WIKI
Precious metal
A major use of silver is as a precious metal, and it has long been used for making high-value objects reflecting the wealth and status of the owner.
Jewellery and silverware are traditionally made from sterling silver (standard silver), an alloy of 92.5% silver with 7.5% copper. In the United States,
only an alloy consisting of at least 92.5% fine silver can be marketed as "silver". Sterling silver is harder than pure silver, and has a lower melting
point (893 °C) than either pure silver or pure copper. Britannia silver is an alternative hallmark-quality standard containing 95.8% silver, often used to
make silver tableware and wrought plate. With the addition of germanium, the patented modified alloy Argentium Sterling Silver is formed, with improved
properties including resistance to firescale.
Sterling silver jewelry is often plated with a thin coat of .999 fine silver to give the item a shiny finish. This process is called "flashing". Silver
jewelry can also be plated with rhodium (for a bright, shiny look) or gold.
Silver is a constituent of almost all colored carat gold alloys and carat gold solders, giving the alloys paler colour and greater hardness.[7] White 9
carat gold contains 62.5% silver and 37.5% gold, while 22 carat gold contains up to 8.4% silver or 8.4% copper.[7]
Silver is used in medals, denoting second place. Some high-end musical instruments are made from sterling silver, such as the flute.
Dentistry
Silver can be alloyed with mercury, tin and other metals at room temperature to make amalgams that are widely used for dental fillings. To make dental amalgam, a mixture of powdered silver and other metals is mixed with mercury to make a stiff paste that can be adapted to the shape of a cavity. The dental amalgam achieves initial hardness within minutes but sets hard in a few hours.
Photography and electronics
Photography used 30.98% of the silver consumed in 1998 in the form of silver nitrate and silver halides, and in 2001, 23.47% for photography, while 20.03%
was used in jewelry, 38.51% for industrial uses, and only 3.5% for coins and medals. The use of silver in photography has rapidly declined, due to the
lower demand for consumer colour film from the advent of digital technology, since in 2007 of the 894.5 million ounces of silver in supply, just 128.3
million ounces (14.3%) were consumed by the photographic sector, and the total amount of silver consumed in 2007 by the photographic sector compared to
1998 is just 50%.[8]
Some electrical and electronic products use silver for its superior conductivity, even when tarnished. For example, printed circuits are made using silver
paints,[9] and computer keyboards use silver electrical contacts. Some high-end audio hardware (DACs, preamplifiers, etc.) are fully silver-wired, which is
believed to cause the least loss of quality in the signal. Silver cadmium oxide is used in high voltage contacts because it can withstand arcing.
During World War II the short supply of copper brought about the government's use of silver from the Treasury vaults for conductors at Oak Ridge National Laboratory.
Solder and brazing
Silver is used to make solder and brazing alloys, electrical contacts, and high-capacity silver-zinc and silver-cadmium batteries. Silver in a thin layer
on top of a bearing material can provide a significant increase in galling resistance and reduce wear under heavy load, particularly against steel.
Mirrors and optics
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. Silver is usually reserved for coatings of specialized optics, and the silvering most often seen in architectural glass and tinted windows on vehicles is produced by sputtered aluminium, which is cheaper and less susceptible to tarnishing and corrosion.
Nuclear reactors
Because silver readily absorbs free neutrons, it is commonly used to make control rods that regulate the fission chain reaction in pressurized water
nuclear reactors, generally in the form of an alloy containing 80% silver, 15% indium, and 5% cadmium.
Catalyst
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 and water.
Silver coin and Silver standard
Silver, in the form of electrum (a gold-silver alloy), was coined to produce money in around 700 BCE by the Lydians. Later, silver was refined and coined in its pure form. Many nations used silver as the basic unit of monetary value. The words for "silver" and "money" are the same in at least 14 languages.
In the modern world, silver bullion has the ISO currency code XAG. The name of the United Kingdom monetary unit "pound" reflects the fact that it originally represented the value of one troy pound of sterling silver. In the 1800s, many nations, such as the United States and Great Britain, switched from silver to a gold standard of monetary value, then in the 20th century to fiat currency.
Medicine
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 are normally associated with these other metals. Its germicidal effects kill many microbial organisms in vitro, but testing and standardization of silver products is difficult.[11]
Hippocrates, the father of modern medicine, wrote that silver had beneficial healing and anti-disease properties[cite this quote], 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.[12] Its germicidal effects increase its value in utensils and as jewellery. The exact process of silver's germicidal effect is still not well understood, although theories exist. One of these is the oligodynamic effect, which explains the effect on microorganisms but would not explain antiviral effects.
Silver compounds were used 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 (SSD Cream),[13] which was generally the "standard of care" for the antibacterial and antibiotic treatment of
serious burns until the late 1990s.Now, other options, such as silver-coated dressings (activated silver dressings), are used in addition to SSD cream. However, the evidence for the effectiveness of such silver-treated dressings is mixed and although the evidence is promising it is marred by the poor quality of the trials used to assess these products.[15] Consequently a major systematic review by the Cochrane Collaboration found insufficient evidence to recommend the use of silver-treated dressings to treat infected wounds.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, silver is being used with alginate, a naturally occurring biopolymer derived from seaweed, in a range of products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims.[16] In 2007, AGC Flat Glass Europe introduced the first antibacterial glass to fight hospital-caught infection: it is covered with a thin layer of silver.[17] In addition, Samsung has introduced washing machines with a final rinse containing silver ions to provide several days of antibacterial protection in the clothes.[18] Kohler has introduced a line of toilet seats that have silver ions embedded to kill germs. A company called Thomson Research Associates has begun 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."[19] The U.S. Food and Drug Administration (FDA) has recently approved an endotracheal breathing tube with a fine coat of silver for use in mechanical ventilation, after studies found it reduced the risk of ventilator-associated pneumonia.[20]
It has long been known that antibacterial action of silver is enhanced by the presence of an electric field. Applying a few volts of electricity across silver electrodes drastically enhances the rate that bacteria in solution are killed. It was found recently that the antibacterial action of silver electrodes is greatly improved if the electrodes are covered with silver nanorods.[21]
Medication
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 most colloidal silver preparations are harmless, some people using these home-made solutions excessively have developed argyria over a period of
months or years.[22] High doses of colloidal silver can result in coma, pleural edema, and hemolysis.[23]
Silver is widely used in topical gels and impregnated into bandages because of its wide-spectrum antimicrobial activity. The anti-microbial properties of
silver stem from the chemical properties of its ionized form, Ag+. This ion forms strong molecular bonds with other substances used by bacteria to respire,
such as molecules containing sulfur, nitrogen, and oxygen.[24] Once the Ag+ ion complexes with these molecules, they are rendered unusable by the bacteria,
depriving it of necessary compounds and eventually leading to the bacteria's death.
Food
In India and Pakistan, foods, especially sweets, can be found decorated with a thin layer of silver known as vark. Silver as a food additive is given the E
number E174 and is 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.
Clothing
Silver inhibits the growth of bacteria and fungi. It keeps odor to a minimum and reduces the risk of bacterial and fungal infection. In clothing, the combination of silver and moisture movement (wicking) may help to reduce the harmful effects of prolonged use in active and humid conditions.
Silver is used in clothing in two main forms: A form in which silver ions are integrated into the polymer from which yarns are made (a form of nanotechnology) A form in which the silver is coated onto the yarns.
In both cases the silver prevents the growth of a broad spectrum of bacteria and fungi.
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 during storage, and 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, wound dressings containing silver are well established for clinical wound care and have recently been introduced in consumer products such as sticking plasters.
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