+------+-----------+-----------+----------+---------+----------+-----------+-----------+------------+
| | Austral- | | | | | | United | |
| Year.| asia. | Africa. | Canada. | India. | Mexico. | Russia. | States. | Totals. |
+------+-----------+-----------+----------+---------+----------+-----------+-----------+------------+
| 1881 | 1,475,161 | .. | 52,483 | .. | 41,545 | 1,181,853 | 1,678,612 | 4,976,980 |
| 1882 | 1,438,067 | .. | 52,000 | .. | 45,289 | 1,154,613 | 1,572,187 | 4,825,794 |
| 1883 | 1,333,849 | .. | 46,150 | .. | 46,229 | 1,132,219 | 1,451,250 | 4,614,588 |
| 1884 | 1,352,761 | .. | 46,000 | .. | 57,227 | 1,055,642 | 1,489,950 | 4,902,889 |
| 1885 | 1,309,804 | .. | 53,987 | .. | 46,941 | 1,225,738 | 1,538,325 | 5,002,584 |
| 1886 | 1,257,670 | .. | 66,061 | .. | 29,702 | 922,226 | 1,693,125 | 5,044,363 |
| 1887 | 1,290,202 | 28,754 | 59,884 | 15,403 | 39,861 | 971,656 | 1,596,375 | 5,061,490 |
| 1888 | 1,344,002 | 240,266 | 53,150 | 35,034 | 47,117 | 1,030,151 | 1,604,841 | 5,175,623 |
| 1889 | 1,540,607 | 366,023 | 62,658 | 78,649 | 33,862 | 1,154,076 | 1,587,000 | 5,611,245 |
| 1890 | 1,453,172 | 497,817 | 55,625 | 107,273 | 37,104 | 1,134,590 | 1,588,880 | 5,726,966 |
| 1891 | 1,518,690 | 729,268 | 45,022 | 131,776 | 48,375 | 1,168,764 | 1,604,840 | 6,287,591 |
| 1892 | 1,638,238 | 1,210,869 | 43,905 | 164,141 | 54,625 | 1,199,809 | 1,597,098 | 7,102,172 |
| 1893 | 1,711,892 | 1,478,477 | 44,853 | 207,152 | 63,144 | 1,345,224 | 1,739,323 | 7,772,585 |
| 1894 | 2,020,180 | 2,024,164 | 50,411 | 210,412 | 217,688 | 1,167,455 | 1,910,813 | 8,813,848 |
| 1895 | 2,170,505 | 2,277,640 | 92,440 | 257,830 | 290,250 | 1,397,767 | 2,254,760 | 9,814,505 |
| 1896 | 2,185,872 | 2,280,892 | 136,274 | 323,501 | 314,437 | 1,041,794 | 2,568,132 | 9,950,861 |
| 1897 | 2,547,704 | 2,832,776 | 294,582 | 350,585 | 362,812 | 1,124,511 | 2,774,935 | 11,420,068 |
| 1898 | 3,137,644 | 3,876,216 | 669,445 | 376,431 | 411,187 | 1,231,791 | 3,118,398 | 13,877,806 |
| 1899 | 3,837,181 | 3,532,488 |1,031,563 | 418,869 | 411,187 | 1,072,333 | 3,437,210 | 14,837,775 |
| 1900 | 3,555,506 | 419,503 |1,348,720 | 456,444 | 435,375 | 974,537 | 3,829,897 | 12,315,135 |
| 1901 | 3,719,080 | 439,704 |1,167,216 | 454,527 | 497,527 | 1,105,412 | 3,805,500 | 12,698,089 |
| 1902 | 3,946,374 | 1,887,773 |1,003,355 | 463,824 | 491,156 | 1,090,053 | 3,870,000 | 14,313,660 |
| 1903 | 4,315,538 | 3,289,409 | 911,118 | 552,873 | 516,524 | 1,191,582 | 3,560,000 | 15,852,620 |
| 1904 | 4,245,744 | 4,156,084 | 793,350 | 556,097 | 609,781 | 1,199,857 | 3,892,480 | 16,790,351 |
| 1905 | 4,159,220 | 5,477,841 | 700,863 | 576,889 | 779,181 | 1,063,883 | 4,265,742 | 18,360,945 |
| 1906 | 3,984,538 | 6,449,749 | 581,709 | 525,527 | 896,615 | 1,087,056 | 4,565,333 | 19,620,272 |
| 1907 | 3,659,693 | 7,270,464 | 399,844 | 495,965 | 903,672 | 1,282,635 | 4,374,827 | 19,988,144 |
| 1908 | 3,557,705 | 7,983,348 | 462,467 | 504,309 |1,182,445 | 1,497,076 | 4,659,360 | 21,529,300 |
+------+-----------+-----------+----------+---------+----------+-----------+-----------+------------+
_Alloys._--Gold forms alloys with most metals, and of these many are
of great importance in the arts. The alloy with mercury--gold
amalgam--is so readily formed that mercury is one of the most powerful
agents for extracting the precious metal. With 10% of gold present the
amalgam is fluid, and with 12.5% pasty, while with 13% it consists of
yellowish-white crystals. Gold readily alloys with silver and copper
to form substances in use from remote times for money, jewelry and
