Black copper holding at least 5 or 6 _loths_ (2-1/2 or 3 oz.) of silver
per cwt., and weighing 90 to 96 lbs.
Lead obtained from litharge, 2 cwts. Litharge, 1/2 cwt.
From 30 to 32 cakes are successively worked in one operation, which
lasts about 5 hours; the furnace is brought into action, as usual, with
the aid of slags; then a little litharge is added; when the lead begins
to flow, the copper is introduced, and when the copper flows, lead is
added, so that the mixture of the metals may be effected in the best way
possible.
From 8 to 16 of these cakes (_pains_) are usually placed in the
liquation furnace, _figs._ 692, 693, 694. The operation lasts 3 or 4
hours, in which time about 1-1/2 quintals of charcoal are consumed. The
cakes are covered with burning charcoal, supported, as I have said, by
the iron plates. The workable lead obtained flows off towards the basin
in front of the furnace; whence it is laded out into moulds set
alongside. See _fig._ 693. If the lead thus obtained be not sufficiently
rich in silver to be worth cupellation, it is employed to form new
liquation cakes. When it contains from 5 to 6 loths of silver per cwt.,
it is submitted to cupellation in the said smelting works. See SILVER.
[Illustration: 695 696 697]
The _trompe_, or water-blowing engine, _figs._ 695, 696, 697. _Fig._
695. is the elevation; _fig._ 696. is a vertical section, made at right
angles to the elevation. The machine is formed of two cylindrical pipes,
the bodies of the _trompe_ _b b_, set upright, called the funnels, which
terminate above in a water cistern _a_, and below in a close basin under
_c_, called the _tub_ or _drum_. The conical part _p_, of the funnel
has been called _etranguillon_, being _strangled_, as it were, in order
that the water discharged into the body of the trompe shall not fill the
pipe in falling, but be divided into many streamlets. Below this _narrow
part_, eight holes, _q q_, are perforated obliquely through the
substance of the trompe, called the vent-holes or nostrils, for
admitting the air, which the water carries with it in its descent. The
air afterwards parts from the water, by dashing upon a cast-iron slab,
placed in the _drum_ upon the pedestal _d_. An aperture _l_, at the
bottom of the drum, allows the water to flow away after its fall; but,
to prevent the air from escaping along with it, the water as it issues
is received in a chest _l m o n_, divided into two parts by a vertical
slide-plate between _m n_. By raising or lowering this plate, the water
may be maintained at any desired level within the drum, so as to give
the included air any determinate degree of pressure. The superfluous
water then flows off by the hole _o_.
The air-pipe _e f_, _fig._ 696., is fitted to the upper part of the
_drum_; it is divided, at the point _f_, into three tubes, of which the
principal one is destined for the furnace of cupellation, whilst the
other two _g g_, serve for different melting furnaces. Each of these
tubes ends in a leather pocket, and an iron nose-pipe _k_, adjusted in
the tuyère of the furnace. At Pesey, and in the whole of Savoy, a
floodgate is fitted into the upper cistern _a_, to regulate the
admission of water into the trome; but in Carniola, the funnel _p_ is
closed with a wooden plug, suspended to a cord, which goes round a
pulley mounted upon a horizontal axis, as shewn in _fig._ 697. By the
plug _a_ being raised more or less, merely the quantity of water
required for the operation is admitted. The plug is pierced lengthwise
with an oblique hole _c c_, in which the small tube _c_ is inserted,
with its top some way above the water level, through which air may be
admitted into the heart of the column descending into the trompe _p q_.
The ordinary height of the trompe apparatus is about 26 or 27 feet to
the upper level of the water cistern; its total length is 11 mètres (36
feet 6 inches), and its width 2 feet, to give room for the drums. It is
situated 10 mètres (33-1/3 feet) from the melting furnace. This is the
case at the smelting works of Jauerberg, in Upper Carniola.