Toning - Technical/physical question?

[Regis44]

Most toning seems to be an actual color change "in" the metal itself and some seem to microscopically coat a coin while while not disturbing detail. Is this an optical illusion or can toning happen differently on different coins (different metal composition)? To me some of the toning seems to be like a microscopic layer of corrision. Some of the coins (pennies & dimes) I've had in blue Whitman folders since the 50's show from clean down to obvious crusty corrision. None of what I personally have is beautiful like what I see here on the forum though.

 

Is there a simple answer to actual consistancy of toning or range of toning and what part corrosion plays?

 

I appreciate the wealth of education on this forum. One day perhaps I will be able to contribute an answer and not just questions.

 

Regis

[pendragon1998]

Wiser minds than me could probably explain it to you better, but I'll give it a shot. Toning is a microscopically thin, translucent deposit of an oxidized metal on the surface of the coin. Just like raw copper or silver cookware will "tarnish" or oxidize, so too will any coins made of metal, provided there are oxidizers present in the environment (e.g. atmospheric oxygen, water, acid-containing paper products, etc). When talking about an iron nail oxidizing, we say it's rusting - same process.

 

These oxidizing agents act on the surface layers of metal on a coin, starting with the outer layer of molecules, and working slowly down into the coin, they change the chemical composition of the pure alloys into the oxidized forms of the consituent metals. Were this process to continue beyond the first few molecular layers of metal, the coin would eventually begin to lose significant detal and start to look quite corroded; however, in many cases, the thin 'skin' of oxidation on the outside of a toned coin serves as a sort of barrier to further oxidation, slowing down the process significantly.

 

The reason a little oxidation causes toned coins to be so colorful, is that when light passes into a layer of toning, it is split into two parts: one part of the lightwave bounces off of the surface of the toning and returns to you eye, while a second part passes through the translucent toning, bounces off the coin's surface, and returns through the toning to hit your eye. Because the second beam had to travel farther, it took it a slightly longer time to return to your eye, making it out of phase with the other part of the beam. The angle at which light strikes the coin affects the degree to which the light parts are offset as well. This results in a phenomenon called iridescence - the same reason oil makes pretty rainbow colors on water, and why hummingbirds are so colorful when the light reflects on them from one angle, but appear dark when light hits them from another angle. If you angle the coin differently between the light source and your eye, the colors will appear to change, because of the slightly different offset times between the lightbeams returning to your eye.

 

Now there's supposedly an order in which colors should appear in natural toning, but I'm no expert, so perhaps someone else could chime in on that, and perhaps point out if I'm wrong anywhere above.

 

Interesting topic!

[TJ'S Coins]

Well said, Pendragon! I like your analogy to the hummingbird colors.

[Architecht]

http://boards.collectors-society.com/sho...e=0#Post1118997

[truthteller]

Excellent answer. I might add, two types of toning for silver. An oxidation of the "thin film" on the surface of the metal, and an oxidation of the silver itself. The white haze on proof coins, for example, is silver oxide, which is a direct result of ambient air in contact with metal. "Thin film", which creates the wild colors, is a reaction with outside gases, such as sulfur, with the silver oxide. Coins that are wildly toned either naturally or artificially, have a reaction creating the film. Coins that are toned directly onto the silver metal surface with artificial toning techniques often produce a splotchy, uncontrolled toning pattern, since the metal reaction is random.

 

 

TRUTH

[pendragon1998]

So perhaps a good way to say it would be 'primary oxidation' is where the coin's metal reacts with oxidizers in the environment, whereas 'secondary oxidation' is where the products of the primary oxidation are themselves further altered by environmental chemicals?

[truthteller]

So perhaps a good way to say it would be 'primary oxidation' is where the coin's metal reacts with oxidizers in the environment, whereas 'secondary oxidation' is where the products of the primary oxidation are themselves further altered by environmental chemicals?

 

 

 

 

 

TRUTH

[Regis44]

Great info - thank you

 

So it sounds like the "skin" folks talk about and I believe I see on some unc coins is actually a tone oxidation. And, it's usually nearly clear unless there was other other environmental issues in the metal or surrounding air affecting the refractive properties. So, when someone cleans an uncirculated coin (colorfully toned or not) this skin is removed. If that is true, do people actually clean coins that are un-attractively toned?

 

I also like the humming bird analogy for the different colors.

 

Thanks to all for the help.

 

Regis

[Lamar]

I have a 2000 S silver Dime whereRoosevelt's head is a Red rust color and his nose is almost red red. It is still in the proof set from the mint. I should add that the mirrored surface of this coin has no change in color or texture. Only the raised areas like the lettering but even the lettering is not as distinct as his head.

 

Thank You,

Lamar

[truthteller]

Great info - thank you

 

So it sounds like the "skin" folks talk about and I believe I see on some unc coins is actually a tone oxidation. And, it's usually nearly clear unless there was other other environmental issues in the metal or surrounding air affecting the refractive properties.

 

Correct.

 

As far as "cleaning" coins, often collectors as well as dealers will use a "dip" solution, which is a weak acid(Jeweluster), to remove a microscopic layer of silver, thus removing the toning and leaving a fresh layer of silver/gold/copper. This dipping process, in general, does not affect the luster. However, repeated dippings often make the coin look bleached, bland and lifeless. Once the new layer of silver is exposed, the oxidation process begins again. Induced heat will accelerate the process creating a new silver oxide"skin".

 

 

TRUTH

[Bruceswar]

check out TomB's Site... great information there...

[TomB]

Pendragon, your post is so familiar to me that I had to read my web site to see if it was a direct quote. I guess there are only so many ways these things can be worded, but that was eery.

[pendragon1998]

Tom,

I looked over your site a month or two ago, so some of your thoughts might have been echoed in my post. When I wrote the post, however, I was writing off the cuff with no reference pages open. Most of what I was saying, I was channeling from my physics and chemistry classes and my ornithological studies of how light interacts with feathers to create color, but I agree - there's only so many ways to describe a simple physical/chemical process like toning. Sorry to freak you out!

[TomB]

No problem! I agree that writing scientific explanations leave folks with only limited options. It happened to me when writing papers, my PhD thesis and when I write grants. Sometimes, when I'm writing, I wonder just whose words are being put down on paper.

[Lamar]

I would agree with you but this coin is a silver proof coin that came streight out of the proof case in this red faced condition. If it were a clad, I would almost think that the silver had not covered the face area. Not being a clad, I would think that the red area could be caused by oxidation but is silver oxide red. The coin came back PF69 so it was stamped well. It could not have been cleaned or dipped and be solid (90% ) silver and be a 69. I will have to study on this one a bit.

Maybe check out what the oxide color could be. this is not a transparent layer with silver color in the background, The color is solid.

 

Thank You,

Lamar