Silver
is a chemical element with the symbol Ag. A soft white lustrous transition
metal, it has the highest electrical and thermal conductivity for
a metal, and occurs as a free metal, and in minerals such as argentite
and chlorargyrite. Most silver is produced as a by-product of copper,
gold, lead, and zinc mining.
Silver has been known since antiquity. It has long been valued as
a precious metal and used in currency, ornaments and jewelry, as well
as utensils (hence the term silverware). Today, it is also used in
photographic film, electrical contacts, and mirrors. Elemental silver
is also used to catalyze chemical reactions.
Silver has certain antimicrobial activity. In the past, dilute solutions
of silver nitrate were used as disinfectants, though this has been
supplanted by other treatments. In alternative medicine, there has
been increasing interest in the use of colloidal silver as remedies
for a wide range of ailments, though these claims are disputed. The
consumption of large amounts of silver can lead to a darkening of
the skin known as argyria.
Silver is a very ductile and malleable (slightly harder than gold)
univalent coinage metal with a brilliant white metallic luster that
can take a high degree of polish. It has the highest electrical conductivity
of all metals, even higher than copper, but its greater cost and tarnishability
has prevented it from being widely used in place of copper for electrical
purposes, though it was used in the electromagnets used for enriching
uranium during World War II (mainly because of the wartime shortage
of copper).
Pure silver has the highest thermal conductivity, whitest color, the
highest optical reflectivity, although aluminium slightly outdoes
it in parts of the visible spectrum, and is a poor reflector of ultraviolet
light. Silver also has the lowest contact resistance of any metal.
Silver halides are photosensitive and are remarkable for the effect
of light upon them. This metal is stable in pure air and water, but
does tarnish when it is exposed to ozone, hydrogen sulfide, or air
containing sulfur.
Occurrence and extraction
Silver is found in native form, combined with sulfur, arsenic, antimony,
or chlorine and in various ores such as argentite and horn silver.
Another ore it is found in is pyrargyrite. The principal sources of
silver are copper, copper-nickel, gold, lead and lead-zinc ores obtained
from Canada, Mexico (historically Batopilas), Peru, Australia and
the United States.
This metal can also be produced during the electrolytic refining of
copper and by application of the Parkes process on lead metal obtained
from lead ores that contain small amounts of silver. Commercial grade
fine silver is at least 99.9% pure silver and purities greater than
99.999% are available. Mexico is the world's largest silver producer.
According to the Secretary of Economics of Mexico, it produced 80,120,000
troy ounces (2492 metric tons) in 2000, about 15% of the annual production
of the world.
Applications
A major use of silver is as a precious metal. Sterling silver is 92.5
% silver, alloyed usually with copper. Jewelry and silverware are
traditionally made from this. Silver is used in medals, denoting second
place. Many high end musical instruments are made with silver, which
benefit from a higher tone quality.
The name of United Kingdom monetary unit 'Pound' originally had the
value of one troy pound of sterling silver. Silver has been coined
to produce money since 700 BC by the Lydians, in the form of electrum.
Later, silver was refined and coined in its pure form. The words for
"silver" and "money" are the same in at least
14 languages.
The largest single end use of silver is photography, in the form of
silver nitrate and silver halides are widely used in photography 30
% of US production is used here.
Some electrical and electronic products use silver for its superior
conductivity, even when tarnished. For example, printed circuits are
made using silver paints, and computer keyboards use silver electrical
contacts. Silver cadmium oxide is used in high voltage contacts because
it can minimize arcing. Silver is also used to make solder and brazing
alloys, electrical contacts, and high capacity silver-zinc and silver-cadmium
batteries. Silver in a thin layer of on top of a bearing material
can provide a significant increase in galling resistance and reduce
wear under heavy load, particularly against steel.
Mirrors which need superior reflectivity for visible light are made
with silver as the reflecting material in a process called silvering,
though common mirrors are backed with aluminium. Using a process called
sputtering, silver (and sometimes gold) can be applied to glass at
various thicknesses, allowing different amounts of light to penetrate.
This is most often seen in architectural glass and tinted windows
on vehicles.
