Tarnish Resistant Sterling Silver
Silver objects, including jewellery have long been subject to the tarnish that occurs with time and exposure to certain chemicals.
There are a number of new alloys that have come on to the market in recent years that are a lot more resistant to tarnishing, than pure silver or conventional (925/75) sterling silver.
Sterling silver has been used for many many years in very much the same alloy across all manufacturers. 92.5% silver is alloyed with 7.5% copper to form ‘conventional’ sterling silver.
The copper is added to strengthen the alloy, pure silver is typically too soft for use in jewellery, hence sterling becoming the choice of most jewellers.
Both pure and sterling silvers will tarnish due to exposure to sulphur compounds (more information) and sterling silver can also suffer from a condition known as ‘firescale’ created during casting or soldering.
Fire scale is a reddish/purple discolouration which occurs when the copper in sterling silver is heated in the presence of oxygen. (Pure silver lacks the copper that causes firescale in sterling silver.) Soldering, casting and annealing operations all create temperatures high enough to cause firescale in sterling silver. During soldering, this can be reduced by coating the surrounding areas round the solder joint with a flux that prevents oxygen reaching the metal surface. However some firescale will still occur at the joint and must later be removed by immersing the piece in an acid solution called ‘pickle’ and then being polished.
New tarnish resistant alloys of sterling silver replace some of the copper with germanium (a metal) and this almost entirely prevents firescale from occurring. It also greatly slows the rate at which the silver will tarnish when exposed to the sulphur compounds that create the well known greyish silver sulphide, called ‘tarnish’. Other new alloys may also contain varying amounts of zinc, silica and palladium.
The germanium in the alloy creates a surface oxide layer that forms a physical barrier to sulphur atoms. The atoms in germanium are small and very mobile which allows them to freely move to the surface of the alloy, regenerating the protective barrier in a continuous cycle. This protective barrier is also the reason firescale is so drastically reduced, oxygen is stopped from reaching the copper within the alloy and forming cupric oxide (CuO2) which is reddish in colour and cupric oxide (CuO) which is black.