I got a couple of Perth Mint Koalas recently with some stain marks. They were advertised (with pics) by the seller as to their condition. So, I knew about the stains and still was happy to buy them anyway (all good). Being a lazy Sunday afternoon, I came across a youtube clip about cleaning silver coins and decided to give it a try. What the hell, if it was a botched job, at least the value was still its 1 oz silver content. http://youtu.be/Kp62AvR7kFM Protocol: Dissolve baking soda (sodium bicarbonate NaHCO3), table salt (sodium chloride NaCl) and vinegar (acetic acid CH3COOH) in a non reactive ceramic bowl of hot water. Place a piece of aluminum foil in the dish, and sit the coin in this bath. Swirl it around for a bit and watch the stain marks come off. Rinse in plenty of clean water and pat dry. Side note: It is an oxidative reaction of the sodium bicarbonate with the acetic acid. I reckon the aluminum foil acts like a catalyst. As per the youtube clip, the higher the silver content the better the result. However, they had a caveat and that was "Don't try it on numismatic coins", just bullion that you wouldn't cry a river over if the treatment adversely affected its looks.
Think I saw a similar vid that didnt advise vinegar, just hot water salt and baking soda. didnt end up working, might have to try this. The aluminium is not a catalyst it forms the anode/cathode (i get mixed up between to two im sure someone can sort me out haha) and the silver coin forms the other half of a galvanic cell, so there is current flowing between the alluminuim and the silver coin forcing the contaminates on the surface of the silver to react with the salts in solution. nerdrantend
I've done it without vinegar and had great results. Not sure why someone would sell it without trying to clean it first.
