RAISINS, are grapes allowed to ripen and dry upon the vine. The best
come from the south of Europe, as from Roquevaire in Provence, Calabria,
Spain, and Portugal. Fine raisins are also imported from Smyrna,
Damascus, and Egypt. Sweet fleshy grapes are selected for maturing into
raisins, and such as grow upon the sunny slopes of hills sheltered from
the north winds. The bunches are pruned, and the vine is stripped of its
leaves, when the fruit has become ripe; the sun then beaming full upon
the grapes, completes their saccharification, and expels the superfluous
water. The raisins are plucked, cleansed, and dipped for a few seconds
in a boiling lye of wood ashes and quicklime, at 12 or 13 degrees of
Baumé’s areometer. The wrinkled fruit is lastly drained, dried, and
exposed in the sun upon hurdles of basket-work during 14 or 15 days.
The finest raisins are those of the sun, so called; being the plumpest
bunches, which are left to ripen fully upon the vine, after their stalks
have been half cut through.
The amount of raisins imported for home consumption was, in the year
1836, 156,495 cwts.; in 1837, 152,635 cwts.
RAPE-SEED, imported for home consumption in 1836, 561,457 bushels; in
1837, 937,526 bushels. See OILS, UNCTUOUS.
RASP, MECHANICAL, is the name given by the French to an important
machine much used for mashing beet-roots. See SUGAR.
RATAFIA, is the generic name, in France, of _liqueurs_ compounded with
alcohol, sugar, and the odoriferous or flavouring principles of
vegetables. Bruised cherries with their stones are infused in spirit of
wine to make the ratafia of Grenoble _de Teyssère_. The liquor being
boiled and filtered, is flavoured, when cold, with spirit of _noyau_,
made by distilling water off the bruised bitter kernels of apricots, and
mixing it with alcohol. Syrup of bay laurel and galango are also added.
REALGAR, _Red Orpiment_. (_Arsenic rouge sulphuré_, Fr.; _Rothes
schwefelarsenik_, Germ.) This ore occurs in primitive mountains,
associated sometimes with native arsenic, under the form of veins,
efflorescences, very rarely crystalline; as also in volcanic districts;
for example, at Solfatara near Naples; or sublimed in the shape of
stalactites, in the rents and craters of Etna, Vesuvius, and other
volcanoes. Its spec. grav. varies from 3·3 to 3·6. It has a fine scarlet
colour in mass, but orange-red in powder, whereby it is distinguishable
from cinnabar. It is soft, sectile, readily scratched by the nail; its
fracture is vitreous and conchoidal. It volatilizes easily before the
blowpipe, emitting the garlic smell of arsenic, along with that of
burning sulphur. It consists of, arsenic 70, sulphur 30, in 100 parts.
It is employed sometimes as a pigment. Factitious orpiment is made by
distilling, in an earthen retort, a mixture of sulphur and arsenic, of
orpiment and sulphur, or of arsenious acid, sulphur, and charcoal. It
has not the rich colour of the native pigment, and is much more
poisonous; since, like factitious orpiment, it always contains more or
less arsenious acid.
RECTIFICATION, is a second distillation of alcoholic liquors, to free
them from whatever impurities may have passed over in the first.
RED LIQUOR, is a crude acetate of alumina, employed in calico-printing,
and prepared from pyrolignous acid; which see.
REED, is the well-known implement of the weaver, made of parallel slips
of metal or reeds, called dents. A thorough knowledge of the adaptation
of yarn of a proper degree of fineness to any given measure of reed,
constitutes one of the principal objects of the manufacturer of cloths;
as upon this depends entirely the appearance, and in a great degree the
durability, of the cloth when finished. The art of performing this
properly, is known by the names of _examining_, _setting_, or _sleying_,
which are used indiscriminately, and mean exactly the same thing. The
reed consists of two parallel pieces of wood, set a few inches apart,
and they are of any given length, as a yard, a yard and a quarter, &c.
