Marketable lead 20,907 quintals.
Marketable litharge, containing 90 per cent. of lead 7,555
Silver, about 67
Copper (finally purified in the works of Altenau) 35
------
Total product 28,564
This weight amounts to one twenty-fifth of the weight of ore raised for
the service of the establishment. Eight parts of ore furnish, on an
average, about one of schlich. The bellows are constructed wholly of
wood, without any leather; an improvement made by a bishop of Bamberg,
about the year 1620. After receiving different modifications, they were
adopted, towards 1730, in almost all the smelting-works of the
continent, except in a few places, as Carniola, where local
circumstances permitted a water blowing-machine to be erected. These
pyramidal shaped bellows, composed of movable wooden boxes, have,
however, many imperfections: their size must often be inconveniently
large, in order to furnish an adequate stream of air; they do not drive
into the furnace all the air which they contain; they require frequent
repairs; and, working with great friction, they waste much mechanical
power.
[Illustration: 1014]
_Fig._ 1014. represents such wooden bellows, consisting of two chests or
boxes, fitted into each other; the upper or moving one being called the
_fly_, the lower or fixed one, the _seat_ (_gite_). In the bottom of the
_gite_, there is an orifice furnished with a clack-valve _d_, opening
inwards when the _fly_ is raised, and shutting when it falls. In order
that the air included in the capacity of the two chests may have no
other outlet than the nose-pipe _m_, the upper portion of the _gite_ is
provided at its four sides with small square slips of wood _c_, _c_,
_c_, which are pressed against the sides of the _fly_ by strong springs
of iron wire _b_, _b_, _b_, while they are retained upon the _gite_ by
means of small square pieces of wood _a_, _a_, _a_, _a_. The latter _a_,
_a_, are perforated in the centre, and adjusted upon rectangular stems,
called _buchettes_; they are attached, at their lower ends, to the
upright sides of the _gite_ G. P is the driving-shaft of a water-wheel,
which, by means of cams or tappets, depresses the fly, while the
counterweight Q, _fig._ 1013., raises it again.
_Figs._ 1015, 1016, 1017, 1018. represent the moderately high
(_demihauts_, or _half-blast_,) furnaces employed in the works of the
lower Hartz, near Goslar, for smelting the silvery lead ores extracted
from the mine of Rammelsberg. See its section, in _fig._ 737.
[Illustration: 1015 1016]
_Fig._ 1015. is the front elevation of the twin furnaces, built in one
body of masonry; _fig._ 1016. is a plan taken at the level of the
tuyères, in the line _v_, _l_, 6. of _fig._ 1017.; _figs._ 1017. and
1018. exhibit two vertical sections; the former in the line A, B, the
latter in the line C, D, of _fig._ 1016. In these four figures the
following objects may be distinguished.
_a_, _b_, _c_, _d_, a balcony or platform, which leads to the place of
charging _n_; _e_, _f_, wooden stairs, by which the charging workmen
mount from the ground _p_, _q_, of the works, to the platform; _g_, _h_,
brickwork of the furnaces; _i_, _k_, wall of the smelting-works, against
which they are supported; _l_, upper basin of reception, hollowed out of
the _brasque_ (or ground charcoal bed) 6; _m_, arch of the tuyère _v_,
by which each furnace receives the blast of two bellows; _n_, place of
charging, which takes place through the upper orifice _n_, _o_, of the
basin _n_, _o_, _v_, _t_, of the furnace; _t_, a slab of clay, placed in
such a way that, during the treatment of the lead, a little metallic
zinc may run together in a sloping gutter, seen in _fig._ 1001., formed
of slates cemented together with clay.
[Illustration: 1017]
In _figs._ 1015 and 1017., _l_, _z_, is the brickwork of the
foundations; _m_, conduits (called evaporatory), for the exhalation of
the moisture; 4, a layer of slags, rammed above; 5, a bed of clay,
rammed above the slags; 6, a brasque, composed of one part of clay, and
two parts of ground charcoal, which forms the sole of the furnace.
