones form a torta, by which name they call a heap of humid schlich, which they leave exposed to the open air, and which is frequently from 20 to 30 metres in breadth,* by five or six decimetrest in thickness. They use for amalgamation in a paved court, (en patio) which is the most generally used process in America, the following materials; muriate, of soda, (sal blanca) sulphate of iron and copper, (magistral) lime and vegetable ashes.

The salt used in New Spain is of very unequal purity, according as it comes from the salt marshes which surround the port of Colima on the shores of the South Sea, or the famous laguna del peñon blanco, between San Luis Potosi and Zacatecas. This lake was visited by M. Sonneschmidt. It is situated at the foot of a granite rock, on the slope of the Cordilleras; and it dries up every year in the month of December. It furnishes annually to the revenue nearly 250 thousand fanegas of impure or earthy salt, (sal tierra) which is all sold to the amalgation works. On the spot even the price of a fanega is half a piastre. The districts of mines of the intendancy of Mexico, receive salt from the coast of Vera Cruz, and the springs of Chautla ; and at Tasco the muriate of soda of Vera Cruz, sells for four piastres the quintal.

* From 65 to 98 feet. † 199 or 231 inches.

Trans. Trans.

The magistral is a mixture of pyritous copper, (kupferkies) and sulphuretted salt, roasted for some hours in a reverberating oven, and slowly cooled. If it is roasted longer, it produces an acid sulphate of iron and copper, mixed with iron oxidated to the highest degree. Sometimes*, though seldom, the azogueros (the name given to the persons charged with the amalgamation) add to the pyrites, during their roasting muriate of soda; so that there is forined sulphate of soda, and muriate of copper and iron. I have also seen vitriolic earths, or copperas, (tierras de tinta o de alcaparosa), which are ochreousearths containing, iron oxidated to the maximum, and sulphate of iron, mixed with the magistral. In the district of mines of Real de Moran, they employ in the preparation of the magistral, pyrites of copper of San Juan Sitacora, the carga of which is paid for at the rate of ten piastres. The lime is obtained by calcinating very pure limestone, and extinguishing it with water;

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and very rarely alkaline ashes are substituted to calcinated lime.

By the contact of these different substances, namely; moistened metallick flour, mercury, muriate of soda, sulphates of iron and copper, and lime, that the amalgamation of silver, in. the process of cold amalgamation, (de patio ý por cruto) takes place. They begin at first by mixing' salt with the metallick flour, and they stir (repassa) the paste (torta). According to the purity of the salt used, they give each quintal of schlich, a quantity which varies from two and a half to twenty four pounds. If the muriate of soda is of moderate purity, they take from three to four per cent. They call metales salineros, those which are believed to require a great deal of salt, and in which the silver mineral is found in grains of considerable volume. They leave the mineral mixed with salt (metal ensalmorado) to repose for several days, in order that the latter may dissolve and be equally distributed. If the azoguero judges the metals to be warm, (calientes) that is to say in a state of oxidation, and naturally charged either with sulphates of iron and copper which rapidly decompose in the air, with muriate of silver, he adds líme to cool the mass; and this operation is called curtir los metales con cal. But they use magistral, if the schlich appears too cold (frios), for example, if they

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