9. Compact marl-limestone, or _zechstein_, which changes from a brownish
colour above to a blackish schist below, as it comes nearer the cupreous
schist, which seems to form a part of it.
10. Cupreous schist (_kuperschiefer_), of which the bottom portion, from
4 to 6 inches thick, is that selected for metallurgic operations.
Beneath it, is found the usual wall or bed of sandstone. A vein of
cobalt ore _a_, which is rich only in the grayish-white sandstone
(_weisse liegende_), traverses and deranges all the beds wherever it
comes.
_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.
[Illustration: 736]
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.
Others will then be prepared at a lower level; and the exploitation may
afterwards be driven below this level by pursuing the same plan, by
which the actual depth of excavation has been gained.
In workings by fire we must distinguish, 1. The case where it is
necessary to open a vault immediately from the floor; 2. The case where
the vault having already a certain elevation, it is necessary to
heighten its roof. In the former case, the wall or floor of the mineral
deposit is first penetrated by blasting with gunpowder. As soon as this
penetration is effected over a certain length, parallel to the direction
of the future vault, as happens at _b_, there is arranged on the bottom
a horizontal layer of billets of firwood, over which other billets are
piled in nearly a vertical position, which rest upon the ore, so that
the flame in its expansion comes to play against the mineral mass to be
detached. When after some similar operations, the flame of the pile can
no longer reach the ore of the roof on account of its height, a small
terrace of rubbish must be raised on the floor of the deposit; and over
this terrace, a new pile of faggots is to be heaped up as above
described. The ancient miners committed the fault of constantly placing
such terraces close to the roof, and consequently arranging the faggots
against this portion of the ore, so that the flame circulated from the
roof down to the floor. The result of such procedure was the weakening
of the roof, and the loss of much of the ore which could not be
extracted from so unstable a fabric; and besides, much more wood was
burned than at the present day, because the action of the flame was
dissipated in part against the whole mass of the roof, instead of being
concentred on the portion of the ore which it was desired to dislodge.
Now, the flame is usually made to circulate from the floor to the roof,
in commencing a new vault.
When the vault has already a certain height, care is always taken that
between the roof of the vault and the rubbish on which the pile is
arranged, no more than two yards of space should intervene, in order
that the flame may embrace equally the whole concavity of the vault, and
produce an uniform effect on all its parts. Here, the pile is formed of
horizontal beds, disposed crosswise above one another, and presents four
free vertical faces, whence it has been called a _chest_ by the miners.
It is usually on Saturday that the fire is applied to all the piles of
faggots distributed through the course of the week. Those in the upper
floors of exploitation are first burned, in order that the inferior
piles may not obstruct by their vitiated air, the combustion of the
former. Thus, at 4 o’clock in the morning, the fires are kindled in the
upper ranges; from pile to pile, the fireman and his assistant descend
towards the lower floors, which occupies them till 3 o’clock in the
afternoon. Vainly should we endeavour to describe the majestic and
terrific spectacle which the fire presents, as it unfolds its wings
under its metallic vaults, soon filled with vast volumes of smoke and
flame. Let us mark the useful effect which it produces.
When the flame has beat for a few instants on the beds of ore, a strong
odour of sulphur, and sometimes of arsenic is perceived; and soon
thereafter loud detonations are heard in the vaults. Suddenly the flame
is seen to assume a blue colour, or even a white; and at this period,
after a slight explosion, flakes of the ore, of greater or less
magnitude, usually fall down on the fire, but the chief portion of the
heated mineral still remains fixed to the vault. The ores pass now into
a shattered and divided condition, which allows them afterwards to be
detached by long forks of iron. In this manner the fire, volatilizing
entirely some principles, such as sulphur, zinc, arsenic, and water,
changing the aggregation of the constituent parts of the ore, and
causing fissures by their unequal expansibilities, facilitates the
excavation of such materials as resist by their tenacity the action of
gunpowder.
The combustion goes on without any person entering the mine from
Saturday evening till Monday morning, on which day, the fireman and his
assistants proceed to extinguish the remains of the bonfires. On Monday
also some piles are constructed in the parts where the effect of the
former ones has been incomplete; and they are kindled after the workmen
have quitted the mine. On Tuesday all hands are employed in detaching
the ores, in sorting them, taking them out, and preparing new piles
against the next Saturday.
The labour of a week consists for every man of five posts during the
day, each of 8 hours, and of one post of four hours for Saturday.
Moreover, an extra allowance is made to such workmen as employ
themselves some posts during the night.
The labour of one compartment or _atelier_ of the mine consists
therefore in arranging the faggots, in detaching the ore which has
already experienced the action of the fire, in breaking the blocks
obtained, in separating the ore from the _débris_ of the pile, and
whenever it may be practicable or useful, in boring holes for blasting
with gunpowder. The heat is so great in this kind of mine, that the men
are obliged to work in it without clothing.
