Of working mines by fire.—The celebrated mine worked since the tenth century in the mountain called Rammelsberg, in the Hartz, to the south of Goslar, presents a stratified mass of ores, among the beds of the rock which constitute that mountain. The mineral deposit is situated in the earth, like an enormous inverted wedge, so that its thickness (power), inconsiderable near the surface of the ground, increases as it descends. At about 100 yards from its outcrop, reckoning in the direction of the slope of the deposit, it is divided into two portions or branches, which are separated from each other, throughout the whole known depth, by a mass of very hard clay slate, which passes into flinty slate. The substances composing the workable mass are copper and iron pyrites with sulphuret of lead (galena), accompanied by quartz, carbonate of lime, compact sulphate of baryta, and sometimes gray copper ore, sulphuret of zinc, and arsenical pyrites. The ores of lead and copper contain silver and gold, but in small proportion, particularly as to the last.
A mine so ancient as that of Rammelsberg, and which was formerly divided among several adventurous companies, cannot fail to present a great many shafts and excavations; but out of the 15 pits, only two are employed for the present workings; namely, those marked A B and E F, in [fig. 736.], by which the whole extraction and drainage are executed.—The general system of exploitation by fire, as practised in this mine, consists of the following operations:—
1. An advance is made towards the deposits of ore, successively at different levels, by transverse galleries which proceed from the shaft of extraction, and terminate at the wall of the stratiform mass.
2. There is formed in the level to be worked, large vaults in the heart of the ore, by means of fire, as we shall presently describe.
3. The floor of these vaults is raised up by means of terraces formed from the rubbish, in proportion as the roof is scooped out.
4. The ores detached by the fire from their bed, are picked and gathered; sometimes the larger blocks are blasted with gunpowder.
5. Lastly, the ores thus obtained are wheeled towards the shaft of extraction, and turned out to the day.
Let us now see how the excavation by fire is practised; and in that view, let us consider the state of the workings in the mines of Rammelsberg in 1809. We may remark in [fig. 736.] the regularity of the vaults previously scooped out above the level B C, and the other vaults which are in full activity of operation. It is, therefore, towards the lower levels that the new workings must be directed. For this purpose, the transverse gallery being already completed, there is prepared on the first of these floors a vault of exploitation at b, which eventually is to become similar to those of the superior levels. At the same time, there is commenced at the starting point below it, reached by a small well dug in the line of the mineral deposit, a transverse gallery in the rock, by means of blasting with gunpowder. The rock is also attacked at the starting-point by a similar cut, which advances to meet the first perforation. In this way, whenever the vaults of the level C are exhausted of ore and terraced up with rubbish, those of the level beneath it will be in full activity.