It is the primary cause of insomnia for silver coin collectors the world over. It’s everywhere – an ever present danger hanging in the air – that given the chance will indiscriminately attack the surfaces of silver coins. It is white spot corrosion, and it’s the worst nightmare for the coin enthusiast who, after a relatively short period of time following the receipt of delivery from the mint or their supplier, sees a dreaded white spot appear on their newly-acquired treasure.
Some might say that this is reason enough for collecting gold coins rather than silver, but for many collectors gold is prohibitively expensive and has a completely different aesthetic appeal.
Melodrama aside, a research project undertaken by the Shenyang Mint Co. Ltd appeared in the March 2011 edition of the journal 腐蚀科学与防护技术 (Corrosion Science and Protection Technology). This research was aimed at identifying the causes of white spot corrosion and in light of this knowledge, tried to propose various preventative measures.
Below is a translation (highlighted in bold) of the introduction and conclusion from the published work. The research methodology, discussion and specific results are not included in this translation, since the finer points of chemistry are not the translator’s area of expertise even in English, let alone Chinese. To clarify, we take no credit for this research or its findings. All credit is due to the researchers Kang Jin-zhe and Yu Hong.
Silver is a type of precious metal that has good conductivity, ductability, and is resistant to corrosion. Apart from its use in the television, communications, and jewellery industries, it is also widely used in China’s production of precious metal commemorative coins. Due to their lustre and rich and colourful appearance, silver commemorative coins are widely desired and revered by collectors and enthusiasts among the many numismatists. However, in exposing silver coins to air in the atmosphere, their surfaces often develop white coloured spots (or simply white spots).
White spots not only destroy the complete artistic effect of the appearance of the silver coin, but also greatly affect the coin’s collector value and price potential. According to statistics, silver coins both inside China and abroad suffer from this phenomenon. Over an extended period of time, silver may undergo a different type of colour change or tarnishing, turning yellow or black. This is different from these white spots which are not easy to remove with conventional cleaning methods. The researchers carried out extensive research on the mechanism of yellow and black discolouration, as well as reports of the appearance of white spots on silver surfaces. To achieve this, this paper will investigate the causes of the appearance of white spots on silver coins through research and analysis, as well as proposing appropriate preventative measures.
In a mild and humid climate, Cl–, O2, and H2S corrosion was the main cause that led to the development of white spots on the surface of the silver coins.
White corrosion spots on the surface of the silver coins were primarily composed of AgCl, Ag2O, and Ag2S, with the first two being dominant. Corrosion spots were white in appearance, and the colour did not change as the corrosion spots grew.
The silver surfaces supplied with oxygen and electricity differed in that the white corrosion spots showed nucleation, and growth was induced.
To prevent white corrosion spots on silver coin surfaces during the production process, silver blanks should be thoroughly dried after washing and cleaning to remove as many surface water particles as possible. At the same time it is recommended to vacuum package the finished product, or to take suitable protective measures to safeguard the surfaces of silver coins.
The key point made in the introduction, as far as collectors are concerned, is the recognition that white spots are a very different problem from run-of-the-mill tarnishing. The researchers highlight that white spots are a different issue, and are harder to clean with standard cleaning practices.
They conclude that the appearance of white spots is as a result of mistakes made during the production process where blanks have not been sufficiently dried. They advise thorough drying of washed blanks, and then careful vacuum packaging of the finished product. These measures should serve to protect the silver coin from the reactive ions found in the air that cause this problematic corrosion.
A full version of this paper can be found written in Chinese here: http://china-mint.info/forum/index.php?PHPSESSID=0c63156f9bb361f067a6aa06caa29a6a&action=dlattach;topic=7823.0;attach=18621
Kang, Jinzhe and Yu, Hong. “银纪念币表面滋生白斑机理研究.” 腐蚀科学与防护技术 23 (2011): 186-190.