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Tarnish resistant silver alloys

Tarnishing can be a big problem for jewellers selling sterling silver, but do the ‘tarnish resistant alloys’ now available on the market actually work? Dippal Manchanda MSc CSci CChem FRSC, technical director of Assay Office Birmingham, assesses their properties.

Silver has always been widely used, but is increasingly popular as the price of gold soars. In July 2010 gold averaged £782 per oz compared with silver at under £12 per oz, offering jewellers a much more attractive medium in which to work, especially while the white metal trend continues.

However, many people are wary of buying silver as tarnishing is always a problem, and they know from experience that eventually it will turn a darkish dull shade of yellow, brown and even black.
Tarnishing of silver is a surface discoloration and mainly results from the formation of silver sulphide, Ag2S, although other compounds have been recently identified which contribute to tarnishing. The process is triggered by its reaction with elements such as sulphur, moisture, oxygen and chlorides. The deterioration is further accelerated by oxygen and sulphur compounds in packaging materials, in the atmosphere, and in perspiration, perfume and deodorant sprays.

The jewellery and silverware industry has addressed this long-standing problem with a number of solutions, including tarnish resistant treatments and coatings and various measures to prevent packaging from accelerating tarnishing. The development of a sterling silver alloy, or treatment for sterling silver that is tarnish resistant to a much greater extent than the usual silver-copper alloy, would obviously be highly beneficial. Metallurgists have been questioning whether the process of silver tarnish can be slowed down or eliminated by alloying and metallurgical treatment for over a century.
The problem is complicated by the fact that the legal standard sterling silver alloy must contain 925 parts per thousand (ppt) of fine silver, thus leaving only 75ppt of other metal with which to impart the necessary tarnish resisting properties to a predominately silver alloy.

Many approaches have been taken to adjust the composition of silver alloys to make them more tarnish resistant, while retaining the physical properties of malleability and durability required for jewellery manufacture.

In 1912 pioneering work was being carried out by Mr Harold Turner of Messrs William Turner & Company – the well-known gold and silver refiner of Sheffield. In 1922, this firm placed upon the market, under the name of ‘Silanca’, the first tarnish resisting sterling silver ever produced, but for certain commercial reasons and difficulties connected with its manufacture, Silanca was not taken into use.
In 1988 an alloy was patented in America, which requires the additions to silver-copper, silver-gold and silver-copper-gold alloys of at least one element of the following: chromium, tantalum, aluminium, titanium, or thorium, where the added amounts of these elements does not exceed 1.5 per cent weight as a substitute for silver. These elements were found to form a thin oxide layer, which was stable and did not affect the properties of the silver-based alloy. The elements were found to be self-healing, forming a layer of oxide and also reacting preferentially with sulphur to form sulphides, rather than formation of silver sulphide. In this way, tarnish resistance was improved.

Further patents followed; US Patent 5,039,479 describes the usefulness of the additions of silicon, boron and tin to an alloy. The preferred composition is 1.85 per cent zinc, 0.05 per cent indium, four per cent tin, 1.44 per cent copper, 0.01 per cent boron, and 0.05 per cent silicon. The composition described is sufficiently pure in silver to qualify as sterling silver. A similar approach is taken by US Patent 5,882,441, which describes a tarnish-resistant 4.5 per cent zinc, 2.9 per cent copper, 0.1 per cent sulphur alloy.

The addition of more noble elements (ie other precious metals) increases the cost of sterling silver, but is highly effective. US Patent 5,037,708 describes an alloy containing five per cent palladium, two per cent copper and 0.5 per cent indium or zinc. In this alloy some of the copper has been replaced by palladium to enhance tarnish resistance and corrosion resistance, and also to improve colour stability.
The addition of germanium to sterling silver is also noted to reduce tarnishing and fire stain. This alloy is a cadmium-free alternative to the two and four per cent cadmium bearing grades that have been used in the past. A similar composition was claimed by a German refiner in 1992. This refiner’s alloy contained between 0.5 and three per cent germanium, the balance of copper to give 7.5 per cent alloy addition and 92.5 per cent silver to give sterling silver.

Quantitative research by Assay Office Birmingham has shown that many of these alloys are more tarnish resistant than traditional silver copper alloys. However, to date no one has come up with a guaranteed completely tarnish-resistant silver alloy suitable for jewellery. The researchers continue to apply their knowledge of science to fight against nature.

 

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