Justin H. writes: If I was to purchase platinum bullion coins and store them properly in a dye free cotton bag in a temperature controlled room would they ever corrode, tarnish, get water spots, or lose their mint luster as silver and copper coins do over time? I ask because I am looking for bullion coins to purchase that will not tarnish or corrode like silver bullion coins and have heard that platinum is a very noble metal that does not tarnish and would like to know if this is true. Thank you.
Platinum and gold are almost inert, that is chemically complete. That means that it is very difficult to chemically combine those metals with other elements. That is why they have always been revered. Virtually all the gold that has ever been mined still exists today. Neither gold or platinum metal combines with oxygen and so will never oxidize.
Since gold is so soft, it has been traditionally mixed with other metals which imparts a hardness to the gold. This mixture, or alloy, is not a chemical combination. Copper is often mixed with gold to create a hard alloy that is ideal for coinage (and dental crowns). It is difficult to create a uniform mixture of copper and gold and so sometimes the copper will oxidize. The oxidation sometimes manifests itself as “copper spots” on some coins.
Fineness is a statement of the relationship of precious metal to base metal. The majority of gold coins, that traditionally were used as money, have a fineness of anywhere from .650 – .925 (65% to 92.5%). Some modern bullion coins, such as the Canadian Maple Leaf and Chinese Pandas are not alloyed at all and are .9999 fine (that is about how chemically pure you can get). Note that alloyed one ounce bullion coins have the identical gold content as the pure bullion coins, they just weigh more. Platinum coins are generally not alloyed.
Platinum and gold coins do not require special temperatures or storage mediums to avoid oxidation. They will stay in their original condition indefinitely.