7. Clay veins; of which there are two sets, the more ancient, called
_Cross-Fluckans_; and the more modern, called _Slides_.
There are therefore three systems of copper veins in Cornwall; of which
the first is considered to be the most ancient, because it is always
traversed by the two others, and because, on the contrary, it never cuts
them off. The width of these veins does not exceed 6 feet, though
occasional enlargements to the extent of 12 feet sometimes take place.
Their length is unknown, but the one explored in the _United Mines_ has
been traced over an extent of seven miles. The gangue of these veins is
generally quartz, either pure, or mixed with green particles analogous
to chlorite. They contain iron pyrites, blende, sulphuret, and several
other compounds of copper, such as the carbonate, phosphate, arseniate,
muriate, &c. The most part of the copper veins are accompanied with
small argillaceous veins, called by the miners _fluckan of the lode_.
These are often found upon both sides of the vein, so as to form cheeks
or _salebandes_.
When two veins intersect each other, the direction of the one thrown out
becomes an object of interest to the miner and geologist. In Saxony it
is regarded as a general fact that the rejected portion is always to the
side of the _obtuse angle_; this also holds generally in Cornwall, and
the more obtuse the angle of incidence, the more considerable the
out-throw.
The great copper vein of _Carharack_, in the parish of Gwenap, is a most
instructive example of intersection. The power of this vein is 8 feet;
it runs nearly from east to west, and dips towards the north at an
inclination of 2 feet in a fathom. Its upper part is in the killas, its
lower part in the granite. The vein has suffered two intersections; the
first results from encountering the vein called _Steven’s fluckan_,
which runs from north-east to south-west, throwing it out several
fathoms. The second has been caused by another vein, almost at right
angles to the first, and which has driven it 20 fathoms out to the right
side. The fall of the vein occurs, therefore, in one case to the right,
and in the other to the left; but in both instances, it is to the side
of the obtuse angle. This disposition is very singular; for one portion
of the vein appears to have ascended, while another has sunk.
The mining works in the copper veins are carried on by reverse steps;
see MINES. The grand shafts for drainage and extraction are vertical,
and open upon the roof side of the vein, traversing it to a certain
depth. These pits are sunk to the lowest point of the exploration; and,
in proportion as the workings descend, by means of excavations in the
vein, the pits are deepened and put into communication towards their
bottom with each new gallery of elongation, by means of transverse
galleries. At present, the main shafts are fully 160 fathoms deep. Their
horizontal section is oblong, and is divided into two compartments; the
one destined for extraction, the other for the pumps. Their timbering
has nothing remarkable, but is executed with every attention to economy,
the whole wood employed in these mines being brought from Norway.
The descent of the workmen is effected by inclined shafts scooped out of
the vein; the ladders are slightly inclined; they are interrupted every
10 fathoms by floors; the steps are made of iron, and, to prevent them
from turning under the foot, the form of a miner’s punch or jumper has
been given them, the one end being round, and the other being
wedge-shaped.
The ore is raised either by means of horse-gins, or by steam-engine
power most frequently of high pressure. I shall take the _Consolidated
Mines_ as an example.
The draining, which is one of the most considerable sources of expense,
both from the quantity of water, and from the depth of the mine, is
executed by means of sucking and forcing pumps, the whole piston-rods of
which, 120 feet long, are attached to a main-rod suspended at the
extremity of the working beam of a steam-engine.
On this mine three steam-engines are erected of very great power, for
the purpose of drainage; the one called the _Maria_ engine is of the
first-rate force, and most improved construction. The cylinder is 90
inches in internal diameter, and the length of the stroke is 9 feet 11
inches. It works single stroke, and is encased in a coating of bricks to
prevent dissipation of the heat. The vapour is admitted at the upper end
of the cylinder during the commencement of the fall of the piston, at a
pressure capable of forming an equilibrium with a column of 60 inches of
mercury. The introduction of the steam ceases whenever the piston has
descended through a certain space, which may be increased or diminished
at pleasure. During the remainder of the descent the piston is pressed
merely by this vapour in its progressive expansion, while the under side
of the piston communicates with the condenser. It ascends by the
counterweight at the pump end of the working beam. Hence, it is only
during the descent of the piston, that the effective stroke is exerted.
Frequently the steam is admitted only during the sixth part of the
course of the piston, or 18 inches. In this way the power of the engine
is proportioned to the work to be done; that is, to the body of water to
be raised. The _maximum_ force of the above engine is about 310 horses;
though it is often made to act with only one third of this power.