The division of the yard being into halves, quarters, eighths, and
sixteenths; the breadth of a web is generally expressed by a vulgar
fraction, as 1/4, 4/4, 5/4, 6/4; and the subdivisions by the eighths or
sixteenths, or _nails_, as they are usually called, as 7/8, 9/8, 11/8,
&c., or 13/16, 15/16, 19/16, &c. In Scotland, the splits of cane which
pass between the longitudinal pieces or ribs of the reed, are expressed
by hundreds, porters, and splits. The porter is 20 splits, or 1/5th of
an hundred.
In Lancashire and Cheshire a different mode is adopted, both as to the
measure and divisions of the reed. The Manchester and Bolton reeds are
counted by the number of splits, or, as they are there called, dents,
contained in 24-1/4 inches of the reed. These dents, instead of being
arranged in hundreds, porters, and splits, as in Scotland, are
calculated by what is there termed _hares_ or _bears_, each containing
20 dents, or the same number as the porter in the Scotch reeds. The
Cheshire or Stockport reeds, again, receive their designation from the
number of ends or threads contained in one inch, two ends being allowed
for every _dent_, that being the almost universal number in every
species and description of plain cloth, according to the modern practice
of weaving, and also for a great proportion of fanciful articles.
The number of threads in the warp of a web is generally ascertained with
considerable precision by means of a small magnifying glass, fitted into
a socket of brass, under which is drilled a small round hole in the
bottom plate of the standard. The number of threads visible in this
perforation, ascertains the number of threads in the standard measure of
the reed. Those used in Scotland have sometimes four perforations, over
any one of which the glass may be shifted. The first perforation is 1/4
of an inch in diameter, and is therefore well adapted to the Stockport
mode of counting; that is to say, for ascertaining the number of ends or
threads per inch; the second is adapted for the Holland reed, being
1/200th part of 40 inches; the third is 1/700th of 37 inches, and is
adapted for the now almost universal construction of Scotch reeds; and
the fourth, being 1/200th of 34 inches, is intended for the French
cambrics. Every thread appearing in these respective measures, of course
represents 200 threads, or 100 splits, in the standard breadth; and thus
the quality of the fabric may be ascertained with considerable
precision, even after the cloth has undergone repeated wettings, either
at the bleaching-ground or dye-work. By counting the other way, the
proportion which the woof bears to the warp is also known, and this
forms the chief use of the glass to the manufacturer and operative
weaver, both of whom are previously acquainted with the exact measure of
the reed.
Comparative TABLE of 37-inch reeds, being the standard used throughout
Europe, for linens, with the Lancashire and Cheshire reeds, and the
foreign reeds used for holland and cambric.
+-------+-----------+---------+--------------+---------------+
|Scotch.|Lancashire.|Cheshire.|Dutch holland.|French cambric.|
+-------+-----------+---------+--------------+---------------+
| 600 | 20 | 34 | 550 | 653 |
| 700 | 24 | 38 | 650 | 761 |
| 800 | 26 | 44 | 740 | 870 |
| 900 | 30 | 50 | 832 | 979 |
| 1000 | 34 | 54 | 925 | 1089 |
| 1100 | 36 | 60 | 1014 | 1197 |
| 1200 | 40 | 64 | 1110 | 1300 |
| 1300 | 42 | 70 | 1202 | 1414 |
| 1400 | 46 | 76 | 1295 | 1464 |
| 1500 | 50 | 80 | 1387 | 1602 |
| 1600 | 52 | 86 | 1480 | 1752 |
| 1700 | 56 | 92 | 1571 | 1820 |
| 1800 | 58 | 96 | 1665 | 1958 |
| 1900 | 62 | 104 | 1757 | 2067 |
| 2000 | 66 | 110 | 1850 | 2176 |
+-------+-----------+---------+--------------+---------------+
In the above table, the 37-inch is placed first. It is called Scotch,
not because it either originated or is exclusively used in that country.
It is the general linen reed of all Europe; but in Scotland it has also
been adopted as the regulator of her cotton manufactures.
REFINING OF GOLD AND SILVER; called also _Parting_. (_Affinage
d’argent_, _Départ_, Fr.; _Scheidung in die quart_, Germ.) For several
uses in the arts, these precious metals are required in an absolutely
pure state, in which alone they possess their malleability and peculiar
properties in the most eminent degree. Thus, for example, neither gold
nor silver leaf can be made of the requisite fineness, if the metals
contain the smallest portion of copper alloy. Till within these ten or
twelve years, the parting of silver from gold was effected every where
by nitric acid; it is still done so in all the establishments of this
country, except the Royal Mint; and in the small refining-houses abroad.
