An Introduction on Silver
July 16, 2012 Leave a comment
A Brief History of Silver
Silver was first mined in about 3000 B.C. in Anatolia, now known as Turkey. It was a valuable resource for the Western cultures flourishing in the Near East, Crete and Greece. Ancient Greek and Romans used silver to prevent infection, such as to disinfect water and food during storage, and also for the treatment of burns and wounds as wound dressing.
The chemical symbol, Ag, is taken from the Latin word, argentum, which means silver. Because of its bright luster, silver gradually came to be used, from utensils and other such functional applications, to ornaments and even as the basis of monetary systems. It is now one of the most demanded precious metals, with the total silver fabrication demand in 2011 reaching its second highest level since the millennium at 876.6 Moz.
All That You Need to Know About Silver
General
Proton No.: 47
Classification: Transition Metal
Atomic Weight: 107.868
Oxidation States: 0, 1, 2, 3
Crystal Structure: Face-Centered Cubic
Characteristics: Soft, ductile, tarnishes
Physical
Density (at 293K): 10.49gcm-3
Melting Point: 961.93°C (1235.1K)
Boiling Point: 2212°C (2428K)
Optical
Colour: Silver
L*:
a*:
b*:
c*:
YI:
Colour may be measured using the CIELAB colour scale. It consists of 3 axis organized in a cube manner. The L* axis is the vertical axis, which runs from 0 to 100, representing black and perfect reflecting diffuser respectively. The a*and b* axes have no specific numerical limits. Positive a* is red, while negative a* is green. Positive b* is yellow, while negative b* is blue. Such a colour scale is commonly used for gold. The CIELAB colour space is as shown:
Thermal
Coefficient of Thermal Expansion: 18.9×10-6K-1
Molar Heat Capacity: 25.35Jmol-1K-1
Heat of Fusion: 11.30kJmol-1
Heat of Vaporization: 250.58kJmol-1
Thermal Conductivity: 429Jm-1s-1°C-1
Mechanical
Young’s Modulus: 83GPa
Shear Modulus: 30GPa
Bulk modulus: 100GPa
Poisson Ratio: 0.37
Hardness: 3.25mohs
Electrical
Electrical Resistivity: 1.467 x 10-8 Ωm
Electrical Conductivity: 630.5 mΩ-1cm-1
Chemical
1st ionization energy: 731 kJmol-1
Electronegativity: 1.93
With air: mild, =>Ag2O
With 6M HCl: none
With 6M HCl: none
With 15M HNO3: mild, =>AgNO3
Number of Isotopes: 2
Hydride(s): none
Oxide(s): Ag2O
Chloride(s): AgCl
Occurrence & Extraction
Silver usually occurs as metallic ores associated with gold, copper, lead and zinc. One of the most common ore is argentite, Ag2S.
The major producers of silver are Peru, the United States, Mexico, Chile and Canada. Silver is primarily produced through electrolytic copper refining, gold, nickel and zinc refining, and by application of the Parkes process on lead metal obtained from lead ores that contain small amounts of silver.
In electrolytic refining of copper, two different electro refining techniques are employed: the Moebius and Thum Balbach systems. The silver obtained from electrolysis usually has a purity of three-nines fine or higher. There are also other alternative methods to extract silver, such as assaying and the cyanide process.
Applications
Silver is ubiquitous: in art, science, industry and beyond. This is due to its unique characteristics such as strength (when alloyed), workability, conductivity and ability to endure extreme temperature ranges. More than 95% of annual silver demand comes from industry, investment as well as jewelry and décor.
Industrial
The unique properties of silver makes it an ideal industrial material. The use of silver in soldering or brazing aids the production of smooth, leak-tight and corrosion-resistant joints; providing safety, strength and quality. Silver brazing alloys are found in a myriad of applications such as air conditioning to electric power distribution.
Investing
Due to its scarcity and high demand, silver is regarded as a precious metal. Socially, precious metals have long signified status and wealth. There are many ways one can invest in silver: bars, coins and even silver certificates. In fact, in Utah, United States, silver is used as a legal tender.
Currency
Silver coins were the main circulating currency during the silver standard until the late 19th century. Today, other metals such as copper and nickel are more commonly used. Mexico, however, still uses trace silver in its currency. Many countries also create special edition bullions, commemorative and proof coins using silver which are sold at prices above their face values, which are popular with silver coin collectors.
Jewelry/Silverware
Last but not least, silver jewelry has long been coveted for its brilliant luster and good workability. As pure silver (with a 999 fineness) is too soft for use in jewelry, silversmiths usually alloy it with other metals to harden it. One of the most popular silver alloys would be sterling silver which consists of 92.5% silver and 7.5% copper. Conventional silver alloys are however prone to tarnishing due to sulphuric content in the atmosphere.
Nonetheless, in recent times, high technology materials company like Argentium and Autium have come out with anti-tarnish silver alloys for jewelry and silverware. Some of these silver alloys have tarnish resistance properties that are even better than fine silver.
Types of Silver
Traditional Sterling Silver
Silver alloy of 92.5% silver and 7.5% copper with good hardness. However, it possesses a problem of firescale and tarnish.