[Illustration: 1018 1019 1020]
The excellent refinery furnace, or _treibheerd_, of Frederickshütte,
near Tarnowitz, in Upper Silesia, is represented in _figs._ 1019. and
1020. _a_, is the bottom, made of slag or cinders; _b_, the foundation,
of fire-bricks; _c_, the body of the hearth proper, composed of a
mixture of 7 parts of dolomite, and 1 of fire-clay, in bulk; _d_, the
grate of the air furnace; _e_, the fire-bridge; _f_, the dome or cap,
made of iron plate strengthened with bars, and lined with clay-lute, to
protect the metal from burning; _g_, the door of the fireplace; _h_, the
ash-pit; _i_, the tap-hole; _k_, _k_, the flue, which is divided by
partitions into several channels; _l_, the chimney; _m_, a damper-plate
for regulating the draught; _n_, a back valve, for admitting air to cool
the furnace, and brushes to sweep the flues; _o_, _tuyère_ of copper,
which by means of an iron wedge may be sloped more or less towards the
hearth; _p_, the _schnepper_, a round piece of sheet iron, hung before
the _eye_ of the _tuyère_, to break and spread the blast; _q_, the
outlet for the glassy litharge.
Lime-marl has been found to answer well for making the body of the
hearth-sole, as it absorbs the vitrified litharge freely, without
combining with it. A basin-shaped hollow is formed in the centre, for
receiving the silver at the end of the process; and a gutter is made
across the hearth for running off the _glätte_ or fluid litharge.
[Illustration: 1021 1022 1023]
_Figs._ 1021, 1022. represent the eliquation hearth of Neustadt. _Fig._
1021. is a cross section; _fig._ 1022. is a front view; and _fig._ 1023.
a longitudinal section. It is formed by two walls _a_, _a_, 3-1/2 feet
high, placed from 1/2 to 1 foot apart, sloped off at top with iron
plates, 3 inches thick, and 18 inches broad, called _saigerscharten_, or
refining plates, _b_, _b_, inclined 3 inches towards each other in the
middle, so as to leave at the lowest point a slit 2-1/2 inches wide
between them, through which the lead, as it sweats out by the heat, is
allowed to fall into the space between the two walls _c_, called the
_saigergasse_ (sweating-gutter). The sole of this channel slopes down
towards the front, so that the liquefied metal may run off into a
crucible or pot. Upon one of the long sides, and each of the shorter
ones, of the hearth, the walls _d_, _d_, are raised two feet high, and
upon these the liquation lumps rest; upon the other long side, where
there is no wall, there is an opening for admitting these lumps into the
hearth. The openings are then shut with a sheet or cast iron plate _e_,
which, by means of a chain, pulley, and counterweight, may be easily
raised and lowered. _f_, is a passage for increasing the draught of air.
[Illustration: 1024 1025]
_Figs._ 1024. and 1025. represent the refining furnaces of
Frederickshütte by Tarnowitz; _a_, is the fire-door; _b_, the grate;
_c_, the door for introducing the silver; _d_, the movable test, resting
upon a couple of iron rods _e_, _e_, which are let at their ends into
the brickwork. They lie lower than would seem to be necessary; but this
is done in order to be able to place the surface of the test at any
desired level, by placing tiles _f_, _f_, under it; _g_, the flue,
leading to a chimney 18 feet high. For the refining of 100 marks of
_blicksilber_, of the fineness of 15-1/2 loths (half ounces) per cwt., 3
cubic feet of pitcoal are required. The test or cupel must be heated
before the impure silver and soft lead are put into it.
At these smelting-houses from 150 to 160 cwt. of very pure _workable
lead_ (lead containing merely a little silver) are put into the furnace
at once, and from 10 to 14 cwt. run off in vitrified oxide; the
remainder is then refined with some pure lead, when an alloy containing
from 14-1/2 to 15-1/2 loths of blicksilber per cwt. is obtained.