The copper mines of the isle of Anglesey, those of North Wales, of
Westmoreland, the adjacent parts of Lancashire and Cumberland, of the
south west of Scotland, of the Isle of Man, and of the south east of
Ireland, occur also in primitive or transition rocks. The ores lie
sometimes in masses, but more frequently in veins. The mine of Ecton in
Staffordshire, and that of Cross-gill-burn, near Alston-moor in
Cumberland, occur in transition or metalliferous limestone.
The copper ores extracted both from the granitic and schistose
localities, as well as from the calcareous, are uniformly copper pyrites
more or less mixed with iron pyrites; the red oxide, carbonate,
arseniate, phosphate, and muriate of copper, are very rare in these
districts.
The working of copper in the isle of Anglesey may be traced to a very
remote era. It appears that the Romans were acquainted with the Hamlet
mine near Holyhead; but it was worked with little activity till about 70
years ago. This metalliferous deposit lies in a greenish clay slate,
passing into talc slate; a rock associated with serpentine and euphotide
(_gabbro_ of Von Buch). The veins of copper are from one to two yards
thick; and they converge towards a point where their union forms a
considerable mass of ore. On this mass the mine was first pierced by an
open excavation, which is now upwards of 300 feet deep, and appears from
above like a vast funnel. Galleries are formed at different levels upon
the flank of the excavation to follow the several small veins, which run
in all directions, and diverge from a common centre like so many radii.
The ore receives in these galleries a kind of sorting, and is raised by
means of hand windlasses, to the summit of a hill, where it is cleaned
by breaking and riddling.
The water is so scanty in this mine that it is pumped up by a six-horse
steam-engine. A great proportion of it is charged with sulphate of
copper. It is conveyed into reservoirs containing pieces of old iron;
the sulphate is thus decomposed into copper of cementation. The Anglesea
ore is poor, yielding only from 2 to 3 _per cent._ of copper: a portion
of its sulphur is collected in roasting the ore.
_Mechanical preparation of the copper ores in Cornwall._--The ore
receives a first sorting, either within the mine itself, or at its
mouth, the object of which is to separate all the pieces larger than a
walnut. These are then reduced by the hammer to a smaller size; after
which the whole are sorted into four lots, according to their relative
richness. The fragments of poor ore are pounded in the stamps so that
the metallic portion may be separated by washing.
The rich ore is broken into small bits, of the size of a nut, with a
flat beater, formed of a piece of iron 6 inches square and 1 inch
thick, adapted to a wooden handle. The ore to be broken is placed upon
plates of cast-iron; each about 16 inches square and 1-1/2 inch thick.
These iron plates are set towards the edge of a small mound about a yard
high, constructed with dry stones rammed with earth. The upper surface
of this mound is a little inclined from behind forwards. The work is
performed by women, each furnished with a beater; the ore is placed in
front of them beyond the plates; they break it, and strew it at their
feet, whence it is lifted and disposed of to the smelting-houses.
Inferior ores, containing a notable proportion of stony matters, are
also broken with the beater, and the rich parts are separated by
riddling and washing from the useless matters.
The smaller ore is washed on a sieve by shaking it in a stream of water,
which carries away the lighter stony pieces, and leaves the denser
metalliferous. They are then sorted by hand. Thus by beating, stamping,
and riddling in water, the stony substances are in a great measure
separated. The finer ground matter is washed on a plane table, over
which a current of water is made to flow. Finally, the ore nearly fine
is put into a large tub with water, and briskly stirred about with a
shovel, after which it settles in the order of richness, the pure
metallic ore being nearest the bottom. The stamps used for copper ore in
Cornwall are the same as those used for tin ores, of which we shall
speak in treating of the latter metal, as well as of the boxes for
washing the fine powder or _slime_. These in fact do not differ
essentially from the stamping mills and washing apparatus described in
the article METALLURGY. Crushing rolls are of late years much employed.
See LEAD and TIN.
Cornwall being destitute of coal, the whole copper ore which this county
produces is sent for smelting to South Wales. Here are 15 copper works
upon the Swansea and Neath, which pursue a nearly uniform and much
improved process, consisting in a series of calcinations, fusions, and
roastings, executed upon the ores and the matters resulting from them.
The furnaces are of the reverberatory construction; they vary in their
dimensions and in the number of their openings, according to the
operations for which they were intended. There are 5 of them:--1. The
calcining furnace or calciner; 2. The melting furnace; 3. The roasting
furnace or roaster; 4. The refining furnace; 5. The heating or igniting
furnace.