Cadmium Silver Alloy
Traditional sterling silver in which the some of the copper content is replaced with cadmium. The resultant alloy is significantly more malleable and ductile, rendering them easier to spin and draw. Cadmium containing silvers are now restricted in many countries because of its toxicity. Some manufacturers now use Britannia silver (95.8% silver, 4.2% copper) as a spinning silver because of its lower hardness.
Deox Silver
Silver alloys containing silicon so as to reduce oxidation and porosity, formulated to enhance casting. Such alloys are generally harder due to the introduction of the deoxidizers.
Platinum Group Metal Additives
Silver alloys with the addition of gold, palladium or platinum. However, such additives significantly increase the intrinsic cost of raw materials and have yet proven to yield excellent anti-tarnish results while enhancing the bright luster of silver.
Germanium Additives
The use of germanium to replace a portion of the copper content in sterling silver enhances its workability and casting characteristics and improving firestain and tarnish resistance. This is the metallurgical theory behind Argentium silver.
Silver Problems
Firestain
Firestain is the reddish purple stain that appears on silver alloys containing copper when they are heated in the presence of oxygen. It may even occur in gold alloys with high copper content. Due to its uneven blotchy appearance, it is often regarded as a blemish, especially where aesthetics are concerned, in jewelry for instance.
When the silver alloy is exposed to oxygen at high temperatures, copper at the surface is first oxidized into red copper(I) oxide, Cu2O, then into black copper(II) oxide, CuO. This phenomenon extends further into the metal with prolonged heating, over heating or repeated heating.
Firestain may be prevented by eliminating oxygen from the metal surface during heating. This can be done by heating the silver alloy in a reducing flame or depositing a protective coat such as boric acid solution on it. Heating the silver alloy in a reducing atmosphere such as hydrogen or ammonia will also yield the same result.
Several methods may be employed to remove firestain. They are: abrasion, chemical removal (acid etching), and covering up(plating).
Tarnish
Silver is said to have tarnished when it starts to show a change in colour from a yellowish tint to light brown and then eventually black. It is independent of silver purity.
Tarnish is primarily caused by contact with hydrogen sulfide, H2S, in the air. The reaction is as summarized: 2Ag + H2S —> Ag2S + H2. Other sources of sulfur compounds include wool, rubber bands, latex gloves and even food. A humid environment will also accelerate the tarnish process.
The silver alloy composition may be adjusted so as to achieve anti tarnish properties (e.g. Argentium Silver). It may also be plated with rhodium, gold or even platinum to form a tarnish resistant coating that delays the onset of silver tarnish.
It is best and easiest to clean tarnished silver in the initial stages. Gently wash and dry silver immediately after use with a phosphate-free detergent. Towel dry using a soft cotton dish towel to avoid water spots. A commercial silver cleaner that is least abrasive may be used if the silver is more tarnished.
Silver Turning Green
http://www.youtube.com/watch?v=P6_XC6sygQQ
Ethics
A large portion of silver on the market is obtained through mining. According to the report ‘Dirty Metals: Mining, Communities and the Environment’, by Earthworks and Oxfam America, the metals mining result in environmental and social costs such as high energy consumption, arsenic emissions, cyanide and mercury poisoning, child labour and human rights abuses, as well as vast landscape damage. An “ethical” source would be recycled silver, since there is traceability and no wastage.
Silver Jewellery Techniques
Processing
Investment Casting: A master model of the design is first created. A wax replica of the design is then created using the latex mold made from the master model. The wax replica is later placed into a flask of plaster. It is placed into an oven to remove melt the wax and form a plaster mold which is then filled with molten metal.
http://www.youtube.com/watch?v=TBH2SQumYis
Working
Sheet Rolling: Process in which a metal is passed through a pair of rolls to form sheets. It can be categorized into hot or cold rolling.
Wire Drawing: Process used to reduce the cross-section area of a wire by pulling it through drawing dies.
http://www.youtube.com/watch?v=Y0vMUGIe2co&feature=plcp
Post Processing
Hardening: Process to strengthen a metal.
Annealing: Heat treatment to alter the properties of a material, such as its hardness and ductility.
http://www.youtube.com/watch?v=T2b4dPWUHLM
Pickling: Process by which impurities and dirt is removed from the silver surface by immersing it in a pickle solution such as sulfuric acid.
Finishing
Polishing: Finishing process to smoothen out a piece or to leave it with a specular reflection.
Tumbling: A rotary tumbler together with stainless steel shots is used to polish jewelry and remove burrs from jump rings or other metal components. The metals also become more durable and better able to hold its shape after tumbling.
http://www.youtube.com/watch?v=iC1WWGvKDMk
Plating: Surface deposition of metal on a conductive surface. Rhodium plating is commonly used on silver to prevent tarnishing.
http://www.youtube.com/watch?v=rNxFBfk4SWI
Joining
Brazing: A method of joining metals by heating a filler metal above its melting point. The filler metal is placed between the metals to be joined by capillary action. In the case of silver, the terms brazing and soldering may be interchangeable due to its high melting point.
http://www.youtube.com/watch?v=J95soMXxhNc
Flux(is flux the medium or method?? Unable to find much info on silver flux): A chemical that may be regarded as a cleaning, flowing or purifying agent. It is used in both extractive metallurgy and metal joining.
Autium Materials 