2. By amalgamation, i.e. forming a gold amalgam, afterwards removing the
mercury by distillation;
3. By chlorination, i.e. forming the soluble gold chloride and then
precipitating the metal;
4. By the cyanide process, i.e. dissolving the gold in potassium cyanide
solution, and then precipitating the metal;
5. Electrolytically, generally applied to the solutions obtained in
processes (3) and (4).
1. _Extraction of Gold by Washing._--In the early days of gold-washing
in California and Australia, when rich alluvial deposits were common
at the surface, the most simple appliances sufficed. The most
characteristic is the "pan," a circular dish of sheet-iron or "tin,"
with sloping sides about 13 or 14 in. in diameter. The pan, about
two-thirds filled with the "pay dirt" to be washed, is held in the
stream or in a hole filled with water. The larger stones having been
removed by hand, gyratory motion is given to the pan by a combination
of shaking and twisting movements so as to keep its contents
suspended in the stream of water, which carries away the bulk of the
lighter material, leaving the heavy minerals, together with any gold
which may have been present. The washing is repeated until enough of
the enriched sand is collected, when the gold is finally recovered by
careful washing or "panning out" in a smaller pan. In Mexico and South
America, instead of the pan, a wooden dish or trough, known as
"batea," is used.
The "cradle" is a simple appliance for treating somewhat larger
quantities, and consists essentially of a box, mounted on rockers, and
provided with a perforated bottom of sheet iron in which the "pay
dirt" is placed. Water is poured on the dirt, and the rocking motion
imparted to the cradle causes the finer particles to pass through the
perforated bottom on to a canvas screen, and thence to the base of the
cradle, where the auriferous particles accumulate on transverse bars
of wood, called "riffles."
The "tom" is a sort of cradle with an extended sluice placed on an
incline of about 1 in 12. The upper end contains a perforated riddle
plate which is placed directly over the riffle box, and under certain
circumstances mercury may be placed behind the riffles. Copper plates
amalgamated with mercury are also used when the gold is very fine, and
in some instances amalgamated silver coins have been used for the same
purpose. Sometimes the stuff is disintegrated with water in a
"puddling machine," which was used, especially in Australia, when the
earthy matters are tenacious and water scarce. The machine frequently
resembles a brickmaker's wash-mill, and is worked by horse or steam
power.
In workings on a larger scale, where the supply of water is abundant,
as in California, sluices were generally employed. They are shallow
troughs about 12 ft. long, about 16 to 20 in. wide and 1 ft. in depth.
The troughs taper slightly so that they can be joined in series, the
total length often reaching several hundred feet. The incline of the
sluice varies with the conformation of the ground and the tenacity of
the stuff to be washed, from 1 in 16 to 1 in 8. A rectangular trough
of boards, whose dimensions depend chiefly on the size of the planks
available, is set up on the higher part of the ground at one side of
the claim to be worked, upon trestles or piers of rough stone-work, at
such an inclination that the stream may carry off all but the largest
stones, which are kept back by a grating of boards about 2 in. apart.
The gravel is dug by hand and thrown in at the upper end, the stones
kept back being removed at intervals by two men with four-pronged
steel forks. The floor of the sluice is laid with riffles made of
strips of wood 2 in. square laid parallel to the direction of the
current, and at other points with boards having transverse notches
filled with mercury. These were known originally as Hungarian riffles.
In larger plant the upper ends of the sluices are often cut in rock or
lined with stone blocks, the grating stopping the larger stones being
known as a "grizzly." In order to save very fine and especially rusty
particles of gold, so-called "under-current sluices" are used; these
are shallow wooden tanks, 50 sq. yds. and upwards in area, which are
placed somewhat below the main sluice, and communicate with it above
and below, the entry being protected by a grating so that only the
finer material is admitted. These are paved with stone blocks or lined
with mercury riffles, so that from the greatly reduced velocity of
flow, due to the sudden increase of surface, the finer particles of
gold may collect. In order to save finely divided gold, amalgamated
copper plates are sometimes placed in a nearly level position, at a
considerable distance from the head of the sluice, the gold which is
retained in it being removed from time to time. Sluices are often made
double, and they are usually cleaned up--that is, the deposit rich in
gold is removed from them--once a week.
The "pan" is now only used by prospectors, while the "cradle" and
"tom" are practically confined to the Chinese; the sluice is
considered to be the best contrivance for washing gold gravels.
2. _The Amalgamation Process._--This method is employed to extract gold
from both alluvial and reef deposits: in the first case it is combined
with "hydraulic mining," i.e. disintegrating auriferous gravels by
powerful jets of water, and the sluice system described above; in the
second case the vein stuff is prepared by crushing and the amalgamation
is carried out in mills.
Hydraulic mining has for the most part been confined to the country of
its invention, California, and the western territories of America,
where the conditions favourable for its use are more fully developed
than elsewhere--notably the presence of thick banks of gravel that
cannot be utilized by other methods, and abundance of water, even
though considerable work may be required at times to make it
available. The general conditions to be observed in such workings may
be briefly stated as follows: (1) The whole of the auriferous gravel,
down to the "bed rock," must be removed,--that is, no selection of
rich or poor parts is possible; (2) this must be accomplished by the
aid of water alone, or at times by water supplemented by blasting; (3)
the conglomerate must be mechanically disintegrated without
interrupting the whole system; (4) the gold must be saved without
interrupting the continuous flow of water; and (5) arrangements must
be made for disposing of the vast masses of impoverished gravel.
The water is brought from a ditch on the high ground, and through a
line of pipes to the distributing box, whence the branch pipes
supplying the jets diverge. The stream issues through a nozzle,
termed a "monitor" or "giant," which is fitted with a ball and socket
joint, so that the direction of the jet may be varied through
considerable angles by simply moving a handle. The material of the
bank being loosened by blasting and the cutting action of the water,
crumbles into holes, and the superincumbent mass, often with large
trees and stones, falls into the lower ground. The stream, laden with
stones and gravel, passes into the sluices, where the gold is
recovered in the manner already described. Under the most advantageous
conditions the loss of gold may be estimated at 15 or 20%, the amount
recovered representing a value of about two shillings per ton of
gravel treated. The loss of mercury is about the same, from 5 to 6
cwt. being in constant use per mile of sluice.
In working auriferous river-beds, dredges have been used with
considerable success in certain parts of New Zealand and on the
Pacific slope in America. The dredges used in California are almost
exclusively of the endless-chain bucket or steam-shovel pattern. Some
dredges have a capacity under favourable conditions of over 2000 cub.
yds. of gravel daily. The gravel is excavated as in the ordinary form
of endless-chain bucket dredge and dumped on to the deck of the
dredge. It then passes through screens and grizzlies to retain the
coarse gravel, the finer material passing on to sluice boxes provided
with riffles, supplied with mercury. There are belt conveyers for
discharging the gravel and tailings at the end of the vessel remote
from the buckets. The water necessary to the process is pumped from
the river; as much as 2000 gallons per minute is used on the larger
dredges.
