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What Causes Toning, Spots Or Discoloration?

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Valued Member

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 Posted 01/05/2020  9:54 pm Show Profile   Bookmark this topic Add BGLI to your friends list Get a Link to this Message
Maybe experts here can discuss what causes toning, black spots and discoloration or crusting on coins? Is rim toning a result of coins from albums or relate to how they were stored? How can you identify artificial toning? Hoping to learn more about coin discoloration and toning, thanks
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 Posted 01/05/2020  11:34 pm  Show Profile   Bookmark this reply Add Sap to your friends list Get a Link to this Reply
"Toning" or "discolouration" is, in a word, corrosion. Usually a very, very thin layer of corrosion, but corrosion none-the-less. Corrosion on coins is caused by exposure to the environment, usually some component of the atmosphere. If we're talking silver coins, then the atmospheric culprit is sulfur - usually in the form of sulfur dioxide or hydrogen sulfide.

Sulfur in the atmosphere can come from several sources. There is a small amount present in "natural" air. Living in a polluted city obviously has more. Kitchens where onions and similar foods are prepared cooked also create a large amount of sulfur in the air. Finally, sulfurous gases can leach out of various kinds of wood, paper and other "solid" objects over time. Natural Rubber, in particular, is very high in sulfur and gives off a lot of sulfurous gases as it slowly decomposes.

The vibrant colours of "toning" that coin collectors seem to appreciate are caused by thin film interference, where the "thin film" in question is a layer of corrosion by-products sitting on the surface of the metal. The thinnest films cause red colour, then it moves down through the rainbow: yellow, orange, green blue. Once you get past blue, the corrosion layer becomes too thick for light to pass through it, and it becomes opaque - which we see as black. At this point, people usually start to call it "tarnish" rather than "toning" and it suddenly becomes undesirable.

Not all discolouration is caused by atmospheric effects, however. Black spots, often called "carbon spots", are caused by small droplets of water landing on a coin. If the drops are generated by someone coughing or sneezing, then it ain't water, it's saliva - which contains all kinds of enzymes and chemicals which can accelerate the formation of a corrosion patch where the droplet landed. Fingerprints, of course, are comprised of oil from your skin, and that oil is quite rich in sulfur compounds - which is why fingerprints turn black on coins.

Both "natural toning" and "artificial toning" are caused by chemistry, where the metal reacts with something in the atmosphere to create a corrosion by-product. Using the terms "natural toning" and "artificial toning" can be deceptive; if identical chemicals are used to create "artificial" toning, then there is logically no way to tell the difference. The coins, and the chemicals reacting with the coins, cannot measure or preserve the intent of the person causing the coins to be exposed to the chemicals. If my Grandma kept some coins in a yellow envelope for a few decades and they came out all lovely greenish-orange toned, that's considered "natural". But if I did it to some otherwise-identical coins with the specific intent of harvesting such colours in a couple of decades time, that would logically be "artificial" - yet the chemicals and circumstances are identical. The only difference is in the intent: my Grandma wasn't trying to create pretty-coloured coins, but I was.

When describing the more criminally-deceptive kinds of coin alteration, it is perhaps better to use "accelerated toning", rather than "artificial toning", because it more clearly describes the intent and effect: to attempt to reproduce the effect of years of natural aging, but to do so in hours or days, rather than years and decades. Accelerated toning does indeed require the use of chemicals not found in nature, and it is those chemicals and their by-products which can most easily be detected.

Hope this helps.
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
Bedrock of the Community
13014 Posts
 Posted 01/06/2020  12:01 am  Show Profile   Bookmark this reply Add basebal21 to your friends list Get a Link to this Reply

How can you identify artificial toning?

Aside from the most blatant you really can't 100 percent. Anything that can be done in a lab or by man nature can do too.

Then theres the fact the hobby can't even agree what artificial means. If I leave something on a window ledge or desk is that artificial, if so why isn't album toning considered artificial as well etc. There's a million scenarios you can run with that.

Market acceptable vs not market acceptable is what you want to focus on. That really is just do things have the right look. It's also going to be different for different coins, ASEs will tone different than Morgans which will both different than nickels and copper and so on.
Pillar of the Community
United States
1448 Posts
 Posted 01/06/2020  07:34 am  Show Profile   Bookmark this reply Add Steelers72 to your friends list Get a Link to this Reply
Pretty much what numismatic student said in terms of the chemistry & physics behind the toning process. You have to train your eye by looking at many examples and then you can easily discern AT from NT coins. For starters, if the colors are too fluorescent on business strike coins, it's usually AT. Coin doctors try to accelerate the toning by gassing the coins from within slabs or by heating/torching the coin.

Many coins toned from old Whitman albums, holders, or canvas bags which contain a high sulfur content. Pair that with a hot damp environment or enclosed in a wood drawer you'll see more dramatic colors. The old canvas bags from the late 19th/early 20th century caused the coins stored within to toned extravagantly. Read into the Battle Creek, Economite Hoards. Coins from these hoards toned nicely due to their storage location:

Like basebal said, the term market acceptable when it comes to toned coins is an ongoing debate within the hobby. Buy what you like, but try to stick to graded coins until you get the gist.

Edited by Steelers72
01/06/2020 08:43 am
Bedrock of the Community
United States
12569 Posts
 Posted 01/06/2020  08:43 am  Show Profile   Bookmark this reply Add panzaldi to your friends list Get a Link to this Reply
one thing to add to artificial toning. when the coin appears to be re-colored (this happens in copper coins) it pretty much stands out. the color may be an ugly dull red and is pretty consistent across both sides of the coin. one other way a coin can be AT is by cooking it in an oven. you can learn more by just doing a search on Artificially toned coins where you can see many examples and the cause
Valued Member
United States
228 Posts
 Posted 01/06/2020  2:22 pm  Show Profile   Bookmark this reply Add BGLI to your friends list Get a Link to this Reply
Thanks and great info and very helpful!
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