We have already remarked, that besides the working by fire, which is
chiefly used here, recourse is sometimes had to blasting by gunpowder.
This is done in order either to recover the bottom part or ground of the
vaults on which the fire can act but imperfectly, to clear away some
projections which would interfere with the effect of the pile, or lastly
to strip the surrounding rock from the mass of the ore, and thence to
obtain schist proper for the construction of the rubbish-terraces.
The blasting process is employed when the foremen of the workshop or
mine-chamber judge that a hole well placed may separate enough of ore to
pay the time, the repair of tools, and the gunpowder expended. But this
indemnification is rarely obtained. The following statement will give an
idea of the tenacity which the mineral deposit often presents.
In 1808, in a portion of the Rammelsberg mine, the ore, consisting of
extremely compact iron and copper pyrites, was attacked by a single man,
who bored a mining hole. After 11 posts of obstinate labour, occupying
altogether 88 hours, the workman, being vigilantly superintended, had
been able to advance the hole to a depth of no more than 4 inches; in
doing which he had rendered entirely unserviceable 126 punches or
borers, besides 26 others which had been re-tipped with steel, and 201
which had been sharpened; 6-1/4 pounds of oil had been consumed in
giving him light; and half a pound of gunpowder was required for
blasting the bore. It was found from a calculation made upon these facts
by the administration of mines, that every inch deep of this hole cost,
at their low price of labour, nearly a florin, value two shillings and
sixpence.
It is therefore evident that though the timber, of which the consumption
is prodigiously great, were much less abundant and dearer than it still
is at Rammelsberg, mining by fire would be preferable to every other
mode of exploitation. It is even certain, that on any supposition, the
employment of gunpowder would not be practicable for every part of the
mine; and if fuel came to fail, it would be requisite to renounce the
workings at Rammelsberg, although this mountain still contains a large
quantity of metals.
If in all mines the free circulation of air be an object of the highest
importance, we must perceive how indispensable it must be in every part
of a mine where the mode of exploitation maintains the temperature of
the air at 112° Fahr., when the workmen return into it after the
combustion of the piles, and in which besides it is necessary that this
combustion be effected with activity in their absence. But in
consequence of the extent and mutual ramifications of the workings, the
number of the shafts, galleries, and their differences of level, the
ventilation of the mine is in a manner spontaneously maintained. The
high temperature is peculiarly favourable to it. The aid of art consists
merely in placing some doors judiciously, which may be opened or shut at
pleasure, to carry on the circulation of the air.
In considering the Rammelsberg from its summit, which rises about 400
yards above the town of Goslar, we observe, first, beds of slaty
sandstone, which become the more horizontal the nearer they approach to
the surface. At about 160 yards below the top level there occurs, in the
bosom of the slaty graywacke, a powerful stratum of shells impasted in a
ferruginous sandstone. See D, _fig._ 730. In descending towards the face
of the ore, the parallel stratification of the clay-slate which forms
its walls and roof grows more and more manifest. Here the slate is
black, compact, and thinly foliated. The inclination of the different
beds of rock is indicated at B. The substance of the workable mass is
copper and iron pyrites, along with sulphuret of lead, accompanied by
quartz, carbonate of lime, compact sulphate of baryta, and occasionally
gray copper (_fahlerz_), sulphuret of zinc, and arsenical pyrites.
The ores are argentiferous and auriferous, but very slightly so,
especially as to the gold. It is the ores of lead and copper which
contain the silver, and in the latter the gold is found, but without its
being well ascertained in what mineral it is deposited. Sometimes the
copper occurs in the native state, or as copper of cementation.
Beautiful crystals of sulphate of lime are found in the old workings.
[Illustration: 737]
In _figs._ 736. 737., A B is the shaft of extraction, called
the _Kahnenkuhler_; N is the ventilation shaft, called
_Breitlingerwetterschacht_; P is the extraction shaft, called
_Innier-schacht_.
E F, is a new extraction-shaft, called _Neuer treibschacht_, by which
also the water is pumped up; by A B, and E F, the whole extraction and
draining are carried on. The ores are raised in these shafts to the
level of the waggon-gallery (_galerie de roulage_) _i_, by the whims
_l_, _q_, provided with ropes and buckets. 1, 2, 3, 4, _fig._ 736.,
represent the positions of four water-wheels for working the whims; the
first two being employed in extracting the ores, the last two in
draining. The driving stream is led to the wheel 1, along the drift _l_;
whence it falls in succession upon the wheels 2, 3, 4. The general
system of working consists of the following operation;--
1. The bed of ore is got at by the transverse galleries, _m_, _n_, _o_,
_q_, _r_, _s_, which branch off from the extraction shaft, and terminate
at the wall of the main bed;
2. Great vaults are scooped out at the level of the workings, by means
of fire;
3. The roofs of these vaults are progressively propped with mounds of
rubbish;
4. The ores thus detached, or by blasting with gunpowder, are then
collected;