The dressing or mechanical preparation of vein stuff containing gold
is generally similar to that of other ores (see ORE-DRESSING), except
that the precious metal should be removed from the waste substances as
quickly as possible, even although other minerals of value that are
subsequently recovered may be present. In all cases the quartz or
other vein stuff must be reduced to a very fine powder as a
preliminary to further operations. This may be done in several ways,
e.g. either (1) by the Mexican crusher or _arrastra_, in which the
grinding is effected upon a bed of stone, over which heavy blocks of
stone attached to cross arms are dragged by the rotation of the arms
about a central spindle, or (2) by the Chilean mill or _trapiche_,
also known as the edge-runner, where the grinding stones roll upon the
floor, at the same time turning about a central upright--contrivances
which are mainly used for the preparation of silver ores; but by far
the largest proportion of the gold quartz of California, Australia and
Africa is reduced by (3) the stamp mill, which is similar in principle
to that used in Europe for the preparation of tin and other ores.
The stamp mill was first used in California, and its use has since
spread over the whole world. In the mills of the Californian type the
stamp is a cylindrical iron pestle faced with a chilled cast iron
shoe, removable so that it can be renewed when necessary, attached to
a round iron rod or lifter, the whole weighing from 600 to 900 lb.;
stamps weighing 1320 lb. are in use in the Transvaal. The lift is
effected by cams acting on the under surface of tappets, and formed by
cylindrical boxes keyed on to the stems of the lifter about one-fourth
of their length from the top. As, however, the cams, unlike those of
European stamp mills, are placed to one side of the stamp, the latter
is not only lifted but turned partly round on its own axis, whereby
the shoes are worn down uniformly. The height of lift may be between 4
and 18 in., and the number of blows from 30 to over 100 per minute.
The stamps are usually arranged in batteries of five; the order of
working is usually 1, 4, 2, 5, 3, but other arrangements, e.g. 1, 3,
5, 2, 4, and 1, 5, 2, 4, 3, are common. The stuff, previously broken
to about 2-in. lumps in a rock-breaker, is fed in through an aperture
at the back of the "battery box," a constant supply of water is
admitted from above, and mercury in a finely divided state is added at
frequent intervals. The discharge of the comminuted material takes
place through an aperture, which is covered by a thin steel plate
perforated with numerous slits about 1/50th in. broad and 1/2 in.
long, a certain volume being discharged at every blow and carried
forward by the flushing water over an apron or table in front, covered
by copper plates filled with mercury. Similar plates are often used to
catch any particles of gold that may be thrown back, while the main
operation is so conducted that the bulk of the gold may be reduced to
the state of amalgam by bringing the two metals into intimate contact
under the stamp head, and remain in the battery. The tables in front
are laid at an incline of about 8 deg. and are about 13 ft. long; they
collect from 10 to 15% of the whole gold; a further quantity is
recovered by leading the sands through a gutter about 16 in. broad and
120 ft. long, also lined with amalgamated copper plates, after the
pyritic and other heavy minerals have been separated by depositing in
catch pits and other similar contrivances.
When the ore does not contain any considerable amount of free gold
mercury is not, as a rule, used during the crushing, but the
amalgamation is carried out in a separate plant. Contrivances of the
most diverse constructions have been employed. The most primitive is
the rubbing together of the concentrated crushings with mercury in
iron mortars. Barrel amalgamation, i.e. mixing the crushings with
mercury in rotating barrels, is rarely used, the process being
wasteful, since the mercury is specially apt to be "floured" (see
below).
At Schemnitz, Kerpenyes, Kreuzberg and other localities in Hungary,
quartz vein stuff containing a little gold, partly free and partly
associated with pyrites and galena, is, after stamping in mills,
similar to those described above, but without rotating stamps, passed
through the so-called "Hungarian gold mill" or "quick-mill." This
consists of a cast-iron pan having a shallow cylindrical bottom
holding mercury, in which a wooden muller, nearly of the same shape as
the inside of the pan, and armed below with several projecting blades,
is made to revolve by gearing wheels. The stuff from the stamps is
conveyed to the middle of the muller, and is distributed over the
mercury, when the gold subsides, while the quartz and lighter
materials are guided by the blades to the circumference and are
discharged, usually into a second similar mill, and subsequently pass
over blanket tables, i.e. boards covered with canvas or sacking, the
gold and heavier particles becoming entangled in the fibres. The
action of this mill is really more nearly analogous to that of a
centrifugal pump, as no grinding action takes place in it. The amalgam
is cleaned out periodically--fortnightly or monthly--and after
filtering through linen bags to remove the excess of mercury, it is
transferred to retorts for distillation (see below).
Many other forms of pan-amalgamators have been devised. The Laszlo is
an improved Hungarian mill, while the Piccard is of the same type. In
the Knox and Boss mills, which are also employed for the amalgamation
of silver ores, the grinding is effected between flat horizontal
surfaces instead of conical or curved surfaces as in the previously
described forms.
One of the greatest difficulties in the treatment of gold by
amalgamation, and more particularly in the treatment of pyrites,
arises from the so-called "sickening" or "flouring" of the mercury;
that is, the particles, losing their bright metallic surfaces, are no
longer capable of coalescing with or taking up other metals. Of the
numerous remedies proposed the most efficacious is perhaps sodium
amalgam. It appears that amalgamation is often impeded by the tarnish
found on the surface of the gold when it is associated with sulphur,
arsenic, bismuth, antimony or tellurium. Henry Wurtz in America (1864)
and Sir William Crookes in England (1865) made independently the
discovery that, by the addition of a small quantity of sodium to the
mercury, the operation is much facilitated. It is also stated that
sodium prevents both the "sickening" and the "flouring" of the mercury
which is produced by certain associated minerals. The addition of
potassium cyanide has been suggested to assist the amalgamation and to
prevent "flouring," but Skey has shown that its use is attended with
loss of gold.
_Separation of Gold from the Amalgam._--The amalgam is first pressed
in wetted canvas or buckskin in order to remove excess of mercury.
Lumps of the solid amalgam, about 2 in. in diameter, are introduced
into an iron vessel provided with an iron tube that leads into a
condenser containing water. The distillation is then effected by
heating to dull redness. The amalgam yields about 30 to 40% of gold.
Horizontal cylindrical retorts, holding from 200 to 1200 lb. of
amalgam, are used in the larger Californian mills, pot retorts being
used in the smaller mills. The bullion left in the retorts is then
melted in black-lead crucibles, with the addition of small quantities
of suitable fluxes, e.g. nitre, sodium carbonate, &c.
The extraction of gold from auriferous minerals by fusion, except as
an incident in their treatment for other metals, is very rarely
practised. It was at one time proposed to treat the concentrated black
iron obtained in the Ural gold washings, which consists chiefly of
magnetite, as an iron ore, by smelting it with charcoal for auriferous
pig-iron, the latter metal possessing the property of dissolving gold
in considerable quantity. By subsequent treatment with sulphuric acid
the gold could be recovered. Experiments on this point were made by
Anossow in 1835, but they have never been followed in practice.