[Illustration: 296 297 298]
1. The calcining furnace rests upon a vault, C, into which the ore is
raked down after being calcined; it is built of bricks, and bound with
iron bars, as shown in the elevation, _fig._ 296. The hearth, B B,
_figs._ 296. and 298. is placed upon a level with the lower horizontal
binding bar, and has nearly the form of an ellipse, truncated at the two
extremities of its great axis. It is horizontal, bedded with fire-bricks
set on edge, so that it may be removed and repaired without disturbing
the arch upon which it reposes. Holes, not visible in the figure, are
left in the shelves before each door, _c c_, through which the roasted
ore is let fall into the subjacent vault. The dimensions of the hearth B
B are immense, being from 17 to 19 feet in length, and from 14 to 16 in
breadth. The fire-place, A, _fig._ 298., is from 4-1/2 to 5 feet long,
and 3 feet wide. The bridge or low wall, _b_, _fig._ 302., which
separates the fire-place from the hearth, is 2 feet thick; and in Mr.
Vivian’s smelting-works is hollow, as shown in the figure, and
communicates at its two ends with the atmosphere, in order to conduct a
supply of fresh air to the hearth of the furnace. This judicious
contrivance will be described in explaining the _roasting_ operation.
The arched roof of the furnace slopes down from the bridge to the
beginning of the chimney, _f_, _fig._ 296, 298., its height above the
hearth being at the first point about 26 inches, and from 8 to 12 at the
second.
Such great calcining furnaces have 4 or 5 doors, _c c c c_, _fig._ 298.,
one for the fire-place, as shown at the right hand in _fig._ 297., and 3
or 4 others for working the ore upon the reverberatory hearth. If there
be 3, 2 of them are placed between the vertical binding bars upon one
side, and a third upon the opposite side of the furnace; if there be 4,
2 are placed upon each side, facing one another. These openings are 12
inches square, and are bound with iron frames. The chimney is about 22
feet high, and is placed at one angle of the hearth, as at _f_, _fig._
298., being joined by an inclined flue to the furnace.
For charging it with ore there is usually placed above the upper part of
the vault 2 hoppers, E E, in a line with the doors; they are formed of 4
plates of iron, supported in an iron frame. Beneath each of them there
is an orifice for letting the ore down into the hearth.
These furnaces serve for calcining the ore, and the _matts_ or _crude
coppers_: for the latter purpose, indeed, furnaces of two stories are
sometimes employed, as represented in _fig._ 301. The dimensions of each
floor in this case are a little less than the preceding. Two doors, _c
c_, correspond to each hearth, and the workmen, while employed at the
upper story, stand upon a raised movable platform.
[Illustration: 299, 300]
2. _Melting furnace_, _figs._ 299 and 300.--The form of the hearth is
also elliptical, but the dimensions are smaller than in the calcining
furnace. The length does not exceed 11 or 11-1/2 feet, and the breadth
varies from 7 to 8. The fire-place is however larger in proportion, its
length being from 3-1/2 feet to 4, and its breadth from 3 to 3-1/2; this
size being requisite to produce the higher temperature of this furnace.
It has fewer openings, there being commonly three; one to the fire-place
at D, a second one, O, in the side, kept generally shut, and used only
when incrustations need to be scraped off the hearth, or when the
furnace is to be entered for repairs; and the third or working-door, G,
placed on the front of the furnace beneath the chimney. Through it the
scoriæ are raked out, and the melted matters are stirred and puddled,
&c.
The hearth is bedded with infusible sand, and slopes slightly towards
the side door, to facilitate the discharge of the metal. Above this door
there is a hole in the wall of the chimney (_fig._ 300.) for letting the
metal escape. An iron gutter, O, leads it into a pit, K, bottomed with
an iron receiving-pot, which may be lifted out by a crane. The pit M is
filled with water, and the metal becomes granulated as it falls into the
receiver. The melting furnaces are surmounted by a hopper, L, as shown
in _fig._ 299.
[Illustration: 301]
Melting furnaces are sometimes used also for calcination. There are some
such near Swansea, which serve this double purpose; they are composed of
3 floors (_fig._ 301.) The floor A is destined for melting the calcined
ore; the other two, B C, serve for calcination. The heat being less
powerful, upon the upper sole C, the ore gets dried upon it, and begins
to be calcined--a process completed on the next floor. Square holes,
_d_, left in the hearths B and C, put them in communication with each
other, and with the lower one A; these perforations are shut during the
operation by a sheet of iron, removable at pleasure.
