Though known from remote times, the date of the first opening of the famous mines of quicksilver of Almadén has not been precisely determined. Almost all the writers on the subject agree that cinnabar, from Spain, was already known in the times of Theophrastus, three hundred years before the Christian era, although there is evidence in the writings of Vitruvius that they were worked at a still earlier date, Spanish ore being sent to Rome for the manufacture of vermilion. Such ore constituted a part of the tribute which Spain paid to Rome emperors, and there are records of its receipt until the first century after Christ. The history of Almaden during the reign of the Moors is so much involved in doubt that some writers deny altogether that the Arabs worked the deposit; still the very name it now bears, which means "the mine," and many of the technical terms still in use, give evidence that they knew and worked that famous deposit. As for their Christian conquerors, there are stray indications that they extracted mercury during the twelfth and thirteen centuries. In 1417, Almaden was given the privileges of a city, and from 1525 to 1645 the working of mines was contracted for by the wealthy family of Fugger, of Augsburg, Germany. Since then, the mine has been worked by the state, though the Rothschilds have controlled the sale of the product.

According to Vitruvius, the works for manufacturing vermilion from Spanish ore in Rome were situated between the temple of Flora and Quirino. The ore was dried and treated in furnaces, to remove the native mercury it contained, and was then ground in iron mortars and washed. In addition, small quantities of quicksilver and vermilion were made at Almaden. The ancients describe other methods, among which Theophrastus speaks of using vinegar, which, however, appears from modern investigations to have been an erroneous account. Nothing definite is known concerning the methods of the Moors; we possess only as a proof that they produced mercury, an account of a quicksilver fountain in the marvelous palace of Abderrahman III., at Medina-Zahara, and the works of Rasis, an Arab. The Moors probably extracted mercury at Almaden, from the eighth to the twelfth century, by the use of furnaces called "xabecas," which latter, in the fourteenth century, were still employed by the Christians, who continued them till the seventeenth century, when German workmen replaced them by "reverberatory" furnaces, which in turn were superseded in 1646 by aludel or Bustamente furnaces. There is an anonymous description of the working with xabecas as practiced at Almaden in 1543, and later accounts in 1557 and 1565. The ore was put into egg-shaped vessels with a lid, the mineral being covered over with ashes. The vessels were packed in a furnace heated with wood, about 60 pounds being used per pound of quicksilver made. This system was also applied at the Guancavelica mines, discovered in Peru in 1566, where the xabecas were abandoned in 1633, being replaced by the furnaces invented by Lope Saavedra Barba, which there were called "busconiles," while in Spain they were named Bustamente furnaces, and elsewhere aludel furnaces. They were introduced at Almaden thirteen years after their first use in Peru by Juan Alfonso de Bustamente, Barba and his son having been lost at sea on their way to the Peninsula. In 1876, there were at Almaden, at the works at Buitrones, twenty such aludel furnaces and two Idria furnaces. D. Luis de la Escosura y Morrogh, from whose work we take the above notes, has followed the historical details of the growth of Almaden closely, and from his account of the method of working in 1878 we take some data:

It is not an easy matter to explain the classification of the ore at Almaden. Metal is there called the richest mineral, composed of quartz impregnated with crystalline cinnabar. Requiebro are middlings of medium richness, China are smalls, and Vaciscos the finest ore. Besides native mercury, which the ores of Almaden contain in greater or smaller quantity, the most abundant mineral is cinnabar, which is always crystalline and is often crystallized. The ores have, besides, a small quantity of selenium and iron pyrites intimately mixed with the cinnabar. The gangue is quartz, occasionally argillaceous and bituminous. The following are assays of some of the ores made by Escosura:

Metal.Requiebro.Vaciscos.China.
12345678
Cinnabar29.121.213.310.25.12.81.20.86
Iron pyrites.2.22.02.01.912.31.52.12.80
Bituminous matter0.61.01.01.24.60.73.40.90
Gangue67.574.882.176.577.593.390.293.50
- - - - - - - - - - - - - - - -
Total99.494.098.898.999.598.398.798.06
Quicksilver25.0518.2811.478.644.402.411.030.75

