Are All Rainbow Toned Coins Preciously Dipped?

First and foremost forget about natural and artificial. Natural toning can look artificial and artificial can look natural. Anything humans can do nature can do as well, lots of natural is also very ugly.

That said there are right looks and wrong looks.

Dipping also isn't a bad word or bad thing to do when done right.




100 percent false claim. Any 100 percent absolute claim about coloring is immediately incorrect. You may have misinterpreted what they were trying to convey, but I would hope that someone with a respected name would not make such absolute claims on toning.

The effect we call "toning" is caused by a combination of chemistry and physics. The physics is the concept of thin film interference. The chemistry is the process whereby a thin film of metal-corrosion byproducts is created on the surface of a coin.

"Natural" toning is created when certan compunds in the air - for silver and bronze coins, that's usually sulfur-containing compunds like sulfur dioxide or hydrogen sulfide - chemically react with the metal surface, to create a layer of metal sulfide. On copper and bronze coins, some copper oxide is likely to be made as well.

"Artificial" toning usually uses the exact same chemical reactions, so it is perhaps better to call it "accelerated toning" rather than "artificial", as it happens within days or weeks rather than years.

So yes, a coin can acquire "rainbow hues" through natural means - that is, the coin can attain those colours by accident, without someone deliberately trying to create those colours. But it does require that the coin be kept undisturbed in a specific, sulfur-rich environment for years. Perhaps you live in a mining or refinery town with lots of sulfur in the air (this isn't as common in these more enlighteded times as it was in the previous two centuries), or perhaps there's sulfur fumes slowly leaching out of some nearby paper, wood or rubber. The bags of stockpiled silver Morgan dollars unearthed some years ago all qualify for this, as the bags were literally untouched for decades before being opened and sold.

But how, exactly, does one determine whether toning is "natural" or "accelerated", when the chemical processes are much the same? If my grandma puts some silver coins in a paper envelope for a decade, and they get rainbow toning because of sulfur in the paper, it's considered "natural" because she wasn't trying to make rainbow-toned coins. But if I do it, is it considered "artificial", because I know what sulfurated paper will do to coins? If I leave a bunch of coins for a few weeks on the windowsill of the kitchen where some friends are constantly using garlic in their cooking (garlic contains lots of organosulfur compounds), is that considered "Accelerated toning"? It's "natural" - but one could argue, it is not "normal".

As you can see, much of our distinction between "natural/normal toning" and "artificial/accelerated toning" is in the intent of the person handling the coins - and chemistry doesn't record your intent.

Personally, I can't look at a monster-toned coin and not think "that is one seriously corroded coin".

Sap, Thanks for the scientific analysis. It satisfied a lot of my curiosity.
The whole "accelerated toning" process you describe makes me think of dyeing Easter eggs. If this effect can essentially be obtained on demand, then it seems a lot less special. And on closer inspection, I see porosity accompanying those unusual colors, so I appreciate your point about corrosion.

Accelerated toning can look different to the eye, especially under magnification, and coins are judged primarily by appearance: does the toning "look fake" or not. The TPGs also claim to have some analytical chemistry tools (perhaps a GCMS, and/or an FTIR; I don't think an XRF will do much good for this purpose) that can detect some of the chemicals the tone-doctors often use for certain kinds of accelerated toning, but I do not know how often they'd use them - presumably only for the most valuable items, since running such tests is neither cheap nor quick. The human nose can be just as powerful as the "chemical sniffers" for certain chemicals - the whiff of hydrogen sulfide (rotten egg gas) or tobacco smoke will be certain giveaways for those two common acceleration techniques.

This article on the PCGS website is old (dating from 2000) but still an interesting read on the subject.

SAP, Thanks for the PCGS link. It seems like a rainbow toned no details TPG certified coin should find marketplace acceptance if it has good eye appeal.
But then there's that corrosion thing again. After you called it out, I see little pits everywhere.

Spruett,
I liked chemistry a lot in high school and took two years of it at that level, so I find your and Sap's posts extremely interesting. But that was over 40 years ago and I know that what I think I remember could be partially or completely incorrect.
My recollection is that all or almost all substances that are not inert are subject to oxidation and accompanying color change and ultimate disintegration. I think a copper or bronze coin turning from mint red to chocolate brown is due to oxidation, which we call toning. I think the same thing happens when a silver coin turns gray. Because silver coins are alloyed with 10% copper (or a little less copper for the earlier dates) natural oxidation of a coin can result in multiple colors. I have a nice original Liberty Seated half with a blue green ring (the color of copper oxide) around the obverse rim. I have always understood gold to be inert (it doesn't tarnish) and believed toning on gold coins to be due to their 10% copper composition.
I agree that what we see as unremarkable toning (non-rainbow) doesn't usually deteriorate the surface quality to the point of pitting. But a rusted-out car (iron oxide) can corrode so much it crumbles. In the case of iron or non-stainless steel water is a well-known oxidation accelerant. I would guess that so-called "monster toned" coins have been subjected to another substance (like water with iron) that accelerates oxidation and resulting corrosion and introduces additional colors. Sulfer-containing paper is one substance I know of.
I have some beautifully toned coins with excellent surfaces that don't have the rainbow colors. But examining pictures of rainbow toned coins, I do see the obvious beginnings of corrosion Sap mentions.
A little knowledge is a dangerous thing, and I have spouted science here that my memory may have failed me on, so if I have things wrong I would welcome being corrected.