Silver is widely diffused but is rarely found in the native
state.
Silver is originally as widespread as gold, occurring in nearly
all of the volcanic rocks. Whereas gold remains unaltered by the
action of the elements and is often carried long distances from
its original place of occurrence, silver on the contrary is only
to be found in the rocks where it originally occurs. When these
rocks are broken down or worn away, the silver is either driven
into new mineral combinations, or more often dissipated and
lost. Silver, therefore, is only to be obtained by subterranean
mining. Shafts are driven and the ore brought to the surface,
and by use of various processes the silver is extracted, refined
and made ready for commercial purposes.
An old process and one still employed extensively throughout
Mexico where a large quantity of silver is produced, is to take
the ore after it has been crushed or reduced to a fine mud or
puddle and spread it about two feet deep over the floor of a
large courtyard. Powered sulfate of copper is spread over the
mass and then horses or mules are driven around in circle to
tread the sulfate in and mix it thoroughly with the ore. After
about one day’s treading a quantity of common salt is added and
after two days more treading quicksilver is added. This mass is
trodden over for a period of about fifteen days, and is then
shoveled into a large tank through which a rapid stream of water
is passed. This washes away all but the silver and quicksilver,
which is then poured into cone-shaped canvas bags. Most of the
quicksilver runs out leaving the silver, which is then retorted.
The quicksilver is used over and over again to assist in
recovering the silver.
Pure silver has a beautiful white color and luster; it is almost
as plastic as pure gold and like it very soft. Silver does not
tarnish in natural air, but when it comes in contact with sulfur
compounds it readily forms black silver sulfide. The sulfur
compounds which act on silver are found in small quantities in
the air as a result of burning coal and illuminating gas, while
larger amounts occur in vulcanized rubber, wool, and foods like
eggs.
Pure silver is too soft to make durable objects that require
lightness and stability of form. This defect is overcome by
alloying it with a little copper.
An alloy of 925 parts fine silver and 75 parts copper is called
925-1000 fine or what is commonly known as sterling silver. This
alloy is used almost universally for jewelry and the best
silverware.