The following apparatus may be advantageously employed in this
operation. It will serve the double purpose of manufacturing nitric acid
of the utmost purity, and of separating silver from gold by its means.
[Illustration: 931]
1. _On procuring nitric acid for parting._--_a_ is a platinum retort or
alembic; _b_ is its capital, terminating above in a tubulure, to which a
kneed tube of platinum, about 2 feet long, is adapted; _c_ is the
tubulure of the retort, for supplying acid during the process, and for
inspecting its progress. It is furnished with a lid ground air-tight,
which may be secured in its place by a weight. _e_ is a stoneware pipe,
about two inches diameter, and several feet long, according to the
locality in which the operation is to be carried on. It is made in
lengths fitted to one another, and secured at the joints with loam-lute.
The one bend of this earthenware hard salt-glazed pipe is adapted to
receive the platinum tube, and the other bend is inserted into a
tubulure in the top of the stoneware drum _f_. The opening _l_, _l_, in
the middle of the top of _f_, is for inspecting the progress of the
condensation of acid; and the third tubulure terminates in a prolonged
pipe _i_, _i_, consisting of several pieces, each of which enters from
above conically into the one below. The joinings of the upper pieces
need not be tightly luted, as it is desirable that some atmospherical
oxygen should enter, to convert the relatively light nitrous gas into
nitrous or nitric acid vapour, which when supplied with moisture will
condense and fall down in a liquid state. To supply this moisture in the
most diffusive form, the upright stoneware pipes _i_, _i_, _l_, _l_, (at
least 3 inches diameter, and 12 feet high,) should be obstructed
partially with flint nodules, or with siliceous pebbles; and water
should be allowed to trickle upon the top pebble from a cistern placed
above. Care must be taken to let the water drop so slowly as merely to
preserve the pebbles in a state of humidity. _h_ is a stopcock, of glass
or stoneware, for drawing off the acid from the cistern _f_. _k_ is a
section of a small air-furnace, covered in at top with an iron ring, on
which the flat iron ring of the platinum frame rests.
_g_, _g_, is a tub in which the stoneware cistern stands, surrounded
with water, kept constantly as cold as possible by passing a stream
through it; the spring water entering by a pipe that dips near to the
bottom, and the hot water escaping at the upper edge.
With the above apparatus, the manufacture of pure nitric acid is
comparatively easy and economical. Into the alembic _a_, 100 pounds (or
thereby) of pure nitre, coarsely bruised if the crystals be large, are
to be put; the capital is then to be adapted, and the platinum tube (the
only movable one) luted into its place. Twenty pounds of strong
sulphuric acid are now to be introduced by the tubulure _c_, and then
its lid must be put on. No heat must yet be applied to the alembic. In
about an hour, another ten pounds of acid may be poured in, and so every
hour, till 60 pounds of acid have been added. A few hours after the
affusion of the last portion of acid, a slight fire may be kindled in
the furnace _k_.
By judicious regulation of the heat, the whole acid may be drawn off in
24 hours; its final expulsion being aided by the dexterous introduction
of a quart or two of boiling water, in small successive portions, by the
tubulure _c_, whose lid must be instantly shut after every inspersion.
The most convenient strength of acid for the parting process, is when
its specific gravity is about 1·320, or when a vessel that contains 16
ounces of pure water, will contain 21-1/8 of the aquafortis. To this
strength it should be brought very exactly by the aid of a hydrometer.
Its purity is easily ascertained by letting fall into it a few drops of
solution of silver; and if no perceptible milkiness ensues, it may be
accounted good. Should a white cloud appear, a few particles of silver
may be introduced, to separate whatever muriatic acid may be present, in
the form of chloride of silver. Though a minute quantity of sulphuric
acid should exist in the nitric, it will be of no consequence in the
operation of parting.