Gold in galena or other lead ores is invariably recovered in the
refining or treatment of the lead and silver obtained. Pyritic ores
containing copper are treated by methods analogous to those of the
copper smelter. In Colorado the pyritic ores containing gold and
silver in association with copper are smelted in reverberatory
furnaces for regulus, which, when desilverized by Ziervogel's method,
leaves a residue containing 20 or 30 oz. of gold per ton. This is
smelted with rich gold ores, notably those containing tellurium, for
white metal or regulus; and by a following process of partial
reduction analogous to that of selecting in copper smelting, "bottoms"
of impure copper are obtained in which practically all the gold is
concentrated. By continuing the treatment of these in the ordinary way
of refining, poling and granulating, all the foreign matters other
than gold, copper and silver are removed, and, by exposing the
granulated metal to a high oxidizing heat for a considerable time the
copper may be completely oxidized while the precious metals are
unaltered. Subsequent treatment with sulphuric acid renders the copper
soluble in water as sulphate, and the final residue contains only gold
and silver, which is parted or refined in the ordinary way. This
method of separating gold from copper, by converting the latter into
oxide and sulphate, is also used at Oker in the Harz.
_Extraction by Means of Aqueous Solutions._--Many processes have been
suggested in which the gold of auriferous deposits is converted into
products soluble in water, from which solutions the gold may be
precipitated. Of these processes, two only are of special importance,
viz. the chlorination or Plattner process, in which the metal is
converted into the chloride, and the cyanide or MacArthur-Forrest
process, in which it is converted into potassium aurocyanide.
(3) _Chlorination or Plattner Process._--In this process moistened
gold ores are treated with chlorine gas, the resulting gold chloride
dissolved out with water, and the gold precipitated with ferrous
sulphate, charcoal, sulphuretted hydrogen or otherwise. The process
originated in 1848 with C. F. Plattner, who suggested that the
residues from certain mines at Reichenstein, in Silesia, should be
treated with chlorine after the arsenical products had been extracted
by roasting. It must be noticed, however, that Percy independently
made the same discovery, and stated his results at the meeting of the
British Association (at Swansea) in 1849, but the Report was not
published until 1852. The process was introduced in 1858 by Deetken at
Grass Valley, California, where the waste minerals, principally
pyrites from tailings, had been worked for a considerable time by
amalgamation. The process is rarely applied to ores direct;
free-milling ores are generally amalgamated, and the tailings and
slimes, after concentration, operated upon. Three stages in the
process are to be distinguished: (i) calcination, to convert all the
metals, except gold and silver, into oxides, which are unacted upon by
chlorine; (ii.) chlorinating the gold and lixiviating the product;
(iii.) precipitating the gold.
The calcination, or roasting, is conducted at a low temperature in
some form of reverberatory furnace. Salt is added in the roasting to
convert any lime, magnesia or lead which may be present, into the
corresponding chlorides. The auric chloride is, however, decomposed at
the elevated temperature into finely divided metallic gold, which is
then readily attacked by the chlorine gas. The high volatility of gold
in the presence of certain metals must also be considered. According
to Egleston the loss may be from 40 to 90% of the total gold present
in cupriferous ores according to the temperature and duration of
calcination. The roasted mineral, slightly moistened, is introduced
into a vat made of stoneware or pitched planks, and furnished with a
double bottom. Chlorine, generally prepared by the interaction of
pyrolusite, salt and sulphuric acid, is led from a suitable generator
beneath the false bottom, and rises through the moistened ore, which
rests on a bed of broken quartz; the gold is thus converted into a
soluble chloride, which is afterwards removed by washing with water.
Both fixed and rotating vats are employed, the chlorination proceeding
more rapidly in the latter case; rotating barrels are sometimes used.
There have also been introduced processes in which the chlorine is
generated in the chloridizing vat, the reagents used being dilute
solutions of bleaching powder and an acid. Munktell's process is of
this type. In the Thies process, used in many districts in the United
States, the vats are rotating barrels made, in the later forms, of
iron lined with lead, and provided with a filter formed of a finely
perforated leaden grating running from one end of the barrel to the
other, and rigidly held in place by wooden frames. Chlorine is
generated within the barrel from sulphuric acid and chloride of lime.
After charging, the barrel is rotated, and when the chlorination is
complete the contents are emptied on a filter of quartz or some
similar material, and the filtrate led to settling tanks.
After settling the solution is run into the precipitating tanks. The
precipitants in use are: ferrous sulphate, charcoal and sulphuretted
hydrogen, either alone or mixed with sulphur dioxide; the use of
copper and iron sulphides has been suggested, but apparently these
substances have achieved no success.
In the case of ferrous sulphate, prepared by dissolving iron in dilute
sulphuric acid, the reaction follows the equation AuCl3 + 3FeSO4 =
FeCl3 + Fe2(SO4)3 + Au. At the same time any lead, calcium, barium and
strontium present are precipitated as sulphates; it is therefore
advantageous to remove these metals by the preliminary addition of
sulphuric acid, which also serves to keep any basic iron salts in
solution. The precipitation is carried out in tanks or vats made with
wooden sides and a cement bottom. The solutions are well mixed by
stirring with wooden poles, and the gold allowed to settle, the time
allowed varying from 12 to 72 hours. The supernatant liquid is led
into settling tanks, where a further amount of gold is deposited, and
is then filtered through sawdust or sand, the sawdust being afterwards
burnt and the gold separated from the ashes and the sand treated in
the chloridizing vat. The precipitated gold is washed, treated with
salt and sulphuric acid to remove iron salts, roughly dried by
pressing in cloths or on filter paper, and then melted with salt,
borax and nitre in graphite crucibles. Thus prepared it has a fineness
of 800-960, the chief impurities usually being iron and lead.
Charcoal is used as the precipitant at Mount Morgan, Australia. Its
use was proposed as early as 1818 and 1819 by Hare and Henry; Percy
advocated it in 1869, and Davis adopted it on the large scale at a
works in Carolina in 1880. The action is not properly understood; it
may be due to the reducing gases (hydrogen, hydrocarbons, &c.) which
are invariably present in wood charcoal. The process consists
essentially in running the solution over layers of charcoal, the
charcoal being afterwards burned. It has been found that the reaction
proceeds faster when the solution is heated.
Precipitation with sulphur dioxide and sulphuretted hydrogen proceeds
much more rapidly, and has been adopted at many works. Sulphur
dioxide, generated by burning sulphur, is forced into the solution
under pressure, where it interacts with any free chlorine present to
form hydrochloric and sulphuric acids. Sulphuretted hydrogen, obtained
by treating iron sulphide or a coarse matte with dilute sulphuric
acid, is forced in similarly. The gold is precipitated as the
sulphide, together with any arsenic, antimony, copper, silver and lead
which may be present. The precipitate is collected in a filter-press,
and then roasted in muffle furnaces with nitre, borax and sodium
carbonate. The fineness of the gold so obtained is 900 to 950.
4. _Cyanide Process._--This process depends upon the solubility of
gold in a dilute solution of potassium cyanide in the presence of air
(or some other oxidizing agent), and the subsequent precipitation of
the gold by metallic zinc or by electrolysis. The solubility of gold
in cyanide solutions was known to K. W. Scheele in 1782; and M.
Faraday applied it to the preparation of extremely thin films of the
metal. L. Eisner recognized, in 1846, the part played by the
atmosphere, and in 1879 Dixon showed that bleaching powder, manganese
dioxide, and other oxidizing agents, facilitated the solution. S. B.