plate. Other metals which find application in the metallurgy of gold
by virtue of their property of extracting the gold as an alloy are
lead, which combines very readily when molten, and which can
afterwards be separated by cupellation, and copper, which is separated
from the gold by solution in acids or by electrolysis; molten lead
also extracts gold from the copper-gold alloys. The relative amount of
gold in an alloy is expressed in two ways: (1) as "fineness," i.e. the
amount of gold in 1000 parts of alloy; (2) as "carats," i.e. the
amount of gold in 24 parts of alloy. Thus, pure gold is 1000 "fine" or
24 carat. In England the following standards are used for plate and
jewelry: 375, 500, 625, 750 and 916.6, corresponding to 9, 12, 15, 18
and 22 carats, the alloying metals being silver and copper in varying
proportions. In France three alloys of the following standards are
used for jewelry, 920, 840 and 750. A greenish alloy used by
goldsmiths contains 70% of silver and 30% of gold. "Blue gold" is
stated to contain 75% of gold and 25% of iron. The Japanese use for
ornament an alloy of gold and silver, the standard of which varies
from 350 to 500, the colour of the precious metal being developed by
"pickling" in a mixture of plum-juice, vinegar and copper sulphate.
They may be said to possess a series of bronzes, in which gold and
silver replace tin and zinc, all these alloys being characterized by
patina having a wonderful range of tint. The common alloy,
Shi-ya-ku-Do, contains 70% of copper and 30% of gold; when exposed to
air it becomes coated with a fine black patina, and is much used in
Japan for sword ornaments. Gold wire may be drawn of any quality, but
it is usual to add 5 to 9 dwts. of copper to the pound. The "solders"
used for red gold contain 1 part of copper and 5 of gold; for light
gold, 1 part of copper, 1 of silver and 4 of gold.
_Gold and Silver._--Electrum is a natural alloy of gold and silver.
Matthiessen observed that the density of alloys, the composition of
which varies from AuAg6 to Au6Ag, is greater than that calculated from
the densities of the constituent metals. These alloys are harder, more
fusible and more sonorous than pure gold. The alloys of the formulae
AuAg, AuAg2, AuAg4 and AuAg20 are perfectly homogeneous, and have been
studied by Levol. Molten alloys containing more than 80% of silver
deposit on cooling the alloy AuAg9, little gold remaining in the
mother liquor.
_Gold and Zinc._--When present in small quantities zinc renders gold
brittle, but it may be added to gold in larger quantities without
destroying the ductility of the precious metal; Peligot proved that a
triple alloy of gold, copper and zinc, which contains 5.8% of the
last-named, is perfectly ductile. The alloy of 11 parts gold and 1
part of zinc is, however, stated to be brittle.
_Gold and Tin._--Alchorne showed that gold alloyed with 1/37th part of
tin is sufficiently ductile to be rolled and stamped into coin,
provided the metal is not annealed at a high temperature. The alloys
of tin and gold are hard and brittle, and the combination of the
metals is attended with contraction; thus the alloy SnAu has a density
14.243, instead of 14.828 indicated by calculation. Matthiessen and
Bose obtained large crystals of the alloy Au2Sn5, having the colour of
tin, which changed to a bronze tint by oxidation.
_Gold and Iron._--Hatchett found that the alloy of 11 parts gold and 1
part of iron is easily rolled without annealing. In these proportions
the density of the alloy is less than the mean of its constituent
metals.