2. _On parting by the nitric acid, called by the Mexicans, “Il
apartado.”_--The principle on which this process is founded, is the fact
of silver being soluble in nitric acid, while gold is insoluble in that
menstruum. If the proportion of gold to that of silver be greater than
one to two, then the particles of the former metal so protect or envelop
those of the latter, that the nitric acid, even at a boiling heat,
remains quite inactive on the alloy. It is indispensable, therefore,
that the weight of the silver be at least double that of the gold. 100
pounds of silver take 38 pounds of nitric acid, of specific gravity
1·320, for oxidizement, and 111 for solution of the oxide; being
together 149; but the refiner often consumes, in acid of the above
strength, more than double the weight of silver, which shows great
waste, owing to the imperfect means of condensation employed for
recovering the vapours of the boiling and very volatile acid.
By the apparatus above delineated, the 38 pounds of acid expended in
oxidizing the silver, become nitrous gas in the first place, and are
afterwards reconverted in a great measure into nitric acid by absorption
of atmospherical oxygen; so that not one-fifth need be lost, under good
management. As the acid must be boiled on the granulated _garble_, or
alloy, to effect the solution of the silver, by proper arrangements the
vapours may be entirely condensed, and nearly the whole acid be
recovered, except the 111 parts indispensable to constitute nitrate of
silver. Hence, with economical management, 120 pounds of such acid may
be assigned as adequate to dissolve 100 of silver associated with 50 of
gold.
It must here be particularly observed, that 100 pounds of copper require
130 pounds of the above acid for oxidizement; and 390 for solution of
the oxide; being 520 pounds in whole, of which less than 1/4 part could
be recovered by the above apparatus. It is therefore manifest that it is
desirable to employ silver pretty well freed from copper by a previous
process; and always, if practicable, a silver containing some gold.
These data being assumed as the bases of the parting operation, 60
pounds of gold and silver alloy or _garble_ finely granulated,
containing not less than 40 pounds of silver, are to be introduced into
the ten-gallon alembic of platinum, _fig._ 931., and 80 pounds of nitric
acid, of 1·320, is to be poured over the alloy; a quantity which will
measure 6 gallons imperial. As for the bulk of the alloy, it is
considerably less than half a gallon. Abundance of space therefore
remains in the alembic for effervescence and ebullition, provided the
fire be rightly tempered.
By the extent of stoneware conducting pipe _e_, which should not be less
than 40 feet, by the dimensions and coldness of the cistern _f_, and by
the regenerating influence of the vertical aerial pipe filled with moist
pebbles _i_, _i_, it is clear, that out of the 80 pounds of nitric acid,
specific gravity 1·320, introduced at first, from 20 to 30 will be
recovered.
Whenever the effervescence and disengagement of nitrous red fumes no
longer appear on opening the orifice _c_, the fire must be removed, and
the vessel may be cooled by the application of moist cloths. The alembic
may be then disengaged from the platinum tube, and lifted out of its
seat. Its liquid contents must be cautiously decanted off, through the
orifice _c_, into a tub nearly filled with soft water. On the heavy
pulverulent gold which remains in the vessel, some more acid should be
boiled, to carry off any residuary silver. This metallic powder, after
being well washed with water, is to be dried, fused along with a little
nitre or borax, and cast into ingots.
Plates of copper being immersed in the nitric solution contained in
wooden or stoneware cisterns, will throw metallic silver down, while a
solution of nitrate of copper, called blue water, will float above. The
pasty silver precipitate is to be freed from the nitrate of copper,
first, by washing with soft water, and next, by strong hydraulic
pressure in cast-iron cylinders. The condensed mass, when now melted in
a crucible along with a little nitre and borax, is fine silver.
The above apparatus has the further advantage of enabling the operator
to recover a great portion of his nitric acid, by evaporating the blue
water to a state approaching to dryness, with the orifices at _c_, and
at the top of the capital, open. In the progress of this evaporation,
nothing but aqueous vapour escapes. Whenever the whole liquid is
dissipated, the pipe _d_ is to be re-adjusted, and the lid applied
closely to _c_. The heat being now continued, and gradually increased,
the _whole_ nitric acid will be expelled from the copper oxide, which
will remain in a black mass at the bottom of the alembic. The
contrivance for letting water trickle upon the pebbles, must be
carefully kept in play, otherwise much of the evolved acid would be
dissipated in nitrous fumes. With due attention to the regenerative
plan, a great part of the acid may be recovered, at no expense but that
of a little fuel.