Christy (_Trans. A.I.M.E._, 1896, vol. 26) has shown that the solution
is hastened by many oxidizing agents, especially sodium and manganese
dioxides and potassium ferricyanide. According to G. Bodlander (_Zeit.
f. angew. Chem._, 1896, vol. 19) the rate of solution in potassium
cyanide depends upon the subdivision of the gold--the finer the
subdivision the quicker the solution,--and on the concentration of the
solution--the rate increasing until the solution contains 0.25% of
cyanide, and remaining fairly stationary with increasing
concentration. The action proceeds in two stages; in the first
hydrogen peroxide and potassium aurocyanide are formed, and in the
second the hydrogen peroxide oxidizes a further quantity of gold and
potassium cyanide to aurocyanide, thus (1) 2Au + 4KCN + O2 + 2H2O =
2KAu(CN)2 + 4KOH + H2O2; (2) 2Au + 4KCN + 2H2O2 = 2KAu(CN)2 + 4KOH.
The end reaction may be written 4Au + 8KCN + 2H2O + O2 = 4KAu(CN)2 +
4KOH.
The commercial process was patented in 1890 by MacArthur and Forrest,
and is now in use all over the world. It is best adapted for
free-milling ores, especially after the bulk of the gold has been
removed by amalgamation. It has been especially successful in the
Transvaal. In the Witwatersrand the ore, which contains about 9 dwts.
of gold to the metric ton (2000 lb.), is stamped and amalgamated, and
the slimes and tailings, containing about 3-1/2 dwts. per ton, are
cyanided, about 2 dwts. more being thus extracted. The total cost per
ton of ore treated is about 6s., of which the cyaniding costs from 2s.
to 4s.
The process embraces three operations: (1) Solution of the gold; (2)
precipitation of the gold; (3) treatment of the precipitate.
The ores, having been broken and ground, generally in tube mills,
until they pass a 150 to 200-mesh sieve, are transferred to the
leaching vats, which are constructed of wood, iron or masonry; steel
vats, coated inside and out with pitch, of circular section and
holding up to 1000 tons, have come into use. The diameter is generally
26 ft., but may be greater; the best depth is considered to be a
quarter of the diameter. The vats are fitted with filters made of
coco-nut matting and jute cloth supported on wooden frames. The
leaching is generally carried out with a strong, medium, and with a
weak liquor, in the order given; sometimes there is a preliminary
leaching with a weak liquor. The strengths employed depend also upon
the mode of precipitation adopted, stronger solutions (up to 0.25%
KCN) being used when zinc is the precipitant. For electrolytic
precipitation the solution may contain up to 0.1% KCN. The liquors are
run off from the vats to the electrolysing baths or precipitating
tanks, and the leached ores are removed by means of doors in the sides
of the vats into wagons. In the Transvaal the operation occupies 3-1/2
to 4 days for fine sands, and up to 14 days for coarse sands; the
quantity of cyanide per ton of tailings varies from 0.26 to 0.28 lb.,
for electrolytic precipitation, and 0.5 lb. for zinc precipitation.
The precipitation is effected by zinc in the form of bright turnings,
or coated with lead, or by electrolysis. According to Christy, the
precipitation with zinc follows equations 1 or 2 according as
potassium cyanide is present or not:
(1) 4KAu(CN)2 + 4Zn + 2H2O = 2Zn(CN)2 + K2Zn(CN)4 + Zn(OK)2 + 4H + 4Au;
(2) 2KAu(CN)2 + 3Zn + 4KCN + 2H2O = 2K2Zn(CN)4 + Zn(OK)2 + 4H + 2Au;
one part of zinc precipitating 3.1 parts of gold in the first case,
and 2.06 in the second. It may be noticed that the potassium zinc
cyanide is useless in gold extraction, for it neither dissolves gold
nor can potassium cyanide be regenerated from it.
The precipitating boxes, generally made of wood but sometimes of
steel, and set on an incline, are divided by partitions into
alternately wide and narrow compartments, so that the liquor travels
upwards in its passage through the wide divisions and downwards
through the narrow divisions. In the wider compartments are placed
sieves having sixteen holes to the square inch and bearing zinc
turnings. The gold and other metals are precipitated on the under
surfaces of the turnings and fall to the bottom of the compartment as
a black slime. The slime is cleaned out fortnightly or monthly, the
zinc turnings being cleaned by rubbing and the supernatant liquor
allowed to settle in the precipitating boxes or in separate vessels.
The slime so obtained consists of finely divided gold and silver
(5-50%), zinc (30-60%), lead (10%), carbon (10%), together with tin,
copper, antimony, arsenic and other impurities of the zinc and ores.
After well washing with water, the slimes are roughly dried in
bag-filters or filter-presses, and then treated with dilute sulphuric
acid, the solution being heated by steam. This dissolves out the zinc.
Lime is added to bring down the gold, and the sediment, after washing
and drying, is fused in graphite crucibles.
5. _Electrolytic Processes._--The electrolytic separation of the gold
from cyanide solutions was first practised in the Transvaal. The
process, as elaborated by Messrs. Siemens and Halske, essentially
consists in the electrolysis of weak solutions with iron or steel
plate anodes, and lead cathodes, the latter, when coated with gold,
being fused and cupelled. Its advantages over the zinc process are
that the deposited gold is purer and more readily extracted, and that
weaker solutions can be employed, thereby effecting an economy in
cyanide.
In the process employed at the Worcester Works in the Transvaal, the
liquors, containing about 150 grains of gold per ton and from 0.08 to
0.01% of cyanide, are treated in rectangular vats in which is placed a
series of iron and leaden plates at intervals of 1 in. The cathodes,
which are sheets of thin lead foil weighing 1-1/2 lb. to the sq. yd.,
are removed monthly, their gold content being from 0.5 to 10%, and
after folding are melted in reverberatory furnaces to ingots
containing 2 to 4% of gold. Cupellation brings up the gold to about
900 fine. Many variations of the electrolytic process as above
outlined have been suggested. S. Cowper Coles has suggested aluminium
cathodes; Andreoli has recommended cathodes of iron and anodes of lead
coated with lead peroxide, the gold being removed from the iron
cathodes by a brief immersion in molten lead; in the Pelatan-Cerici
process the gold is amalgamated at a mercury cathode (see also below).
_Refining or Parting of Gold._--Gold is almost always silver-bearing,
and it may be also noticed that silver generally contains some gold.
Consequently the separation of these two metals Is one of the most
important metallurgical processes. In addition to the separation of the
silver the operation extends to the elimination of the last traces of
lead, tin, arsenic, &c. which have resisted the preceding cupellation.
The "parting" of gold and silver is of considerable antiquity. Thus
Strabo states that in his time a process was employed for refining and
purifying gold in large quantities by cementing or burning it with an
aluminous earth, which, by destroying the silver, left the gold in a
state of purity. Pliny shows that for this purpose the gold was placed
on the fire in an earthen vessel with treble its weight of salt, and
that it was afterwards again exposed to the fire with two parts of
salt and one of argillaceous rock, which, in the presence of moisture,
effected the decomposition of the salt; by this means the silver
became converted into chloride.
