Pure silver is too soft a metal to make jewellery articles out of it therefore it is often alloyed with copper as conventional hardening agent. Moreover, silver cannot carry much weight and the load managing is also left to copper. Sterling Silver, which has become much popular as a jewellery metal consists of the binary alloy of silver and copper. Quite a number of negatives have surfaced on its wake. Here is a review of the issues surrounding Sterling Silver.
Beauty of Sterling Silver jewellery lies not only on the main metal, but the kind of alloy that are mixed to silver. As you are aware an article of 92.5% silver is composed with an alloy of 7.5% of other metals chiefly copper and trace element of Zinc. By adding copper the original beauty, elegance, brightness, reflective capacity of pure silver are protected. Furthermore silver–copper alloy continues to sustain solderability, malleability, ductility of silver. Cosmics of silver as conductor of heat and electricity is not impaired by alloying it with copper.
Disadvantages come along bringing major headaches to jewellers. Casting porosity arises when the copper alloy is heated resulting in empty spaces. Firescale is another ill when silver-copper ally oxidizes to form black or red coloured blemishes. Copper is notorious for tarnishing effects as it reacts to many elements. In the contrary silver does not tarnish in air or water, but reacts slightly to human sweat. Combining these metals, of course leads to a degree of tarnish and discoloration. To offset these minuses the industry has come out with few alternatives. One is to introduce non-tarnish Sterling Silver using metals such as Metalloid germanium, Zinc, Platinum, Tin and additives such as Indium, Silicon and Boron. Magnesium and Cadmium are also used judiciously as trace elements.
One such alloy of improved Sterling Silver is the US Patent 4973446 A, assigned to United Precious Metal Refining Company. The composition of the alloy and the functionality of metals used are described briefly. The breakdown is given in percentages for easy reading
Silver: Main metal at 92.5
Copper: 0.5 for hardening and load carrying of the alloy
Zinc: 4.25 to reduce melting point of alloy, to add whiteness, to act as copper substitute, and to de-oxidise & improve fluidity of alloy
Indium: 0.02 to act as grain refiner and to improve wettability of alloy
Tin: 0.48 to provide tarnish resistance as well as hardening
Boron - copper alloy: 1.25 where composition of Boron is 2 % and Copper is @ 98%. Boron is added in order to reduce surface tension of alloy and to allow blending homogenously
Silicon - copper alloy: 1.00 where composition of Silicon is 10% and Copper is @ 90%. Silicon is added for multi-functionality in de-oxidizing, reducing porosity and slight hardening.
Using this alloy, Sterling silver gem & jewellery pieces can be fabricated where the overall content of copper is reduced from a whopping 7.5 % to manageable 2.625% just about one third of the original proportion. If it baffles you how this percentage of 2.625% has been arrived at just do a little bit of arithmetic.
Copper in pure form 0.5 %
Add part of copper from the Boron-copper alloy which is worked out as 1.225%
Add part of copper from Silicon-copper alloy which is worked out as 0.90%
When you add all these three items you get 2.625%
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