Coin Toning

Most naturally-toned coins tone from the edge in, usually as the result of sitting in an album. With silver, the toning follows a color progression with age, yellow being the earliest color and progressing through reds, blues and to black as the toning gets thicker. It's a function of the thickness of the layer of silver sulfide.

Here's some reading:

http://rg.ancients.info/guide/toning.html

Interesting question, which also makes me realize I haven't seen Hadleydog post in a while. He is what I would consider one of the toning experts. You could probably look though his most recent posts and potentially find something on this subject....

Here's another good site from Hadley's home page.

http://www.tonedcoins.org/

I don't care for toned coins. They just look damaged to me.

As far as I know Toning means tha the metal is starting to Tarnish then it will corode. In another words it reacts with the surrounding enviromental factors such as humidity and temperature changes to form an oxide layer on the surface of the metal.

Different metal oxides have different colors for example:

Titanium oxide has a yellow-brown color
Iron oxide has a brown-black color
Cobalt oxide has a blue color
Chromium oxide has a green color....etc

the mixing of these colors together may result in another color for example yellow and blue gives green and so on.

Depending on the composition of the alloy from which the coin is made of, the different ingredients (when starting the oxidation process due to humidity or rise of temperature..etc) will form these metal oxides on the surface of the metal and due to light reflection from the surface of the metal, these oxides will absorb all the light and reflects only its color.

These oxides we use in dentistry during manufacturing of different shades of Porcelain veneering materials inorder to match the natural teeth color.

I hope that I am right and please correct me if I'm wrong.

I hope this could help.