The methods of parting can be classified into "dry," "wet" and
electrolytic methods. In the "dry" methods the silver is converted
into sulphide or chloride, the gold remaining unaltered; in the "wet"
methods the silver is dissolved by nitric acid or boiling sulphuric
acid; and in the electrolytic processes advantage is taken of the fact
that under certain current densities and other circumstances silver
passes from an anode composed of a gold-silver alloy to the cathode
more readily than gold. Of the dry methods only F. B. Miller's
chlorine process is of any importance, this method, and the wet
process of refining by sulphuric acid, together with the electrolytic
process, being the only ones now practised.
The conversion of silver into the sulphide may be effected by heating
with antimony sulphide, litharge and sulphur, pyrites, or with sulphur
alone. The antimony, or _Guss und Fluss_, method was practised up till
1846 at the Dresden mint; it is only applicable to alloys containing
more than 50% of gold. The fusion results in the formation of a
gold-antimony alloy, from which the antimony is removed by an
oxidizing fusion with nitre. The sulphur and litharge, or
_Pfannenschmied_, process was used to concentrate the gold in an alloy
in order to make it amenable to "quartation," or parting with nitric
acid. Fusion with sulphur was used for the same purpose as the
Pfannenschmied process. It was employed in 1797 at the St Petersburg
mint.
The conversion of the silver into the chloride may be effected by
means of salt--the "cementation" process--or other chlorides, or by
free chlorine--Miller's process. The first process consists
essentially in heating the alloy with salt and brickdust; the latter
absorbs the chloride formed, while the gold is recovered by washing.
It is no longer employed. The second process depends upon the fact
that, if chlorine be led into the molten alloy, the base metals and
the silver are converted into chlorides. It was proposed in 1838 by
Lewis Thompson, but it was only applied commercially after Miller's
improvements in 1867, when it was adopted at the Sydney mint. Sir W.
C. Roberts-Austen introduced it at the London mint; and it has also
been used at Pretoria. It is especially suitable to gold containing
little silver and base metals--a character of Australian gold--but it
yields to the sulphuric acid and electrolytic methods in point of
economy.
The separation of gold from silver in the wet way may be effected by
nitric acid, sulphuric acid or by a mixture of sulphuric acid and
_aqua regia_.
Parting by nitric acid is of considerable antiquity, being mentioned
by Albertus Magnus (13th cent.), Biringuccio (1540) and Agricola
(1556). It is now rarely practised, although in some refineries both
the nitric acid and the sulphuric acid processes are combined, the
alloy being first treated with nitric acid. It used to be called
"quartation" or "inquartation," from the fact that the alloy best
suited for the operation of refining contained 3 parts of silver to 1
of gold. The operation may be conducted in vessels of glass or
platinum, and each pound of granulated metal is treated with a pound
and a quarter of nitric acid of specific gravity 1.32. The method is
sometimes employed in the assay of gold.
Refining by sulphuric acid, the process usually adopted for separating
gold from silver, was first employed on the large scale by d'Arcet in
Paris in 1802, and was introduced into the Mint refinery, London, by
Mathison in 1829. It is based upon the facts that concentrated hot
sulphuric acid converts silver and copper into soluble sulphates
without attacking the gold, the silver sulphate being subsequently
reduced to the metallic state by copper plates with the formation of
copper sulphate. It is applicable to any alloy, and is the best method
for parting gold with the exception of the electrolytic method.
The process embraces four operations: (1) the preparation of an alloy
suitable for parting; (2) the treatment with sulphuric acid; (3) the
treatment of the residue for gold; (4) the treatment of the solution
for silver.
It is necessary to remove as completely as possible any lead, tin,
bismuth, antimony, arsenic and tellurium, impurities which impair the
properties of gold and silver, by an oxidizing fusion, e.g. with
nitre. Over 10% of copper makes the parting difficult; consequently in
such alloys the percentage of copper is diminished by the addition of
silver free from copper, or else the copper is removed by a chemical
process. Other undesirable impurities are the platinum metals, special
treatment being necessary when these substances are present. The
alloy, after the preliminary refining, is granulated by being poured,
while molten, in a thin stream into cold water which is kept well
agitated.
The acid treatment is generally carried out in cast iron pots;
platinum vessels used to be employed, while porcelain vessels are only
used for small operations, e.g. for charges of 190 to 225 oz. as at
Oker in the Harz. The pots, which are usually cylindrical with a
hemispherical bottom, may hold as much as 13,000 to 16,000 oz. of
alloy. They are provided with lids, made either of lead or of wood
lined with lead, which have openings to serve for the introduction of
the alloy and acid, and a vent tube to lead off the vapours evolved
during the operation. The bullion with about twice its weight of
sulphuric acid of 66 deg. Be is placed in the pot, and the whole
gradually heated. Since the action is sometimes very violent,
especially when the bullion is treated in the granulated form (it is
steadier when thin plates are operated upon), it is found expedient to
add the acid in several portions. The heating is continued for 4 to 12
hours according to the amount of silver present; the end of the
reaction is known by the absence of any hissing. Generally the
reaction mixture is allowed to cool, and the residue, which settles to
the bottom of the pot, consists of gold together with copper, lead and
iron sulphates, which are insoluble in strong sulphuric acid; silver
sulphate may also separate if present in sufficient quantity and the
solution be sufficiently cooled. The solution is removed by ladles or
by siphons, and the residue is leached out with boiling water; this
removes the sulphates. A certain amount of silver is still present
and, according to M. Pettenkofer, it is impossible to remove all the
silver by means of sulphuric acid. Several methods are in use for
removing the silver. Fusion with an alkaline bisulphate converts the
silver into the sulphate, which may be extracted by boiling with
sulphuric acid and then with water. Another process consists in
treating a mixture of the residue with one-quarter of its weight of
calcined sodium sulphate with sulphuric acid, the residue being
finally boiled with a large quantity of acid. Or the alloy is
dissolved in _aqua regia_, the solution filtered from the insoluble
silver chloride, and the gold precipitated by ferrous chloride.
The silver present in the solution obtained in the sulphuric acid
boiling is recovered by a variety of processes. The solution may be
directly precipitated with copper, the copper passing into solution as
copper sulphate, and the silver separating as a mud, termed "cement
silver." Or the silver sulphate may be separated from the solution by
cooling and dilution, and then mixed with iron clippings, the
interaction being accompanied with a considerable evolution of heat.
Or Gutzkow's method of precipitating the metal with ferrous sulphate
may be employed.
The electrolytic parting of gold and silver has been shown to be more
economical and free from the objections--such as the poisonous
fumes--of the sulphuric acid process. One process depends upon the
fact that, with a suitable current density, if a very dilute solution
of silver nitrate be electrolysed between an auriferous silver anode
and a silver cathode, the silver of the anode is dissolved out and
deposited at the cathode, the gold remaining at the anode. The silver
is quite free from gold, and the gold after boiling with nitric acid
has a fineness of over 999.