_Gold and Palladium._--These metals are stated to alloy in all
proportions. According to Chenevix, the alloy composed of equal parts
of the two metals is grey, is less ductile than its constituent metals
and has the specific gravity 11.08. The alloy of 4 parts of gold and 1
part of palladium is white, hard and ductile. Graham showed that a
wire of palladium alloyed with from 24 to 25 parts of gold does not
exhibit the remarkable retraction which, in pure palladium, attends
its loss of occluded hydrogen.
_Gold and Platinum._--Clarke states that the alloy of equal parts of
the two metals is ductile, and has almost the colour of gold.
_Gold and Rhodium._--Gold alloyed with 1/4th or 1/5th of rhodium is,
according to Wollaston, very ductile, infusible and of the colour of
gold.
_Gold and Iridium._--Small quantities of iridium do not destroy the
ductility of gold, but this is probably because the metal is only
disseminated through the mass, and not alloyed, as it falls to the
bottom of the crucible in which the gold is fused.
_Gold and Nickel._--Eleven parts of gold and 1 of nickel yield an
alloy resembling brass.
_Gold and Cobalt._--Eleven parts of gold and 1 of cobalt form a
brittle alloy of a dull yellow colour.
_Compounds._--Aurous oxide, Au2O, is obtained by cautiously adding
potash to a solution of aurous bromide, or by boiling mixed solutions
of auric chloride and mercurous nitrate. It forms a dark-violet
precipitate which dries to a greyish-violet powder. When freshly
prepared it dissolves in cold water to form an indigo-coloured
solution with a brownish fluorescence of colloidal aurous oxide; it is
insoluble in hot water. This oxide is slightly basic. Auric oxide,
Au2O3, is a brown powder, decomposed into its elements when heated to
about 250 deg. or on exposure to light. When a concentrated solution
of auric chloride is treated with caustic potash, a brown precipitate
of auric hydrate, Au(OH)3, is obtained, which, on heating, loses water
to form auryl hydrate, AuO(OH), and auric oxide, Au2O3. It functions
chiefly as an acidic oxide, being less basic than aluminium oxide, and
forming no stable oxy-salts. It dissolves in alkalis to form
well-defined crystalline salts; potassium aurate, KAuO2.3H2O, is very
soluble in water, and is used in electro-gilding. With concentrated
ammonia auric oxide forms a black, highly explosive compound of the
composition AuN2H3.3H2O, named "fulminating gold"; this substance is
generally considered to be Au(NH2)NH.3H2O, but it may be an ammine of
the formula [Au(NH3)2(OH)2]OH. Other oxides, e.g. Au2O2, have been
described.
Aurous chloride, AuCl, is obtained as a lemon-yellow, amorphous
powder, insoluble in water, by heating auric chloride to 185 deg. It
begins to decompose into gold and chlorine at 185 deg., the
decomposition being complete at 230 deg.; water decomposes it into
gold and auric chloride. Auric chloride, or gold trichloride, AuCl3,
is a dark ruby-red or reddish-brown, crystalline, deliquescent powder
obtained by dissolving the metal in aqua regia. It is also obtained by
carefully evaporating a solution of the metal in chlorine water. The
gold chloride of commerce, which is used in photography, is really a
hydrochloride, chlorauric or aurichloric acid, HAuCl4.3H2O, and is
obtained in long yellow needles by crystallizing the acid solution.
Corresponding to this acid, a series of salts, named chloraurates or
aurichlorides, are known. The potassium salt is obtained by
crystallizing equivalent quantities of potassium and auric chlorides.
Light-yellow monoclinic needles of 2KAuCl4.H2O are deposited from
warm, strongly acid solutions, and transparent rhombic tables of
KAuCl4.2H2O from neutral solutions. By crystallizing an aqueous
solution, red crystals of AuCl3.2H2O are obtained. Auric chloride
combines with the hydrochlorides of many organic bases--amines,
alkaloids, &c.--to form characteristic compounds. Gold dichloride,
probably Au2Cl4, = Au.AuCl4, aurous chloraurate, is said to be
obtained as a dark-red mass by heating finely divided gold to 140
deg.-170 deg. in chlorine. Water decomposes it into gold and auric
chloride. The bromides and iodides resemble the chlorides. Aurous
bromide, AuBr, is a yellowish-green powder obtained by heating the
tribromide to 140 deg.; auric bromide, AuBr3, forms reddish-black or
scarlet-red leafy crystals, which dissolve in water to form a
reddish-brown solution, and combines with bromides to form bromaurates
corresponding to the chloraurates. Aurous iodide, AuI, is a
light-yellow, sparingly soluble powder obtained, together with free
iodine, by adding potassium iodide to auric chloride; auric iodide,
AuI3, is formed as a dark-green powder at the same time, but it
readily decomposes to aurous iodide and iodine. Aurous iodide is also
obtained as a green solid by acting upon gold with iodine. The
iodaurates correspond to the chlor- and bromaurates; the potassium
salt, KAuI4, forms highly lustrous, intensely black, four-sided
prisms.
