1. The metals most extensively employed in the arts are gold, silver,
copper, lead, tin, and iron. These are sometimes found uncombined with
any other substance, or combined only with each other; in either of
these cases, they are said to be in a _native state_. But they are
more frequently found united with some substances which, in a great
measure, disguise their metallic qualities, or, in other words, in a
state of _ore_. The mode of separating the metals from their ores,
will be noticed in connexion with some of the trades in which they are
prepared for, or practically applied in, the arts.
2. Gold is a metal of a yellow color, a characteristic by which it is
distinguished from all other simple metallic bodies. As a
representative of property, it has been used from time immemorial;
and, before coinage was invented, it passed for money in its native
state. In this form, gold is still current in some parts of Africa;
and even in the Southern states of our own country, in the vicinity of
the gold mines, the same practice, in a measure, prevails.
3. Gold is rarely employed in a state of perfect purity, but is
generally used in combination with some other metal, which renders it
harder, and consequently more capable of enduring the friction to
which it is exposed. The metal used for this purpose is called an
_alloy_, and generally consists of silver or copper.
4. For convenience in commerce, this precious metal is supposed to be
divided into twenty-four equal parts, called _carats_. If perfectly
pure, it is denominated gold 24 carats fine; if alloyed with one part
of any other metal or mixture of metals, it is said to be 23 carats
fine. The standard gold coin of the United States and Great Britain is
22 carats fine; or, in other words, it contains one-twelfth part of
alloy. Gold, made standard by equal parts of copper and silver,
approaches in color more nearly to pure gold than when alloyed in any
other manner.
5. Gold is found in veins in mountains, most usually associated with
ores of silver, sulphurets of iron, copper, lead, and other metals. It
is often so minutely distributed, that its presence is detected only
by pounding and washing the ores in which it exists. But the greatest
part of the gold in the possession of mankind, has been found in the
form of grains and small detached masses, amid the sands of rivers and
in alluvial lands, where it had been deposited by means of water,
which had detached it from its original position in the mountains.
6. To separate or extract gold from the foreign matters with which it
may be combined, the whole is first pounded fine, and then washed by
putting it in a stream of water, which carries off the stony
particles, while the gold, by its specific gravity, sinks to the
bottom. To render the separation still more perfect, this sediment is
mixed with ten times its weight of quicksilver, and put into a leather
bag, in which it is submitted to a pressure that forces the fluid part
through its pores; while the more solid part of the amalgam, which
contains most of the gold, remains.
7. To separate the quicksilver from the gold, the mass is subjected to
the process of _sublimation_ in earthen retorts, which, as applied to
metals, is similar in its effects to distillation, as applied to
liquids. When gold is contained in the ores of other metals, they are
roasted, in order to drive off the volatile parts, and to oxydize the
other metals. The gold is then extracted by amalgamation, by
liquefaction with lead, by the aid of nitric acid, or by other methods
adapted to the nature of the ore.
8. Gold obtained in any of these methods is always more or less
alloyed with some other metal, especially with silver or copper; but a
separation is produced, so far as it is required for the purposes of
commerce, by two processes, one of which is called _cupellation_, and
the other _parting_. The former of these operations consists in
melting the gold with a quantity of lead, which readily oxydizes and
vitrifies, and which causes the same changes to take place in the
metal to be detached from the mass of gold. The operation is called
cupellation, because it is usually performed on a _cupel_, a vessel
formed of bone-ashes, or sometimes of wood-ashes.
9. Cupellation is effectual in removing copper, but not so with regard
to silver; the latter is separated by means of a process called
_parting_. The metal is rolled out into thin sheets or strips, and cut
into small pieces. These are put into diluted nitric acid, which, by
the aid of a moderate heat, dissolves the silver, leaving the gold in
a porous state.
10. Another process, called _cementation_, is also sometimes used. It
is performed by beating the alloyed metal into thin plates, and
arranging them in alternate layers with a cement containing nitrate of
potash, and sulphate of iron. The whole is then exposed to heat, until
a great part of the baser metals has been removed by the action of the
nitric acid liberated by the nitre. Cementation is often employed by
goldsmiths, to refine the surface of articles in which the gold has
been combined, in too small a proportion, with metals of less value.
11. The average amount of gold annually obtained in every part of the
globe cannot fall far short of twenty-millions of dollars in value, of
which South America supplies about one half, and Europe, about one
twenty-fifth part. The amount yielded by the Southern states of our
Union, cannot be accurately ascertained, but the whole sum coined at
the United States' Mint in 1834, from gold obtained in this quarter,
amounted to $898,000, and since 1824 to that time, to $3,679,000. In
1824, the sum was but $5000. Our Southern mines will probably continue
to increase in productiveness.