Silver's catalytic properties make it ideal for use as a catalyst
in oxidation reactions; for example, the production of formaldehyde
from methanol and air by means of silver screens or crystallites containing
a minimum 99.95 weight-percent silver. Silver (upon some suitable
support) is probably the only catalyst available today to convert
ethylene to ethylene oxide (later hydrolyzed to ethylene glycol, used
for making polyesters) a very important industrial reaction.
Oxygen dissolves in silver relatively easily compared to other gases
present in air. Attempts have been made to construct silver membranes
of only a few monolayers thickness. Such a membrane could be used
to filter pure oxygen from air.
In medicine
Hippocrates, the father of modern medicine, wrote that silver had
beneficial healing and anti-disease properties, and the Phoenicians
used to store water, wine, and vinegar in silver bottles to prevent
spoiling. In the early 1900s people would put silver dollars in milk
bottles to prolong the milk's freshness. Silver compounds were used
successfully to prevent infection in World War I before the advent
of antibiotics. Silver nitrate solution was a standard of care but
was largely replaced by silver sulfadiazine cream which was generally
the "standard of care" for the antibacterial/antibiotic
treatment of serious burns until the late 1990's. Now, other options
such as silver coated dressings (activated silver dressings) are used
in addition to SSD cream, and may present advantages such as pain
reduction and capacity for treatment at home.
The widespread use of silver went out of fashion with the development
of modern antibiotics. However, recently there has been renewed interest
in silver as a broad spectrum antimicrobial. In particular, it is
being used with alginate, a naturally occurring biopolymer derived
from seaweed, in a range of silver alginate products designed to prevent
infections as part of wound management procedures, particularly applicable
to burn victims. In addition, Samsung has introduced washing machines
with a final rinse containing silver ions to provide several days
of antibacterial protection in the clothes. Additionally, Kohler has
introduced a line of toilets that have silver ions embedded in the
porcelain to kill germs. A company called Thomson Research Associates
has began treating products with Ultra Fresh, an anti-microbial technology
involving "proprietary nano-technology to produce the ultra-fine
silver particles essential to ease of application and long-term protection."
The malleability, non-toxicity and beauty of silver make it useful
in dental alloys for fittings and fillings.
Health precautions
Silver plays no known natural biological role in humans, and possible
health effects of silver are a subject of dispute. Silver itself is
not toxic but most silver salts are, and some may be carcinogenic.
Silver and compounds containing silver (like colloidal silver) can
be absorbed into the circulatory system and become deposited in various
body tissues leading to a condition called argyria which results in
a blue-grayish pigmentation of the skin, eyes, and mucous membranes.
Although this condition does not harm a person's health, it is disfiguring
and usually permanent. Argyria is rare and mild forms are sometimes
mistaken for cyanosis.
Silver-ions and silver compounds show a toxic effect on some bacteria,
viruses, algae and fungi typical for heavy metals like lead or mercury,
but without the high toxicity to humans that is normally associated
with them. Its germicidal effects kill many microbial organisms in
vitro (i.e. in a test tube or a petri dish). The exact process by
which this is done is still not well understood, although different
theories exist. One of these is a process generally known for heavy
metals called the oligodynamic effect, which goes a long way explaining
the effect on microbial lifeforms but does not explain certain antiviral
functions.
Alternative medicine
Today, various kinds of silver compounds, or devices to make solutions
or colloids containing silver, are sold as remedies for a wide variety
of diseases. Although mostly harmless, some people using these home-made
solutions use far too much and develop argyria over a period of months
or years, and several have been documented in the last few years in
the medical literature, including one possible case of coma associated
with a high intake of silver (see medical references). It is strongly
advised to notify a doctor when taking silver as a form of self-medication.
In food
In India, foods can be found decorated with a thin layer of silver,
known as Varak. Silver as a food additive is given the E number E174
and classed as a food coloring. It is used solely for external decoration,
such as on chocolate confectionery, in the covering of dragées
and the decoration of sugar-coated flour confectionery. In Australia,
it is banned as a food additive.
In clothing
Silver is a natural antimicrobial ingredient that has been used for
years by professionals in the field of wound healing, mainly because
of its scientifically proven antimicrobial efficacy. Silver inhibits
the growth of bacteria and of fungus. It keeps odour to a minimum
and reduces the risk of bacterial and fungal infection. In clothing
the combination of silver and moisture movement (wicking) is the best
combination to reduce the harmful effects of prolonged use in active
and humid conditions.
