Yes, I know you're never supposed to clean a coin. I have a 1964 half. The obverse is in great condition, but the bottom of the coin is covered in this black mildew gunk. Yes, I know you're never supposed to clean a coin. If it were patina, I wouldn't care because that adds to the look of the coin, but this is just nasty mildew as the coin was underneath some built in drawers in the end of the hall in our old house for over 40 years. If it's impossible to get off, I'll accept it, but if I can get this gunk off it would be great. Yes, I know you're never supposed to clean a coin. I don't want to damage the coin but I'd like to have this nastiness removed if at all possible. Yes, I know you're never supposed to clean a coin.
It's a 1964 half that's been sitting under furniture. You probably don't need to lose sleep over whatever you do to it. That being said, though, it sounds like the first stage would be a soak in distilled water. Based on my experience with shower surfaces, I thing gentle swabbing with rubbing alcohol (get some without coloring or scent) would probably remove the mildew pretty quickly.
This coin is a family heirloom. That's why I want to get the nastiness off. The house that I grew up in was built in '64 and this coin was found when we moved out somewhere around 2006-7. Apparently it was once tradition to hide a coin of the year the house was built in.
Yes, I remember the story, and I'm glad you're interested in preserving the coin. For mildew, I normally scrub with bleach, but that's really really the wrong thing to do with a coin. I haven't done research on this, but it seems like rubbing alcohol dissolves whatever it is that "biofilms" use to stick to surfaces. (I use 91% isopropanol, but I'm guessing regular 70% alcohol would do the trick.)
I find it hard to imagine mildew or any other variety of molds attaching or growing on a silver dollar, since silver is a chemical inhibitor of microorganisms. More likely, it is probably very advanced tarnish/corrosion. If you still think the first, I would soak for several minutes in acetone or the 91% isopropyl alcohol which should remove attached organic material , but not tarnish. If you do not rub with something, the surface of the coin should not be affected.
The general procedure for cleaning debris from a coin is to go from the gentlest to the most potent BUT avoiding rubbing and dips. IMO use these solvents in this order: 1. distilled water 2. isopropyl alcohol 3. acetone (or xylene) Use a glass bowl. Rinse in distilled water (my experience shows that 70% isopropyl alcohol can leave a haze; acetone or xylene shouldn't unless contaminated)
The more I look at it, the more I think it is indeed just tarnish and not mildew. I tried the alcohol treatment and it didn't make a difference. It's still in good enough shape and is priceless to me.
Mildew cannot grow on a coin, impossible. What you likely have is corrosion, aka toning. Silver coins are best treated with an acid-thiourea dip like EZest.