The black oxide of copper thus obtained, is an economical form of
employing that metal for the production of the sulphate; 100 pounds of
it, with 122-1/2 of sulphuric acid diluted with water, produce 312-1/2
pounds of crystallized sulphate of copper. A leaden boiler is best
adapted for that operation. 100 pounds of silver are precipitable from
its solution in nitric acid, by 29 of copper. If more be needed, it is a
proof that a wasteful excess of acid has existed in the solution.
In parting by nitric acid, the gold generally retains a little silver;
as is proved by the cloud of chloride of silver which it affords, at the
end of some hours, when dissolved in aqua regia. And on the other hand,
the silver retains a little gold. These facts induced M. Dizé, when he
was inspector of the French mint, to adopt some other process, which
would give more accurate analytical results; and after numerous
experiments, he ascertained that sulphuric acid presented great
advantages in this point of view, since with it he succeeded in
detecting, in silver, quantities of gold which had eluded the other plan
of parting. The suggestion of M. Dizé has been since universally adopted
in France. M. Costell, about nine or ten years ago, erected in
Pomeroy-street, Old Kent-road, a laboratory upon the French plan, for
parting by sulphuric acid; but he was not successful in his enterprise;
and since he relinquished the business, Mr. Matheson introduced the same
system into our Royal Mint, under the management of M. Costell’s French
operatives. In the Parisian refineries, gold, to the amount of
one-thousandth part of the weight, has been extracted from all the
silver which had been previously parted by the nitric acid process;
being 3500 francs in value upon every thousand kilogrammes of silver.
I shall give first a general outline of the method of parting by
sulphuric acid, and then describe its details as I have lately seen them
executed upon a magnificent scale in an establishment near Paris.
The most suitable alloy for refining gold, by the sulphuric-acid
process, is the compound of gold, silver, and copper, having a standard
quality, by the cupel, of from 900 to 950 millièmes, and containing
one-fifth of its weight of gold. The best proportions of the three
metals are the following:--silver, 725; gold, 200; copper, 75; = 1000.
It has been found that alloys which contain more copper, afford
solutions that hold some anhydrous sulphate of that metal in solution,
which prevents the gold from being readily separated; and that alloys
containing more gold, are not acted on easily by the sulphuric acid. The
refiner ought, therefore, when at all convenient, to reduce the alloys
that he has to treat, to the above-stated proportions. He may effect
this purpose either by fusing the coarser alloys with nitre in a
crucible, or by adding finer alloy, or even fine silver, or finally, by
subjecting the coarser alloys to a previous cupellation with lead on
the great scale. As to gold or silver bullion, which contains lead and
other easily oxidizable metals besides copper, the refiner ought always
to avoid treating them by sulphuric acid; and should separate, first of
all, these foreign metals by the agency of nitre, if they exist in
minute quantity; but if in larger, he should have recourse to the cupel.
Great advantage will therefore be derived from the judicious preparation
of the alloy to be refined.
For an alloy of the above description, the principal Parisian refiners
are in the habit of employing thrice its weight of sulphuric acid, in
order to obtain a clear solution of sulphate of silver, which does not
too suddenly concrete on cooling, so as to obstruct its discharge from
the alembic by decantation. A small increase in the quantity of copper,
calls for a considerable increase in the quantity of acid.
Generally speaking, one-half of the sulphuric acid strictly required for
converting the silver and copper into sulphates, is decomposed into
sulphurous acid, which is lost to the manufacturer, unless he has
recourse to the agency of nitrous acid.
The process for silver containing but little gold, consists of five
different operations.
