Zinc oxide is white. Zinc sulfide is white, too. In fact, unlike most other metals, most zinc compounds are white or colourless. The great mystery to me is not, "why do some zinc coins turn white?" but, "why do most zinc coins turn black?", because they shouldn't.
These guys don't know; they think it might be some kind of surface effect, rather than actually forming a black deposit. I would assume that for coins, it's reacting with acidic skin oils from people's fingers and hands.
So, to answer your questions:
1. Zinc is a highly reactive metal; any chemical agent (eg acid) strong enough to remove zinc oxides, sulfides etc would most likely destroy the coin. You'll probably be forced to resort to physical removal (rubbing, scraping, etc) - things which you generally shouldn't do to a coin unless the corrosion has already destroyed it's value.
2. It will, in all likelihood, come back, especially if kept in the same environment that turned it white in the first place. Some of the bad white corrosion spots I've seen on zinc coins seem to have been the zinc equivalent of bronze disease; capable of spreading and consuming an entire coin.
3. See above. Removal, especially by chemical means, will likely leave the surface "activated" and much more prone to reaction with it's environment.
4. Can't say I've tried it. I'm not sure about Vaseline, but some petroleum jellies contain sulfur compounds. It might prevent the worse kinds of corrosion, but it wouldn't stop it changing colour.
5. If it's just a thin film evenly over the surface, keep it like that. White coins are just as good as black ones.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis
Posted 01/17/2010 4:31 pm
