Holders Prevent Unsightly Toning|
May 22, 2013
As a numismatist, I see lots of silver coins that have been saved over the years but many have not been protected from hydrogen sulfide that is liberated into the air by a variety of chemical reactions.
The silver coins that are tarnished vary in coloration depending on how many micrograms of sulfur per square centimeter have reacted with the silver to form silver sulfide on the surfaces of the coins. A light golden coloration has about 7 micrograms of sulfide per square centimeter on its surface and a bluish tarnish coloration has around 11 micrograms per square centimeter on its surface. More detailed information can be found in my book, “Coin Chemistry.”
Why did the author of this article do a tarnishing experiment by allowing three 1958 Roosevelt dimes to become tarnished? I wanted to show coin collectors how important good coin care is in preventing corrosive sulfur from tarnishing numismatic properties. Basically, tarnish is a form of chemical wear. The rusting of iron, a metal component in cars, is also a form of chemical wear. When silver coins become tarnished, the silver gets oxidized and loses electrons. Below is the equation that occurs when hydrogen sulfide reacts with silver to tarnish it:
4Ag + O2 + 2H2S ––––> 2Ag2S + 2H2O
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The experiment in this article started two and a half years ago when I took four snow white Roosevelt dimes that were in brilliant uncirculated condition and placed them on a shelf in the kitchen. To show how important good coin care is, one specimen was put in an Intercept Shield case and placed next to the other three unprotected dimes.
Over the 2.5 years, I saw the unprotected dimes slowly start to tarnish. They started out showing yellowish tarnish from the exposure to the hydrogen sulfide in the air. As the tarnish thickened, the dimes turned a reddish coloration and then to a dark spotted blue color plus traces of green as seen in the showcased photographs. By contrast, the brilliant uncirculated dime in the Intercept Shield case remained snow white.
There are several good ways to protect silver coins from the sulfide in the air. Certified coins in slabs from the grading services can help protect coins. Other available commercial holders will also protect coins and the more air tight they are, the better they serve to keep out airborne sulfur gases.
Again, my book, “Coin Chemistry,” will help educate those who are interested in coin preservation and also how to carefully remove unwanted tarnish or toning from coins.
The book can be purchased by calling (585) 924-4250, extension 300.
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