1. Upon several furnaces, one foot in diameter, egg-shaped alembics of
platinum are mounted, into each of which are put 3 kilogrammes (8 lbs.
troy) of the granulated silver, containing a few grains of gold per
pound, and 6 kilogrammes of concentrated sulphuric acid. The alembics
are covered with conical capitals, ending in bent tubes, which conduct
the acid vapours into lead pipes of condensation; and the furnaces are
erected under a proper hood. As the cold acid is inoperative, it must be
set a boiling, at which temperature it gives up one atom of its oxygen
to the metal, and is transformed into sulphurous acid, which escapes in
a gaseous state. Some of the undecomposed sulphuric acid immediately
combines with the oxide into a sulphate, which subsides, in the state of
a crystalline powder, to the bottom of the vessel. The solution goes on
vigorously, with a copious disengagement of sulphurous acid gas, only
during the two or three first hours; after which it proceeds slowly, and
is not completed till after a digestion of nearly twelve hours more.
During the ebullition a considerable quantity of sulphuric acid vapour
escapes along with the sulphurous acid gas; the former of which is
readily condensed in a large leaden receiver immersed in a cistern of
cold water, if need be. It has been proposed to condense the sulphurous
acid, by leading it over extensive surfaces of lime-pap, as in the
coal-gas purifiers.
2. When the whole silver has been converted into sulphate, this is to be
emptied out of the alembic into water contained in a round-bottomed
receiver lined with lead, and diluted till the density of the solution
marks from 15° to 20° Baumé. The small portion of gold, in the form of a
brown powder, which remains undissolved, having been allowed to settle
to the bottom, the supernatant solution of silver is to be decanted
carefully off into a leaden cistern, and the powder being repeatedly
edulcorated with water, the washings are to be added to it. The silver
is now to be precipitated by plunging plates of copper in the solution,
and the magma which falls is to be well washed, and freed from the
residuary particles of sulphate of copper by powerful compression.
3. The silver, precipitated and dried as above described, is melted in a
crucible, and cast into an ingot.
4. The gold powder is also dried and cast into an ingot, a little nitre
being added in the fusion, to oxidize and separate any minute particles
of copper that may perchance have been protected from the solvent action
of the acid.
5. As the sulphate of copper is of considerable value, its solution is
to be neutralized, evaporated in leaden pans to a proper strength, and
set aside to crystallize in leaden cisterns. The farmers throughout
France consume an immense quantity of this salt. They sprinkle a weak
solution of it (at 2° or 3° Baumé) over their grain before sowing it, in
order to protect it against the ravages of birds and insects.
The pure gold, at the instant of its separation from the alloy by the
action of sulphuric acid, being in a very fine powder, and lying in
close contact with the platinum, under the influence of a boiling
menstruum, which brightens the surfaces of the two metals, and raises
their temperature to fully the 600th degree of Fahrenheit’s scale, tends
to become partially soldered to the platinum, and may thus progressively
thicken the bottom of the still. The importance of preserving this
vessel entire, and of economizing the fuel requisite to heat its
contents, induces the refiner to detach the crust of gold from time to
time, by passing over the bottom of the still, in small quantities, a
dilute nitro-muriatic acid, which acts readily on gold, but not on
platinum. But as this operation is a very delicate one, it must be
conducted with great circumspection. The danger of such adhering
deposits is much increased by using too high a heat, and too small a
body of acid, relatively to the metals dissolved. Hence it is
advantageous to employ alembics of large size. Should any lead or tin
get into the platinum still, while the hot acid is in it, the precious
vessel would be speedily destroyed; an accident which has not
unfrequently happened. Each operation may be conveniently finished in
twelve hours; so that each alembic may refine with ease 160 marcs
daily. Some persons work more rapidly, but such haste is hazardous.
The Parisian refiners restore to the owners the whole of the gold and
silver contained in the ingots, reserving to themselves the copper which
formed the alloy, and charging only the sum of 5-1/2 francs per
kilogramme (2·68 lbs. troy) for the expense of the parting of the
metals.
If they are employed to refine an ingot of silver containing less than
one-tenth of gold, they retain for themselves a two-thousandth part of
the gold, and all the copper, existing in the alloy; return all the rest
of the gold, with the whole of the silver, in the ingot; and give,
besides, to the owners a _premium_ or _bonus_, which amounted lately to
3/4 of a franc on the kilogramme of metal. Should the owner desire to
have the whole of the gold and silver contained in his ingot, the
refiner then demands from him 2 francs and 68 centimes per kilogramme,
retaining the copper of the alloy. As to silver ingots of low standard,
the perfection of the refining processes is such, that the mere copper
contained in them pays all the costs; for in this case, the refiner
restores to the proprietor of the ingot as much fine silver as the assay
indicated to exist in the ingot, contenting himself with the copper of
the alloy. See _infrà_.