[Illustration: 1026 1027]
_English refining furnaces._--The refining of lead is well performed in
some works in the neighbourhood of Alston-moor, in reverberatory
furnaces, _figs._ 1026. and 1027., whose fireplace is 22 inches square,
and is separated from the sole by a fire-bridge, 14 inches in breadth.
The flame, after having passed over the surface of the lead in the
cupel, enters two flues _e_, _e_, on the opposite side of the furnace,
which terminate in a chimney _i_, _i_, _i_, _i_, 40 feet high. At the
bottom of the chimney are openings _f_, _f_, for taking out the metallic
dust deposited within. These openings are shut during the process.
[Illustration: 1028 1029]
The cupel or test, which constitutes, in fact, the sole of the hearth in
which the operation takes place, is movable. It consists of a vertical
elliptical ring of iron, A, B, C, D, _figs._ 1028. and 1029., 3-3/4
inches high, the greatest diameter of the ellipse being 4 feet, and the
smallest 2-1/2. Four iron bars (A D, _m_, _m´_, B C, _n_, _n´_,) are
fixed across its bottom, which are also 3-3/4 inches broad, and an inch
thick. The first of these bars is placed 9 inches from the end of the
elliptic ring nearest the fireplace, and the three others are equally
distributed between this bar and the back end.
In forming the cupel, several layers of a mixture of moistened bone
ashes, and fern ashes, in very fine powder, are put into the
_test-frame_. The bone ash constitutes from 1/8 to 1/16 of the bulk of
the mixture, according to the purity of the fern ashes employed,
estimated by the proportion of potash they contain, which has the
property of semi-vitrifying the powder of burnt bones, of thus removing
its friability, and rendering it more durable. The layers of ashes are
strongly beat down, till the frame is entirely filled. The mass thus
formed is then hollowed out by means of a little spade, made on purpose,
till it is only three quarters of an inch thick above the iron bars near
the centre of the bottom. A flange, 2 inches broad, is made at the upper
part, and 2-1/2 inches at the lower part, except on the front or
_breast_, which is 5 inches thick. In this anterior part, there is
hollowed out an opening of an inch and a quarter broad, and 6 inches
long, with which the outlet or _gateway_ of the litharge communicates.
The cupel thus prepared is placed in the refining furnace. It rests in
an iron ring built into the brickwork. The arched roof of the furnace is
12 inches above the cupel near the fire-bridge, and 9 inches near the
flue at the other end.
The tuyère is placed in the back of the furnace, opposite to the side at
which the litharge is allowed to overflow.
Openings _g_, _g_, are left at the sides of each cupel, either for
running off or for introducing melted lead.
_Refining of lead to extract its silver._--This operation, which the
lead of Derbyshire cannot be submitted to with advantage, is performed
in a certain number of the smelting-houses at Alston-moor, and always
upon leads reduced in the Scotch furnace.
The cupel furnace above described, must be slowly heated, in order to
dry the cupel without causing it to crack, which would infallibly be
produced by sudden evaporation of the moisture in it. When it has been
thus slowly brought to the verge of a red heat, it is almost completely
filled with lead previously melted in an iron pot. The cupel may be
charged with about 5 cwt. At the temperature at which the lead is
introduced, it is immediately covered with a gray pellicle of oxide; but
when the heat of the furnace has been progressively raised to the proper
pitch, it becomes whitish-red, and has its surface covered over with
litharge. Now is the time to set in action the blowing-machine, the
blast of which, impelled in the direction of the great axis of the
cupel, drives the litharge towards the _breast_ of the cupel, and makes
it flow out by the _gateway_ prepared for it, through which it falls
upon a cast-iron plate, on a level with the floor of the apartment, and
is dispersed into tears. It is carried in this state to the furnace of
reduction, and revived. As by the effect of the continual oxidization
which it undergoes, the surface of the metal necessarily falls below the
level of the gateway of the litharge, melted lead must be added anew by
ladling it into the furnace from the iron boiler, as occasion may
require. The operation is carried on in this manner till 84 cwt. or 4
Newcastle _fodders_ of lead have been introduced, which takes from 16 to
18 hours, if the tuyère has been properly set. The whole quantity of
silver which this mass of lead contains, is left in combination with
about 1 cwt. of lead, which, under the name of rich lead, is taken out
of the cupel.