Gold is left in the anode slime when copper or silver are refined by
the usual processes, but if the gold preponderate in the anode these
processes are inapplicable. A cyanide bath, as used in electroplating,
would dissolve the gold, but is not suitable for refining, because
other metals (silver, copper, &c.) passing with gold into the solution
would deposit with it. Bock, however, in 1880 (_Berg- und
huttenmannische Zeitung_, 1880, p. 411) described a process used at
the North German Refinery in Hamburg for the refining of gold
containing platinum with a small proportion of silver, lead or
bismuth, and a subsequent patent specification (1896) and a paper by
Wohlwill (_Zeits. f. Elektrochem._, 1898, pp. 379, 402, 421) have
thrown more light upon the process. The electrolyte is gold chloride
(2.5-3 parts of pure gold per 100 of solution) mixed with from 2 to 6%
of the strongest hydrochloric acid to render the gold anodes readily
soluble, which they are not in the neutral chloride solution. The bath
is used at 65 deg. to 70 deg. C. (150 deg. to 158 deg. F.), and if
free chlorine be evolved, which is known at once by its pungent smell,
the temperature is raised, or more acid is added, to promote the
solubility of the gold. The bath is used with a current-density of 100
amperes per sq. ft. at 1 volt (or higher), with electrodes about 1.2
in. apart. In this process all the anode metals pass into solution
except iridium and other refractory metals of that group, which remain
as metals, and silver, which is converted into insoluble chloride;
lead and bismuth form chloride and oxychloride respectively, and these
dissolve until the bath is saturated with them, and then precipitate
with the silver in the tank. But if the gold-strength of the bath be
maintained, only gold is deposited at the cathode--in a loose powdery
condition from pure solutions, but in a smooth detachable deposit from
impure liquors. Under good conditions the gold should contain 99.98%
of the pure metal. The tank is of porcelain or glazed earthenware, the
electrodes for impure solutions are 1/2 in. apart (or more with pure
solutions), and are on the multiple system, and the potential
difference at the terminals of the bath is 1 volt. A high
current-density being employed, the turn-over of gold is rapid--an
essential factor of success when the costliness of the metal is taken
into account. Platinum and palladium dissolved from the anode
accumulate in the solution, and are removed at intervals of, say, a
few months by chemical precipitation. It is essential that the bath
should not contain more than 5% of palladium, or some of this metal
will deposit with the gold. The slimes are treated chemically for the
separation of the metals contained in them.
AUTHORITIES.--Standard works on the metallurgy of gold are the
treatises of T. Kirke Rose and of M. Eissler. The cyanide process is
especially treated by M. Eissler, _Cyanide Process for the Extraction
of Gold_, which pays particular attention to the Witwatersrand
methods; Alfred James, _Cyanide Practice_; H. Forbes Julian and Edgar
Smart, _Cyaniding Gold and Silver Ores_. Gold milling is treated by
Henry Louis, _A Handbook of Gold Milling_; C. G. Warnford Lock, _Gold
Milling_; T. A. Rickard, _Stamp Milling of Gold Ores_. Gold dredging
is treated by Captain C. C. Longridge in _Gold Dredging_, and
hydraulic mining is discussed by the same author in his _Hydraulic
Mining_. For operations in special districts see J. M. Maclaren,
_Gold_ (1908); J. H. Curle, _Gold Mines of the World_; Africa: F. H.
Hatch and J. A. Chalmers, _Gold Mines of the Rand_; S. J. Truscott,
_Witwatersrand Goldfields Banket and Mining Practice_; Australasia: D.
Clark, _Australian Mining and Metallurgy_; Karl Schmeisser,
_Goldfields of Australasia_; A. G. Charleton, _Gold Mining and Milling
in Western Australia_; India: F. H. Hatch, _The Kolar Gold-Field_.
GOLD AND SILVER THREAD. Under this heading some general account may be
given of gold and silver strips, threads and gimp used in connexion with
varieties of weaving, embroidery and twisting and plaiting or lace work.
To this day, in many oriental centres where it seems that early
traditions of the knowledge and the use of fabrics wholly or partly
woven, ornamented, and embroidered with gold and silver have been
maintained, the passion for such brilliant and costly textiles is still
strong and prevalent. One of the earliest mentions of the use of gold in
a woven fabric occurs in the description of the ephod made for Aaron
(Exod. xxxix. 2, 3), "And he made the ephod of gold, blue, and purple,
and scarlet, and fine twined linen. And they did beat the gold into thin
plates, and cut it into wires (strips), to work it in the blue, and in
the purple, and in the scarlet, and in the fine linen, with cunning
work." This is suggestive of early Syrian or Arabic in-darning or
weaving with gold strips or tinsel. In both the _Iliad_ and the
_Odyssey_ allusion is frequently made to inwoven and embroidered golden
textiles. Assyrian sculpture gives an elaborately designed ornament upon
the robe of King Assur-nasir-pal (884 B.C.) which was probably an
interweaving of gold and coloured threads, and testifies to the
consummate skill of Assyrian or Babylonian workers at that date. From
Assyrian and Babylonian weavers the conquering Persians of the time of
Darius derived their celebrity as weavers and users of splendid stuffs.
Herodotus describes the corselet given by Amasis king of Egypt to the
Minerva of Lindus and how it was inwoven or embroidered with gold.
Darius, we are told, wore a war mantle on which were figured (probably
inwoven) two golden hawks as if pecking at each other. Alexander the
Great is said to have found Eastern kings and princes arrayed in robes
of gold and purple. More than two hundred years later than Alexander the
Great was the king of Pergamos (the third bearing the name Attalus) who
gave much attention to working in metals and is mentioned by Pliny as
having invented weaving with gold, hence the historic Attalic cloths.
There are several references in Roman writings to costumes and stuffs
woven and embroidered with gold threads and the Graeco-Roman
_chryso-phrygium_ and the Roman _auri-phrygium_ are evidences not only
of Roman work with gold threads but also of its indebtedness to Phrygian
sources. The famous tunics of Agrippina and those of Heliogabalus are
said to have been of tissues made entirely with gold threads, whereas
the robes which Marcus Aurelius found in the treasury of Hadrian, as
well as the costumes sold at the dispersal of the wardrobe of Commodus,
were different in character, being of fine linen and possibly even of
silken stuffs inwoven or embroidered with gold threads. The same
description is perhaps correct of the reputedly splendid hangings with
which King Dagobert decorated the early medieval oratory of St Denis.
Reference to these and many such stuffs is made by the respectively
contemporary or almost contemporary writers; and a very full and
interesting work by Monsieur Francisque Michel (Paris, 1852) is still a
standard book for consultation in respect of the history of silk, gold
and silver stuffs.
From indications such as these, as well as those of later date, one sees
broadly that the art of weaving and embroidering with gold and silver
threads passed from one great city to another, travelling as a rule
westward. Babylon, Tarsus, Bagdad, Damascus, the islands of Cyprus and
Sicily, Constantinople, Venice and southern Spain appear successively in
the process of time as famous centres of these much-prized manufactures.
During the middle ages European royal personages and high ecclesiastical
dignitaries used cloth and tissues of gold and silver for their state
and ceremonial robes, as well as for costly hangings and decoration; and
various names--ciclatoun, tartarium, naques or nac, baudekin or
baldachin (Bagdad) and tissue--were applied to textiles in the making of
which gold threads were almost always introduced in combination with
others. The thin flimsy paper known as tissue paper is so called because
it originally was placed between the folds of gold "tissue" (or weaving)
to prevent the contiguous surfaces from fraying each other. Under the
articles dealing with carpets, embroidery, lace and tapestry will be
found notices of the occasional use in such productions of gold and
silver threads. Of early date in the history of European weaving are
rich stuffs produced in Southern Spain by Moors, as well as by Saracenic
and Byzantine weavers at Palermo and Constantinople in the 12th century,
in which metallic threads were freely used. Equally esteemed at about
the same period were corresponding stuffs made in Cyprus, whilst for
centuries later the merchants in such fabrics eagerly sought for and
traded in Cyprus gold and silver threads. Later the actual manufacture
of them was not confined to Cyprus, but was also carried on by Italian
thread and trimming makers from the 14th century onwards. For the most
part the gold threads referred to were of silver gilt. In rare instances
of middle-age Moorish or Arabian fabrics the gold threads are made with
strips of parchment or paper gilt and still rarer are instances of the
use of real gold wire.