The chemical works of M. Poizat, called _affinage d’argent_, on the bank
of the _canal de l’Ourcq_, in the vicinity of Paris, are undoubtedly the
most spacious and best arranged for refining the precious metals, which
exist in the world. On being introduced to this gentleman, by my friend
and companion M. Clement-Desormes, he immediately expressed his
readiness to conduct me through his _fabrique_, politely alluding to the
French translation of my Dictionary of Chemistry, which lay upon the
desk of his _bureau_. The principal room is 240 feet long, 40 feet wide,
and about 30 feet high. A lofty chimney rises up through the middle of
the apartment, and another at each of its ends. The one space, 120 feet
long, to the right of the central chimney, is allotted to the processes
of dissolving the silver, and parting the gold; the other, to the left,
to the evaporation and crystallization of the sulphate of copper, and
the concentration of the recovered sulphuric acid.
M. Poizat melts his great masses of silver in pots made of malleable
iron, capable of holding several cwts. each; and granulates it by
pouring it into water contained in large iron pans. The granulated
silver is dried with heat, and carried into a well lighted office
enclosed by glazed casements, to be weighed, registered, and divided
into determinate portions. Each of these is put into a cast-iron pot, of
a flattened hemispherical shape, about 2 feet in diameter, covered with
an iron lid, made in halves, and hinged together in the middle line.
From the top of the fixed lid a bent pipe issues, and proceeds downwards
into an oblong leaden chest sunk beneath the floor. Four of the above
cast-iron pots stand in a line across the room, divided into two ranges,
with an intervening space for passing between them. The bottoms of the
pots are directly heated by the flame, one fire serving for two pots.
Two parts of concentrated sulphuric acid by weight are poured upon every
part of granulated silver, and kept gently boiling till the whole silver
be converted into a pasty sulphate.
From the underground leaden chests, a leaden pipe, 4 inches in diameter,
rises vertically, and enters the side of a leaden chamber, which is
supported upon strong cross-beams or rafters, a little way beneath the
roof of the apartment. This chamber, which is 30 feet long, 10 feet
wide, and 6 feet high, is intended to condense the sulphuric acid
vapours, along with some of the sulphurous acid; that of the latter
being promoted by the admission of nitrous gas and air, which convert it
into sulphuric acid. From the further end of this chamber, a large
square leaden pipe returns with a slight slope towards the middle of the
room, and terminates at the right-hand side of the central chimney, in a
small leaden chest, for receiving the drops of acid which are condensed
in the pipe. From that chest a pipe issues, to discharge into the high
central chimney the incondensable gases, and also to maintain a constant
draught through the whole series of leaden chambers back to the
cast-iron hemispherical pots.
Besides the above cast-iron pots, destined to dissolve only the coarse
cupreous silver, containing a few grains of gold per pound, there are,
in the centre of the apartment, at the right-hand side of the chimney, 6
alembics of platinum, in which the rich alloys of gold and silver are
treated in the process of refining gold.
The pasty sulphate of silver obtained in the iron pots, is transferred
by cast-iron ladles with long handles into large leaden cisterns,
adjoining the pots, and there diluted with a little water to the density
of 36° Baumé. Into this liquor, steam is admitted through a series of
upright leaden pipes arranged along the side of the cistern, which
speedily causes ebullition, and dilutes the solution eventually to the
22d degree of Baumé. In this state, the liquid supersulphate is run off
by leaden syphons into large oblong leaden cisterns, rounded at the
bottom; and is there exposed to the action of ribands of copper, like
thin wood shavings. The metallic silver precipitates in a pasty form;
and the supernatant sulphate of copper is then run off into a cistern,
upon a somewhat lower level, where it is left to settle and become
clear.
The precipitate of silver, called by the English, water-silver, and by
the French, _chaux d’argent_, is drained, then strongly squeezed in a
square box of cast iron, by the action of a hydraulic press; in which 60
pounds of silver are operated upon at once.