When a sufficient number of these pieces of rich lead have been
procured, so that by their respective quality, as determined by
assaying, they contain in whole from 1000 to 2000 ounces of silver, they
are re-melted to extract their silver, in the same furnace, but in a
cupel which differs from the former in having at its bottom a depression
capable of receiving at the end of the process the cake of silver. In
this case a portion of the bottom remains uncovered, on which the scoriæ
may be pushed aside with a little rake, from the edges of the silver.
The experiments of MM. Lucas and Gay Lussac have proved that fine
silver, exposed to the air in a state of fusion, absorbs oxygen gas, and
gives it out again in the act of consolidation. The quantity of oxygen
thus absorbed may amount to twenty-two times the volume of the silver.
The following phenomena are observed when the mass of metal is
considerable; for example, from 40 to 50 pounds.
The solidification commences at the edges, and advances towards the
centre. The liquid silver, at the moment of its passage to the solid
state, experiences a slight agitation, and then becomes motionless. The
surface, after remaining thus tranquil for a little, gets all at once
irregularly perturbed, fissures appear in one or several lines, from
which flow, in different directions, streams of very fluid silver, which
increase the original agitation. The first stage does not yet clearly
manifest the presence of gas, and seems to arise from some intestine
motion of the particles in their tendency to group, on entering upon the
process of crystallization, and thus causing the rupture of the envelop
or external crust, and the ejection of some liquid portions.
After remaining some time tranquil, the metal presents a fresh
appearance, precisely analogous to volcanic phenomena. As the
crystallization continues, the oxygen gas is given out with violence at
one or more points, carrying with it melted silver from the interior of
the surface, producing a series of cones, generally surmounted by a
small crater, vomiting out streams of the metal, which may be seen
boiling violently within them.
These cones gradually increase in height by the accumulation of metal
thrown up, and that which becomes consolidated on their sloping sides.
The thin crust of metal on which they rest, consequently experiences
violent impulses, being alternately raised and depressed by such violent
agitation, that were it not for the tenacity and elasticity of the
metal, there would evidently arise dislocation, fissures, and other
analogous accidents. At length several of the craters permanently close,
while others continue to allow the gas a passage. The more difficult
this is, the more the craters become elevated, and the more their
funnels contract by the adhesion or coagulation of a portion of the
metal. The projection of globules of silver now becomes more violent;
the latter being carried to great distances, even beyond the furnace,
and accompanied by a series of explosions, repeated at short intervals.
It is generally the last of these little volcanoes that attains the
greatest altitude, and exhibits the foregoing phenomena with the
greatest energy. It is, moreover, observable, that these cones do not
all arise at the same time, some having spent their force, when others
commence forming at other points. Some reach the height of an inch,
forming bases of two or three inches in diameter. The time occupied by
this exhibition is at least from half to three quarters of an hour.
During the formation of these cones, by the evolution of gas, portions
of silver are shot forth, which assume, on induration, a form somewhat
cylindrical, and often very fantastic, notwithstanding the
incompatibility which appears to exist between the fluidity of the
silver and these elongated figures. Their appearance is momentary, and
without any symptoms of gas, although it is impossible to decide whether
they may not arise from its influence; they seem, in fact, to resemble
the phenomena of the first volcanic period.
Till very recently the only operations employed for separating silver
from lead in the English smelting-works, were the following:--
1. Cupellation, in which the lead was converted into a vitreous oxide,
which was floated off from the surface of the silver.