In India the preparation of varieties of gold and silver threads is an
ancient and important art. The "gold wire" of the manufacturer has been
and is as a rule silver wire gilt, the silver wire being, of course,
composed of pure silver. The wire is drawn by means of simple
draw-plates, with rude and simple appliances, from rounded bars of
silver, or gold-plated silver, as the case may be. The wire is flattened
into strip, tinsel or ribbon-like form, by passing fourteen or fifteen
strands simultaneously, over a fine, smooth, round-topped anvil and
beating each as it passes with a heavy hammer having a slightly convex
surface. Such strips or tinsel of wire so flattened are woven into
Indian _soniri_, tissue or cloth of gold, the web or warp being composed
entirely of golden strips, and _ruperi_, similar tissue of silver. Other
gold and silver threads suitable for use in embroidery, pillow and
needlepoint lace making, &c., consist of fine strips of flattened wire
wound round cores of orange (in the case of silver, white) silk thread
so as to completely cover them. Wires flattened or partially flattened
are also twisted into exceedingly fine spirals and much used for heavy
embroideries. Spangles for embroideries, &c., are made from spirals of
comparatively stout wire, by cutting them down ring by ring, laying each
C-like ring on an anvil, and by a smart blow with a hammer flattening it
out into a thin round disk with a slit extending from the centre to one
edge. The demand for many kinds of loom-woven and embroidered gold and
silver work in India is immense, and the variety of textiles so
ornamented is also very great, chief amongst which are the golden or
silvery tinsel fabrics known as kincobs.
Amongst Western communities the demand for gold and silver embroideries
and braid lace now exists chiefly in connexion with naval, military and
other uniforms, masonic insignia, court costumes, public and private
liveries, ecclesiastical robes and draperies, theatrical dresses, &c.
The proportions of gold and silver in the gold thread for the woven
braid lace or ribbon trade varies, but in all cases the proportion of
gold is exceedingly small. An ordinary gold braid wire is drawn from a
bar containing 90 parts of silver and 7 of copper, and plated with 3 of
gold. On an average each ounce troy of a bar so plated is drawn into
1500 yds. of wire; and therefore about 16 grains of gold cover 1 m. of
wire. (A. S. C.)
GOLDAST AB HAIMINSFELD, MELCHIOR (1576-1635), Swiss writer, an
industrious though uncritical collector of documents relating to the
medieval history and Constitution of Germany, was born on the 6th of
January 1576 (some say 1578), of poor Protestant parents, near
Bischofszell, in the Swiss Canton of Thurgau. His university career,
first at Ingolstadt (1585-1586), then at Altdorf near Nuremberg
(1597-1598), was cut short by his poverty, from which he suffered all
his life, and which was the main cause of his wanderings. In 1598 he
found a rich protector in the person of Bartholomaeus Schobinger, of St
Gall, by whose liberality he was enabled to study at St Gall (where he
first became interested in medieval documents, which abound in the
conventual library) and elsewhere in Switzerland. Before his patron's
death (1604) he became (1603) secretary to Henry, duke of Bouillon, with
whom he went to Heidelberg and Frankfort. But in 1604 he entered the
service of the Baron von Hohensax, then the possessor of the precious
MS. volume of old German poems, returned from Paris to Heidelberg in
1888, and, partially published by Goldast. Soon he was back in
Switzerland, and by 1606 in Frankfort, earning his living by preparing
and correcting books for the press. In 1611 he was appointed councillor
at the court of Saxe-Weimar, and in 1615 he entered the service of the
count of Schaumburg at Buckeburg. In 1624 he was forced by the war to
retire to Bremen; there in 1625 he deposited his library in that of the
town (his books were bought by the town in 1646, but many of his MSS.
passed to Queen Christina of Sweden, and hence are now in the Vatican
library), he himself returning to Frankfort. In 1627 he became
councillor to the emperor and to the archbishop-elector of Treves, and
in 1633 passed to the service of the landgrave of Hesse-Darmstadt. He
died at Giessen early in 1635.
His immense industry is shown by the fact that his biographer,
Senckenburg, gives a list of 65 works published or written by him, some
extending to several substantial volumes. Among the more important are
his _Paraeneticorum veterum pars i._ (1604), which contained the old
German tales of _Kunig Tyrol von Schotten_, the _Winsbeke_ and the
_Winsbekin; Suevicarum rerum scriptores_ (Frankfort, 1605, new edition,
1727); _Rerum Alamannicarum scriptores_ (Frankfort, 1606, new edition by
Senckenburg, 1730); _Constitutiones imperiales_ (Frankfort, 1607-1613, 4
vols.); _Monarchia s. Romani imperii_ (Hanover and Frankfort, 1612-1614,
3 vols.); _Commentarii de regni Bohemiae juribus_ (Frankfort, 1627, new
edition by Schmink, 1719). He also edited De Thou's _History_
(1609-1610) and Willibald Pirckheimer's works (1610). In 1688 a volume
of letters addressed to him by his learned friends was published.
_Life_ by Senckenburg, prefixed to his 1730 work. See also R. von
Raumer's _Geschichte d. germanischen Philologie_ (Munich, 1870).
(W. A. B. C.)
GOLDBEATING.--The art of goldbeating is of great antiquity, being
referred to by Homer; and Pliny (_N.H._ 33. 19) states that 1 oz. of
gold was extended to 750 leaves, each leaf being four fingers (about 3
in.) square; such a leaf is three times as thick as the ordinary leaf
gold of the present time. In all probability the art originated among
the Eastern nations, where the working of gold and the use of gold
ornaments have been distinguishing characteristics from the most remote
periods. On Egyptian mummy cases specimens of original leaf-gilding are
met with, where the gold is so thin that it resembles modern gilding
(q.v.). The minimum thickness to which gold can be beaten is not known
with certainty. According to Mersenne (1621) 1 oz. was spread out over
105 sq. ft.; Reaumur (1711) obtained 146-1/2 sq. ft.; other values are
189 sq. ft. and 300 sq. ft. Its malleability is greatly diminished by
the presence of other metals, even in very minute quantity. In practice
the average degree of tenuity to which the gold is reduced is not nearly
so great as the last example quoted above. A "book of gold" containing
25 leaves measuring each 3-1/4 in., equal to an area of 264 sq. in.,
generally weighs from 4 to 5 grains.