The silver lumps are dried, melted in black lead crucibles, in a furnace
built near the silver end of the room, where the superintendent sits in
his _bureau_--a closet enclosed by glazed casements, like a green-house.
The whole course of the operations is so planned, that they are made to
commence near the centre with the mixed metals, and progressively
approach towards the office end of the apartment as the parting
processes advance. Here the raw material, after being granulated and
weighed, was given out, and here the pure gold and silver are finally
eliminated in a separate state.
In the other half of the hall, the solutions of sulphate of copper are
evaporated in large shallow leaden pans, placed over a range of
furnaces; from which, at the proper degree of concentration, they are
run off by syphons into crystallizing pans of the same metal. From the
mother-waters, duly evaporated, a second crop of crystals is obtained;
and also a third, the last being anhydrous, from the great affinity for
water possessed by the strong sulphuric acid with which they are now
surrounded. The acid in this way parts with almost the whole of the
cupreous oxide, and is then transferred into a large alembic of platinum
(value 1000_l._), to be rendered fit, by re-concentration, for acting
upon fresh portions of granulated silver. The capital of that alembic is
connected with a leaden-worm, which traverses an oblong vessel, through
which a stream of cold water flows.
The crystallized sulphate of copper fetched, two years ago, 30_l._ a
ton. It is almost all sold to the grocers in the towns of the
agricultural districts of France. In the above establishment of M.
Poizat, silver to the value of 10,000_l._ can be operated upon daily.
There is a steam engine of 6-horse power placed in a small glazed
chamber at one side of the parting hall, which serves to work all his
leaden pumps for lifting the dilute sulphuric acid and acidulous
solutions of copper into their appropriate cisterns of concentration, as
also to grind his old crucibles, and drive his amalgamation mill,
consisting of a pair of vertical round-edged wheels, working upon one
shaft, in a groove formed round a central hemisphere--of cast iron.
After the mercury has dissolved out of the ground crucibles all the
particles of silver which it can find, the residuary earthy matter is
sold to the _sweep-washers_. The floor of the hall around the alembics,
pots, and cisterns, is covered with an iron grating, made of bars having
one of their angles uppermost, to act as scrapers upon the shoes of the
operatives. The dust collects in a vacant space left beneath the
grating, whence it is taken to the amalgamation mill. The processes are
so well arranged and conducted by M. Poizat, that he can execute as much
business in his establishment with 10 workmen as is elsewhere done with
from 40 to 50; and with less than 3 grains of gold, in one Paris pound
or 7561 grains of silver, he can defray the whole expenses of the
parting or refining.
Since 26 parts of copper afford 100 of the crystallized sulphate, the
tenth of copper present in the dollars, and most foreign coins, will
yield nearly four times its weight of blue vitriol; a subsidiary product
of considerable value to the refiner.
The works of M. Poizat are so judiciously fitted up as to be quite
salubrious, and have not those “very mischievous effects upon the
trachea,” which Mr. Matheson states as being common in his refinery
works in the Royal Mint.[48] But, in fact, as refining by sulphuric acid
is always a nuisance to a neighbourhood, it is not suffered in the
_Monnaie Royale_ of Paris; but is best and most economically performed
by private enterprise and fair competition, which is impossible in
London, on account of the anomalous privilege, worth at least 2000_l._ a
year, possessed by Mr. Matheson, who works most extensively for private
profit on a public plant, fitted up with a lofty chimney, platinum
vessels to the value of 3000_l._, and other apparatus, at the cost of
the government. His charge to the crown for refining gold per lb. troy,
is 6_s._ 6_d._; that of the refiners in London, who are obliged, for
fear of prosecution, to employ the more expensive, but more condensable,
nitric acid, is only 4_s._ That of the Parisian refiners is regulated as
follows. For the dealers in the precious metals:--
[48] Report of Committee of House of Commons on the Mint, in 1837, p.
91.
For gold bullion containing silver, and more than 100/1000 of gold, 6
fr. 12 c. per kilogramme, = 2 fr. 29 c. per lb. troy.
For silver bullion, containing from 1/1000 to 100/1000 of gold (called
_dorés_), 3 fr. 27 c. per kilogramme, = 1 fr. 22 c. per lb. troy.