The hearths _b_ and _c_ are made of bricks; they are horizontal at top
and slightly vaulted beneath; they are 2 bricks thick, and their
dimensions are larger than those of the inferior hearths, as they extend
above the fire-place. On the floors destined for calcination the furnace
has two doors on one of its sides: on the lower story there are also
two; but they are differently collocated. The first, being in the front
of the furnace, serves for drawing off the scoriæ, for working the
metal, &c.; and the second, upon the side, admits workmen to make
necessary repairs. Below this door the discharge or tap-hole A is
placed, which communicates by a cast-iron gutter with a pit filled with
water. The dimensions of this furnace in length and breadth are nearly
the same as those of the melting furnace above described; the total
height is nearly 12 feet. It is charged by means of one or two hoppers.
3. _Roasting furnace._--The furnaces employed for this purpose are in
general analogous to the calcining ones; but in the smelting works of
Hafod, the property of Messrs. Vivian, these furnaces, alluded to above,
present a peculiar construction, for the purpose of introducing a
continuous current of air upon the metal, in order to facilitate its
oxidizement. This process was originally invented by Mr. Sheffield, who
disposed of his patent right to Messrs. Vivian.
[Illustration: 302]
The air is admitted by a channel, _c c_, through the middle of the
fire-bridge, _fig._ 302, and extending all its length; it communicates
with the atmosphere at its two ends _c c_; square holes, _b b_, left at
right angles to this channel, conduct the air into the furnace. This
very simple construction produces a powerful effect in the roasting
operation. It not only promotes the oxidizement of the metals, but burns
the smoke, and assists in the vaporisation of the sulphur; while by
keeping the bridge cool it preserves it from wasting, and secures
uniformity of temperature to the hearth.
4. _Refining furnace._--In this, as in the melting furnace, the sole
slopes towards the door in front, instead of towards the side doors,
because in the refining furnace the copper collects into a cavity formed
in the hearth towards the front door, from which it is lifted out by
ladles; whereas, in the melting furnaces, the metal is run out by a
tap-hole in the side. The hearth sole is laid with sand; but the roof is
higher than in the melting furnace, being from 32 to 36 inches. If the
top arch were too much depressed, there might be produced upon the
surface of the metal a layer of oxide very prejudicial to the quality of
the copper. When the metal in that case is run out, its surface
solidifies and cracks, while the melted copper beneath breaks through
and spreads irregularly over the cake. This accident, called the _rising
of the copper_, hinders it from being laminated, and requires it to be
exposed to a fresh refining process, when lead must be added to dissolve
the oxide of copper. This is the only occasion upon which the addition
of lead is proper in refining copper. When the metal to be refined is
mixed with others, particularly with tin, as in extracting copper from
old bells, then very wide furnaces must be employed, to expose the
metallic bath upon a great surface, and in a thin stratum, to the
oxidizing action of the air.
The door G, _fig._ 300., upon the side of the refining furnace, is very
large, and is shut with a framed brick door, balanced by a
counter-weight. This door being open during the refining process, the
heat is stronger at B than at A (_figs._ 299, 300.)
5. _Heating furnaces_, being destined to heat the pigs or bars of copper
to be laminated, as well as the copper sheets themselves, are made much
longer in proportion to their breadth. Their hearth is horizontal, the
vault not much depressed; they have only one door, placed upon the side,
but which extends nearly the whole length of the furnace: this door may
be raised by means of a counter-weight, in the same way as in the
furnaces for the fabrication of sheet-iron and brass.
_Series of operations to which the ore is subjected._--The ores which
are smelted in the Swansea works are cupreous pyrites, more or less
mingled with _gangue_ (vein-stone). The pyrites is composed of nearly
equal proportions of sulphuret of copper and sulphuret of iron.
The earthy matters which accompany the pyrites are usually siliceous,
though in some mines the metalliferous deposit is mixed with clay or
fluate of lime. Along with these substances, pretty uniformly
distributed, tin and arsenical pyrites occur occasionally with the
copper; and though these two metals are not chemically combined, yet
they cannot be separated entirely in the mechanical preparations. The
constituent parts of the ore prepared for smelting are, therefore,
copper, iron, sulphur, with tin, arsenic, and earthy matters in some
cases. The different ores are mixed in such proportions that the average
metallic contents may amount to 8-1/2 per cent. The smelting process
consists in alternate roastings and fusions. The following description
of it is chiefly taken from an excellent paper, published by John
Vivian, esq., in the Annals of Philosophy for 1823.
In the roasting operation the volatile substances are disengaged mostly
in the gaseous state, while the metals that possess a strong affinity
for oxygen become oxidized. In the fusion the earthy substances combine
with these oxides, and form glassy scoriæ or slags, which float upon the
surface of the melted metal.