More About Toning?

To some extent all metals do what is called Toning. Some may call it Staining, Tarnishing, Corroding, Oxydizing, etc. All Metals will react with something to make it look different. Inspite of some stories that Gold is Inert, it is not. Gold as well as many metals will react with someting which makes it look different. The difference is what is called the names mentioned above.
As to climate or moisture issues. YES, differences in climatic situations as well as moisture helps in many of the Toning processes.

Australian base metal alloy gold coloured coins retain their gold colour very well. That is, the $1 and $2 coins. This alloy is usually known as Aluminium bronze.
There is a reason for this.
The alloy is 92% copper, 6% aluminium, and 2% nickel, to harden the alloy.

Both the nickel and aluminium can make copper alloy turn yellow, just a zinc does to make yellow brass.
All three alloying metals are reactive in their own right, but aluminium and nickel in particular, develop a protective oxide coating that is only a few molecules thick. Since the coating is extremely thin, the coin retains a metallic lustre.

When a coin wears, the molecular thin oxide coating re forms instantly. This alloy characteristic severely limits toning. Although toning can develop on aluminium copper coins, it happens more slowly, a metallic sheen is usually retained, depending on storage conditions.

The toning, when it happens, is the yellow equivalent of the gray colour of the white metal copper nickel coins. Radial mint lustre is usually retained on uncirculated examples of aluminium bronze coins that have managed to acquire some toning.

The US mint should have taken notes for the presidential coins.

Pretty much, yes. Earth's atmosphere is not friendly to pure refined metals. They are all trying as hard as they can to turn back into the little piles of metallic ore from whence they came.


Chemically speaking, metallic elements that react very slowly and generally do not interact much with the atmosphere are called noble metals. Coins made of these metals are going to be less likely to tarnish and tone. However, the only noble metals you're likely to find made into coinage are gold, silver, platinum and palladium.

There are other ways to make "toning-resistant" coins. Stainless steel, for example, is used by many countries for coinage these days. Chemically, stainless steels and similar alloys are not "noble"; they still form oxides and other corrosion products, but the corrosion layer made by stainless steel under normal circumstances is very thin (invisible to the eye) and protective, in the fashion outlined by sel_69l.


I would have to disagree with sel on this point: I've found aluminium-bronze coins to be more reactive, and to tone and discolour faster, than cupronickel coins of the same age. As with the composition chosen for American dollars, The alloy was designed for colour and for wear-resistance, not for toning-resistance.


Absolutely. A main cause of toning on base-metal coins is moisture; a droplet of water on a coin will suck in all the other goop in the air and hold it on the coin surface in circumstances that accelerate toning. Dry climates keep coins better then humid ones. The worst combination is humidity plus wildly variable temperature. If a metal object cools down rapidly in humid air after being warm, water will condense on it. If you must keep coins somewhere humid, try to keep the temperature as constant as possible.

Other environmental factors can cause toning. Sulfur in the air is a major one (sulfur is the main culprit for silver tarnish); aerosolized sea salt (or dry lake salt, for that matter) can also cause toning problems.


Well, pure gold (.9999 fine) is chemically inert in any kind of environment humans can survive in. If the dinosaurs had made .9999 gold coins and we dug them up, they'd still look as mint-pristine as the day they were made. But circulation-quality gold (22k in Britain and Australia, .900 fine in America) has copper or silver in it to dilute it, and that diluent can cause toning of the alloy.


You are wise to be wary. "Artificial toning" or "intentional toning" is a significant problem in coin collecting these days, since some collectors have gotten it in their heads that certain kinds of toning are "beautiful" and worth more than an otherwise identical untoned coin. Personally, toning is corrosion, and something I'd rather not have on my coins. Especially not modern coins that could not possibly have acquired a genuine patina "naturally" yet.

There is also a relatively new form of toning.
PVC damage/toning, caused by coins being stored in PVC and the chemicals leaching out of them damaging the coins.
i am amazed that the Australian mints stopped using PVC as part of there packaging on the mint sets but still to this day the PNC issues are still sold in PVC envelopes