Causes Of Verdigris On Copper.

All-
I can't add much about the causes/cures of verdigris, though I can speak to using ethyl alcohol to "clean" coins: don't! At least not copper, bronze, or aluminum-bronze coins- it will alter the patina by turning the coin various shades of purple (due to formation of copper oxides and sulphates, I believe). While the "purple haze" finish is interesting looking, even pretty, the coins stand out like a sore thumb and yell "cleaned!". Copper-nickel coins seemed to do fine, though you have to beware of residues from evaporation(unless you use laboratory grade 100% EtOH, which I doubt you would). Of course you can get rid of the residue by rubbing the coin with a cloth. Of course rubbing the coin with a cloth adds unnatural lustrous highlights to the raised parts of the design. Of course unnatural highlights yell "cleaned", so maybe not using alcohol at all is the best idea...

cheers,
tbg

I realize this thread is old, but acetone will not react with metals, whether in the dark or under bright lighting. It won't even remove tarnish. But it will quickly and completely dissolve PVC (and most every other petroleum based plastic). I have safely used it on several coins in my registry set. It evaporates very quickly, and it is important to avoid breathing any fumes. I suggest using it in a glass covered dish. Personally, I use a glass covered butter dish, and a use a can of compressed air to blow off the still liquid acetone when I remove coins from the soak, as any dissolved PVC will be left behind on the coin after the acetone evaporates. I can provide several examples where heavy PVC residue, (which was green and gooey in the crevasses, but hazy elsewhere), was completely removed and made it through PCGS certification to be part of my Registry Set, so I speak from experience.

Verdigris on the other hand, is a tougher nut to crack, and most people don't have the time o patience to leave a coin in mineral oil or Refined (lowest acidity) Olive Oil for 6 months or longer to soften it for easier removal. However, I am currently experimenting with a process (based upon procedures I have used at work to perform accelerated life testing) to see if the 6 plus month time frame can be significantly shortened without unnaturally affecting toning of bronzes. If it substantially improves the appearance of the coin without producing unnatural toning, I'll post the process with before and after pictures.