It appears to be a difficult matter to determine the average percentage of the various grades of ore. In 1872, a commission classified and sampled a lot of 300 tons with the following results:

Grade.No.Quantity,
kilos.
Per cent.
mercury.
Average
of grade.
Metal1.
2.
81,890
14,970
23.86
22.65
24.80
Requiebro3.
4.
12,240
17,000
15.20
10.50
12.47
China5.
6.
7.
31,890
32,360
28,960
3.84
1.17
0.10
1.75
Vaciscos8.78,3209.249.24

This general average of 12.28 per cent. of mercury is pronounced higher than the usual run of the ore, which, it is stated, does not go above 7 to 8.50 per cent.

The furnace in which the ore is treated is cylindrical, 2 meters in diameter, and 3.70 meters high from a brick grate, supported by three arches to the arched roof. At the level of the grate is a charging orifice, and near the roof are openings into two chambers, from the bottom of which extend 12 lines of aludels, clay vessels, open at both ends, the middle being expanded. The mouth of one fits into the back end of the one following, a channel being thus formed through which the fumes to be condensed are passed. The lines of aludels which are laid on the ground terminate in a chamber, and for half the distance between the furnaces and these chambers the ground slopes downward, while for the other it slopes upward. Two furnaces are always placed side by side, and the pair have from 1,100 to 1,150 aludels.

The operation is as follows: A layer of poor quartz is spread over the brick grate; this is followed by a layer of smalls, and then by a layer of still finer stuff, all of it being low grade ore. On top of this are piled two-thirds of the china of the charge on which the metal is put. Then follows a layer of requiebro, another lot of china, and finally the vaciscos, shaped into balls, the whole charge amounting to about 11½ tons, which is put in from an hour and a half to two hours by three men. The charging orifice is then closed, the aludels are luted, and everything made tight. The fires under the brick grate are lighted and kept going for twelve hours, during which time furnaces, charge, and condensing apparatus are heated up. During this period, the temperature in the condensing-chamber at the end of the line of aludels runs up 40 or 50 degrees Celsius, and some mercury, evidently part of the native quicksilver, is noticed in it.

The temperature of the aludels in the immediate vicinity of the furnaces is about 140 degrees C. During this period, the consumption of fuel is four parts to every part of quicksilver produced. At its close, the fire is drawn, and the second period begins. The air entering through the brick arch is heated to from 200 to 300 degrees by contact with the layer of poor stuff, the cinnabar is ignited, and its sulphur oxidized, and the quicksilver vaporized and, condensing in the aludels, flows toward the depression in the central portion of the line. The temperature goes on increasing, until, twelve hours after the beginning of this period, the thermometer shows 212 degrees C. at the first aludels. This lasts for 18 hours, and then the third or "cooling period" begins, which takes from 24 to 26 hours, and during the beginning of which the temperature in the furnaces still rises. It is then opened and cooled down. A very elaborate series of observations made on the temperatures of various parts of the condensing apparatus of the Almaden furnaces has shown that at the aludels nearest to them the heat increases steadily until it reaches 249 degrees C., 44 hours after the beginning of the operation; that in the middle of the line, at the depression, the maximum is 50 degrees 50 hours after starting the fires; and that at the end it does not surpass 39 degrees. In the final condensing chamber, the temperature varied, running downward from 40 degrees during the heating period to 14 degrees, rising again to 29 degrees toward the close.

The loss of the quicksilver during the operation has been vary variously estimated, some stating that it is 50 per cent. and more, while others place it at 30 per cent. Escosura, in his work, gives the details of an operation checked by a royal commission in 1872, according to which the loss in working ore running 9.55 per cent. was only 4.41 per cent. - a loss which he considered inevitable. In 1806, two Idria furnaces were put up at Almaden, but the engineers are not favorably impressed with them. The first cost is stated to be more than ten times greater than that of an aludel furnace, while the capacity is only 50 per cent. greater. One pair of Idria furnaces in five years produced 120,000 kilogrammes of quicksilver, against 843,000 kilogrammes made by eight sets of the Bustamente furnaces, the cost per kilogramme of quicksilver being respectively 0.121 and 0.056 peseta.