Silver is used in clothing in two main forms: where silver ions are
integrated into the polymer from which yarns are made (nano-technology)
where the silver is physically coated onto the yarns. In both cases
the silver prevents the growth of a broad spectrum of bacteria and
fungi. At the same time, silver is a very skin-friendly and highly
compatible agent to which - unlike many antibiotics - bacteria rarely
build up resistance.
Recorded use of silver to prevent infection dates to ancient Greece
and Rome. It was rediscovered in the Middle Ages where it was used
for several purposes such as to disinfect water and food while storing
it, but also for the treatment of burns and wounds as wound dressing.
In the 19th Century, sailors on long ocean voyages would put silver
coins in barrels of water and wine to keep the liquid pure. Pioneers
in America used the same idea as they made their journey from coast
to coast. Silver solutions were approved in the 1920s by the US Food
and Drug Administration for use as antibacterial agents. Today silver
containing wound dressings are well established for clinical wound
care and have recently been introduced in consumer products such as
sticking plasters.
"In comparison to other antiseptics, silver has its advantage
of being effective on a broad range of germs even in small concentrations
and over a long period of time. And this being non-toxic at the same
time." Prof. Dr. med. Heinz-Peter Werner; Microbiologist
Skinlife (tm) is a trademarked nano-technology; Xstatic is a trademarked
yarn application. Silver is used in clothing by Rohan Designs in broad
travel clothing applications including shoes and by Bridgedale (amongst
others) in socks.
Other silver compounds
" Silver sulfide, also known as Silver Whiskers, is formed when
silver electrical contacts are used in an atmosphere rich in hydrogen
sulfide.
" Silver fulminate is a powerful explosive.
" Silver chloride can be made transparent and is used as a cement
for glass.
" Silver chloride is a widely used electrode for pH testing and
potentiometric measurement.
" Silver iodide has been used in attempts to seed clouds to produce
rain.
" Silver oxide is used as a positive electrode (cathode) in watch
batteries.
Price
Silver is currently about 1/50th the price of gold by mass, and approximately
70 times more valuable than copper. Silver did once trade at 1/6th
to 1/12th the price of gold, prior to the Age of Discovery and the
discovery of great silver deposits in the Americas, most notably the
vast Comstock Lode in Virginia City, Nevada, USA. This then resulted
in the debate over cheap Free Silver to benefit the agricultural sector
was among the most prolongued and difficult in that country's history
and dominated public discourse during the latter decades of the nineteenth
century.
Over the last 100 years the price of silver and the gold/silver price
ratio has fluctuated greatly due to competing industrial and store
of value demands. In 1980 the silver price rose to an all-time high
of US$49.45 per troy ounce. By December 2001 the price had fallen
to US$4.15 per ounce, and in May 2006 it had risen back as high as
US$15.21 per ounce. As of 2006, current silver prices (and most other
metal prices) have been rather volatile, for example quickly dropping
from the May high of US$15.21 per ounce to a June low of US$9.60 per
ounce before rising back above US$12 per ounce by August.
Folklore and mass culture
Because of the mysticism surrounding silver's lunar associations,
as well as the aesthetic qualities of the white, reflective metal
that cause it to be associated with purity, silver in European Folklore
has long been traditionally believed to be an antidote to various
maladies and fictional monsters. Notably, silver was believed to be
a repellant against vampires (this primarily originates from its holy
connotations; also, mirrors were originally polished silver, and as
such, vampires allegedly cannot be seen in them because they are wicked)
and it was believed that a werewolf, in his bestial form, could only
be killed by a weapon or bullet made of silver, and was equally effective
against vampires, as described in Eastern European folklore. This
has given rise to the term "silver bullet," which is used
to describe things that very effectively deal with one specific problem.
The Lone Ranger of radio serials, comic strips, and some TV programs
leaves a silver bullet as a calling card.
In Christian doctrine, Christ's disciple Judas Iscariot is infamous
for having, according to the New Testament, taken a bribe of thirty
pieces of silver from religious leaders in Jerusalem to turn Jesus
Christ over to the Romans.
In heraldry, the argent, in addition to being shown as silver (this
has been shown at times with real silver in official representations),
can also be shown as white. Occasionally, the word "silver"
is used rather than argent; sometimes this is done across-the-board,
sometimes to avoid repetition of the word "argent" in blazon.