Author Topic: Research into White Spots  (Read 3734 times)

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Research into White Spots
« on: June 26, 2015, 02:19:24 AM »
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...

Source: 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.

Offline Slabman

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Re: Research into White Spots
« Reply #1 on: June 26, 2015, 08:38:46 AM »

1.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.

2.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.

3.The silver surfaces supplied with oxygen and electricity differed in that the white corrosion spots showed nucleation, and growth was induced.

4.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."

FWIW - I have stored every single coin, whether or not in slab, in an Intercept bag for many years.  I never once had any white spots develop nor toning.  Whether it was silver or gold.  As the article concluded chlorine and hydrogen sulfide both contribute to the development of white spots.  Intercept has the ability to neutralize and clean the air surrounding the coin of these gases.  Intercept does not act like a magnet but must be kept as close as possible to the coin.  Some like to add copper pennies to their storage container.  However if the copper penny is not close to the coin, which is impossible, it won't do any good.   Also, vacuum sealing does not help nor do any other storage methods since both gases can pass thru the poly very easily.