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Pillar of the Community
United States
3486 Posts |
I am reacquainting myself with the CCF and ebay. I see many coins described as having corrosion. Which metal corrodes? Not gold - inert Not silver - it tarnishes but not corrodes.. Stop - both have 90 percent copper/ Is THIS (copper) what corrodes? Nickels - copper and nickel Cents - copper and nickel Barber/"V"/Liberty nickels suffer from this A LOT..... I am not buying coins at present...just seeing what is being offered.....I see bad coins, and I see BAD COINS!! What is the cause? Edited by matthewvincent
05/25/2019 5:57 pm
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Pillar of the Community
5464 Posts |
Corrosion:
Damage that results when reactive chemicals act upon metal. When toning ceases to be a "protective" coating and instead begins to damage a coin, corrosion is the cause. Usually confined to copper, nickel and silver regular issues, although patterns in aluminum, white metal, tin, etc., also are subject to this harmful process.
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Bedrock of the Community
United States
94367 Posts |
And of course rust on the 1943 steel cents.
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Pillar of the Community
 United States
7935 Posts |
There are lots of other fancy names out there, but to a chemist, corrosion is just a chemical reaction with a metal.
Rust is corrosion of iron.
Verdigris is a type of corrosion of copper. So is "bronze disease." And so is the tarnishing of newly minted copper or bronze to "RB" and to "BR."
Toning and tarnish are both corrosion of silver.
Gold is relatively non-reactive, so it is not prone to corrosion.
It's like the word infection in medicine. It can be viral, it can be bacterial. It can affect any organ or body part. It can be a gumboil, pneumonia or necrotizing fascitis.
USSID18 hits the nail on the head in mentioning "damage." I think in numismatics, corrosion is reserved for situations where the chemical reaction is seen to be a detriment rather than neutral or beneficial.
It's a value judgement of a chemical reaction.
Edited by tdziemia
05/25/2019 8:32 pm
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Bedrock of the Community
United States
20753 Posts |
Gold is really not inert. Gold does combine with other substances. For example Au2S is a BR/BK discoloration on Gold. AuCl3 is Gold Chloride found on gold near swimming pools. HAuCl4 is a Chloride Acid. Etc. Gold also combines with Fluorine, Cyanide and dissolves in Mercury and Aqua Regia. Corrosion is commonly used instead of Tarnishing which is just a form of corrosion. Basically all metals react with something else and form something else in a reaction. With most gasses, metals form what people say is corrosion. Quote:
There are lots of other fancy names out there, but to a chemist, corrosion is just a chemical reaction with a metal. As noted. Edited by just carl
05/26/2019 08:31 am
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Pillar of the Community
United States
1667 Posts |
In my opinion. Corrosion is corrosion a further step of oxidization. But tarnish, toning,ect, is just oxidization and all metals can oxidize as well as all metals can corrode given the right conditions, even gold.
Tarnish or toning could be removed from metal without significant damage to the metal, corrosion can be removed also but it may take an abrasive to do it or just leave behind pitting where the corrosion has eaten away metal.
All of this is just my opinion on the subject. I won't say either is a good thing, desirable, but one isn't eating away at the surface of the metal and for a long time acts as a protective coating until it has the coverage and right conditions to react and start corrosion.
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Moderator
 Australia
16809 Posts |
As a chemist, I would concur with the sentiment that "all chemical reactions with a metal are corrosion". The only difference - the only difference - is that certain chemical by-products, under certain conditons, are considered "acceptable" or even "preferable", in an aesthetic sense. Paying premiums for "monster toning" is not an attitude I persoanlly can condone - to me, corrosion is corrosion, and something for a coin collector to try to avoid happening to their coins.
Re: gold. When we say "gold is inert", we mean "gold, under normal Earth-like atmospheric conditions of temperature, pressure and composition, will not corrode". And this is essentially correct. Regarding the chemical reactions mentioned by JustCarl: gold sulfide is unstable; take it out of the rather extreme chemical conditions in which it formed and it will revert back to pure gold; you won't find it on your coins. Gold chloride can form, slowly, in highly chlorinated environments (such as the air around swimming pools), which are not normally encountered in nature. It is also water soluble, so a piece of affected gold left outside in the rain will be washed clean, leaving pure gold behind. And chloroauric acid is what you get when you dissolve gold in aqua regia - again, a substance not normally encountered in the environment.
Gold submerged in seawater will also dissolve, but very, very slowly, over thousands of years - which is why the gold dredges off the Alaskan coast can suck gold nuggets off the seafloor. I do not think pure gold coins retrieved from shipwrecks after just a few hundred years underwater show evidence of any adverse chemcial reaction (though they might get damaged by underwater lifeforms).
However, as pointed out above, coinage gold is not usually "pure gold" - it is diluted with copper (or sometimes silver) to give the coin additional physical strength, because pure gold is too soft to make a good circulating coin. It is this diluent that is the cause of most of the toning and corrosion that you might find on old gold coins.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis
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Moderator
United States
187862 Posts |
Excellent information, Sap.  |
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Pillar of the Community
 United States
3486 Posts |
Toning protects coins?
Never thought of that.
Corrosion comes later?
OK I am lost.
Let me go back to the golden rule:
If the eye appeal of a
coin does not appeal to you ---
what makes you think that it will appeal to anyone else?
Saves a LOT of bad purchases..
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Bedrock of the Community
United States
20753 Posts |
Quote:
If the eye appeal of a
coin does not appeal to you ---
what makes you think that it will appeal to anyone else? Because people are different. Some like toning and some don't. Some like that old look of tarnishing and corroding and some will not touch it. Many like coins to look like just minted or better if possible. This is why cars are all different. Why houses are different. Why clothing is different. People as a rule don't like every thing to look the same. |
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Pillar of the Community
United States
7935 Posts |
Quote:
If the eye appeal of a
coin does not appeal to you ---
what makes you think that it will appeal to anyone else? Ah, we've now moved from chemistry (and related terminology) to aesthetics, and the related decisions .  All subjective, and plenty of issues of supply/demand/price as well. I own quite a few medieval and renaissance era coins with bits of corrosion because a more perfect, corrosion-free coin carries a large premium that I am often unwilling to pay. And I don't care about resale price. Edited by tdziemia
05/28/2019 08:23 am
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Replies: 10 / Views: 4,433 |
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Posted 05/25/2019 6:20 pm
