Why Do Some Silver Coins Tone Black Instead Of Rainbow?

hmmm sulfur as in matches?

Why does silverware tarnish black? I'm guessing it's the same reason.

I think black toning is the typical path for silver toning. If you set silver out in the air long enough, it will eventually turn black.

Rainbox toning gets introduced by other types of airborne chemicals that get mixed in. So, for example, if a coin was stored in a cedar box or a leather purse or an album, it may have been exposed to a different mix and toned differently.

There's a lot of interest in toning currently - to the point that now people are artificially toning coins. For example, I have heard things such as wrapping a coin in a Taco Bell napkin (it's got to be Taco Bell - something about the chemicals used in them), putting them in a potato and baking them, or buying old albums specifically because they are known to tone the coins. The grading services have begun rejecting coins as being AT or artificially toned.

Ken

There are quite a few toning discussions on the forum, with lots of good information. In a nutshell, the primary toning compound on silver coins is silver sulfide. However, the color on the coin is determined by the thickness of the sulfide layer. When the layer is thin it is still transparent to visible light and colors are produced by diffraction as the light reflects through the layer. The same effect causes the swirls of rainbow color on soap bubbles and oil slicks. If the coin continues to be exposed to sulfur-containing chemicals the toned layer gets thicker, eventually becoming opaque. At that point you have a dark grey or black coin with little or no color.

Toning can be removed using commercial cleaning products, like E-Z-Est coin cleaner. Excessive use of such cleaners however will strip metal from the coin surface and degrade the luster. They need to be used with caution (if you use them at all!).

I find that the black tarnish can be easily removed by rubbing your thumb over it, if caught early enough.

Most silver compounds upon exposure to light turn black. The colors of toning on silver are the result of the thickness of the film of silver sulfide or other compounds on the surface. (This is caused by a phenomenon called thin film interference.) A single layer will result in a monochrome coin, layers of different depths results in a coin with multiple colors (rainbow). If the film becomes too thick the colors become darker and eventually black.