The gold used by the goldbeater is variously alloyed, according to the
colour required. Fine gold is commonly supposed to be incapable of being
reduced to thin leaves. This, however, is not the case, although its use
for ordinary purposes is undesirable on account of its greater cost. It
also adheres on one part of a leaf touching another, thus causing a
waste of labour by the leaves being spoiled; but for work exposed to the
weather it is much preferable, as it is more durable, and does not
tarnish or change colour. The external gilding on many public buildings,
e.g. the Albert Memorial in Kensington Gardens, London, is done with
pure gold. The following is a list of the principal classes of leaf
recognized and ordinarily prepared by British beaters, with the
proportions of alloy per oz. they contain.
+---------------+-----------+-----------+-----------+
| |Proportion |Proportion |Proportion |
| Name of leaf. | of gold. | of Silver.| of Copper.|
+---------------+-----------+-----------+-----------+
| | Grains. | Grains. | Grains. |
| Red | 456-460 | .. | 20-24 |
| Pale red | 464 | .. | 16 |
| Extra deep | 456 | 12 | 12 |
| Deep | 444 | 24 | 12 |
| Citron | 440 | 30 | 10 |
| Yellow | 408 | 72 | .. |
| Pale yellow | 384 | 96 | .. |
| Lemon | 360 | 120 | .. |
| Green or pale | 312 | 168 | .. |
| White | 240 | 240 | .. |
+---------------+-----------+-----------+-----------+
The process of goldbeating is as follows: The gold, having been
alloyed according to the colour desired, is melted in a crucible at a
higher temperature than is simply necessary to fuse it, as its
malleability is improved by exposure to a greater heat; sudden cooling
does not interfere with its malleability, gold differing in this
respect from some other metals. It is then cast into an ingot, and
flattened, by rolling between a pair of powerful smooth steel rollers,
into a ribbon of 1-1/2 in. wide and 10 ft. in length to the oz. After
being flattened it is annealed and cut into pieces of about 6-1/2 grs.
each, or about 75 per oz., and placed between the leaves of a "cutch,"
which is about 1/2 in. thick and 3-1/2 in. square, containing about
180 leaves of a tough paper. Formerly fine vellum was used for this
purpose, and generally still it is interleaved in the proportion of
about one of vellum to six of paper. The cutch is beaten on for about
20 minutes with a 17-lb. hammer, which rebounds by the elasticity of
the skin, and saves the labour of lifting, by which the gold is spread
to the size of the cutch; each leaf is then taken out, and cut into
four pieces, and put between the skins of a "shoder," 4-1/2 in. square
and 3/4 in. thick, containing about 720 skins, which have been worn
out in the finishing or "mould" process. The shoder requires about two
hours' beating upon with a 9-lb. hammer. As the gold will spread
unequally, the shoder is beaten upon after the larger leaves have
reached the edges. The effect of this is that the margins of larger
leaves come out of the edges in a state of dust. This allows time for
the smaller leaves to reach the full size of the shoder, thus
producing a general evenness of size in the leaves. Each leaf is again
cut into four pieces, and placed between the leaves of a "mould,"
composed of about 950 of the finest gold-beaters' skins, 5 in. square
and 3/4 in. thick, the contents of one shoder filling three moulds.
The material has now reached the last and most difficult stage of the
process; and on the fineness of the skin and judgment of the workman
the perfection and thinness of the leaf of gold depend. During the
first hour the hammer is allowed to fall principally upon the centre
of the mould. This causes gaping cracks upon the edges of the leaves,
the sides of which readily coalesce and unite without leaving any
trace of the union after being beaten upon. At the second hour, when
the gold is about the 150,000th part of an inch in thickness, it for
the first time permits the transmission of the rays of light. Pure
gold, or gold but slightly alloyed, transmits green rays; gold highly
alloyed with silver transmits pale violet rays. The mould requires in
all about four hours' beating with a 7-lb. hammer, when the ordinary
thinness for the gold leaf of commerce will be reached. A single ounce
of gold will at this stage be extended to 75 X 4 X 4 = 1200 leaves,
which will trim to squares of about 3-1/4 in. each. The finished leaf
is then taken out of the mould, and the rough edges are trimmed off by
slips of the ratan fixed in parallel grooves of an instrument called a
waggon, the leaf being laid upon a leathern cushion. The leaves thus
prepared are placed into "books" capable of holding 25 leaves each,
which have been rubbed over with red ochre to prevent the gold
clinging to the paper. Dentist gold is gold leaf carried no farther
than the cutch stage, and should be perfectly pure gold.
By the above process also silver is beaten, but not so thin, the
inferior value of the metal not rendering it commercially desirable to
bestow so much labour upon it. Copper, tin, zinc, palladium, lead,
cadmium, platinum and aluminium can be beaten into thin leaves, but
not to the same extent as gold or silver.
The fine membrane called goldbeater's skin, used for making up the
shoder and mould, is the outer coat of the caecum or blind gut of the
ox. It is stripped off in lengths about 25 or 30 in., and freed from fat
by dipping in a solution of caustic alkali and scraping with a blunt
knife. It is afterwards stretched on a frame; two membranes are glued
together, treated with a solution of aromatic substances or camphor in
isinglass, and subsequently coated with white of egg. Finally they are
cut into squares of 5 or 5-1/2 in.; and to make up a mould of 950 pieces
the gut of about 380 oxen is required, about 2-1/2 skins being got from
each animal. A skin will endure about 200 beatings in the mould, after
which it is fit for use in the shoder alone.
The dryness of the cutch, shoder and mould is a matter of extreme
delicacy. They require to be hot-pressed every time they are used,
although they may be used daily, to remove the moisture which they
acquire from the atmosphere, except in extremely frosty weather, when
they acquire so little moisture that a difficulty arises from their
over-dryness, whereby the brilliancy of the gold is diminished, and it
spreads very slowly under the hammer. On the contrary, if the cutch or
shoder be damp, the gold will become pierced with innumerable
microscopic holes; and in the moulds in its more attenuated state it
will become reduced to a pulverulent state. This condition is more
readily produced in alloyed golds than in fine gold. It is necessary
that each skin of the mould should be rubbed over with calcined gypsum
each time the mould may be used, in order to prevent the adhesion of
the gold to the surface of the skin in beating.
GOLDBERG, a town of Germany, in the Prussian province of Silesia,[1] 14
m. by rail S.W. of Liegnitz, on the Katzbach, an affluent of the Oder.
Pop. (1905) 6804. The principal buildings are an old church dating from
the beginning of the 13th century, the Schwabe-Priesemuth institution,
completed in 1876, for the board and education of orphans, and the
classical school or gymnasium (founded in 1524 by Duke Frederick II. of
Liegnitz), which in the 17th century enjoyed great prosperity, and
numbered Wallenstein among its pupils. The chief manufactures are
woollen cloth, flannel, gloves, stockings, leather and beer, and there
is a considerable trade in corn and fruit. Goldberg owes its origin and
name to a gold mine in the neighbourhood, which, however, has been
wholly abandoned since the time of the Hussite wars. The town obtained
civic rights in 1211. It suffered heavily from the Tatars in 1241, from
the plague in 1334, from the Hussites in 1428, and from the Saxon,
Imperial and Swedish forces during the Thirty Years' War. On the 27th of
May 1813 a battle took place near it between the French and the
Russians; and on the 23rd and the 27th of August of the same year
fights between the allies and the French.
See Sturm, _Geschichte der Stadt Goldberg in Schlesien_ (1887).