Aurous cyanide, AuCN, forms yellow, microscopic, hexagonal tables,
insoluble in water, and is obtained by the addition of hydrochloric
acid to a solution of potassium aurocyanide, KAu(CN)2. This salt is
prepared by precipitating a solution of gold in _aqua regia_ by
ammonia, and then introducing the well-washed precipitate into a
boiling solution of potassium cyanide. The solution is filtered and
allowed to cool, when colourless rhombic pyramids of the aurocyanide
separate. It is also obtained in the action of potassium cyanide on
gold in the presence of air, a reaction utilized in the
MacArthur-Forrest process of gold extraction (see below). Auric
cyanide, Au(CN)3, is not certainly known; its double salts, however,
have been frequently described. Potassium auricyanide, 2KAu(CN)4.3H2O,
is obtained as large, colourless, efflorescent tablets by
crystallizing concentrated solutions of auric chloride and potassium
cyanide. The acid, auricyanic acid, 2HAu(CN)4.3H2O, is obtained by
treating the silver salt (obtained by precipitating the potassium salt
with silver nitrate) with hydrochloric acid; it forms tabular
crystals, readily soluble in water, alcohol and ether.
Gold forms three sulphides corresponding to the oxides; they readily
decompose on heating. Aurous sulphide, Au2S, is a brownish-black
powder formed by passing sulphuretted hydrogen into a solution of
potassium aurocyanide and then acidifying. Sodium aurosulphide,
NaAuS.4H2O, is prepared by fusing gold with sodium sulphide and
sulphur, the melt being extracted with water, filtered in an
atmosphere of nitrogen, and evaporated in a vacuum over sulphuric
acid. It forms colourless, monoclinic prisms, which turn brown on
exposure to air. This method of bringing gold into solution is
mentioned by Stahl in his _Observationes Chymico-Physico-Medicae_; he
there remarks that Moses probably destroyed the golden calf by burning
it with sulphur and alkali (Ex. xxxii. 20). Auric sulphide, Au2S3, is
an amorphous powder formed when lithium aurichloride is treated with
dry sulphuretted hydrogen at -10 deg. It is very unstable, decomposing
into gold and sulphur at 200 deg.
Oxy-salts of gold are almost unknown, but the sulphite and
thiosulphate form double salts. Thus by adding acid sodium sulphite
to, or by passing sulphur dioxide at 50 deg. into, a solution of
sodium aurate, the salt, 3Na2SO3.Au2SO3.3H2O is obtained, which, when
precipitated from its aqueous solution by alcohol, forms a purple
powder, appearing yellow or green by reflected light. Sodium
aurothiosulphate, 3Na2S2O3.Au2S2O3.4H2O, forms colourless needles; it
is obtained in the direct action of sodium thiosulphate on gold in the
presence of an oxidizing agent, or by the addition of a dilute
solution of auric chloride to a sodium thiosulphate solution.
_Mining and Metallurgy._
The various deposits of gold may be divided into two classes--"veins"
and "placers." The vein mining of gold does not greatly differ from that
of similar deposits of metals (see MINERAL DEPOSITS). In the placer or
alluvial deposits, the precious metal is found usually in a water-worn
condition imbedded in earthy matter, and the method of working all such
deposits is based on the disintegration of the earthy matter by the
action of a stream of water, which washes away the lighter portions and
leaves the denser gold. In alluvial deposits the richest ground is
usually found in contact with the "bed rock"; and, when the overlying
cover of gravel is very thick, or, as sometimes happens, when the older
gravel is covered with a flow of basalt, regular mining by shafts and
levels, as in what are known as tunnel-claims, may be required to reach
the auriferous ground.
The extraction of gold may be effected by several methods; we may
distinguish the following leading types: