To Jonathon T.,
No, I didn't polish the coin prior to bleaching it but I don't think that would have mattered much. I did make an attempt to use 3% H2O2 after rinsing the coin with Isopropyl Alcohol to remove the oils. (I thought of the oil issue halfway thru the bleaching process.) It actually reacted more than I expected. After 2-3 minutes the coin had a fairly uniform gray cast similar to coins I've purchased that hadn't been shined or polished.
(BTW, referring back to my supposition about Silver Chloride formation causing the gray result in the bleach; I'd forgotten that Silver Chloride isn't naturally gray/black; it's white, but is light sensitive and the Chloride ion re-associates to form chlorine gas that leaves behind pure silver metal that will look gray - it's been a few years since my college classes.)
I used a 7.5% H2O2 solution today and got better results (and much faster ~30 sec. [I'm glad I diluted the 30% solution with which I'd started.]), closer to what I was wanting, but the oxidation wasn't quite as dark as I'd hoped so I decided to hit Google (which would have saved me the time of playing with this - remember kids: research first; then experiment).
The research brought back everything I would have known had I thought about this when I dropped out of my chemistry degree program after 3 1/2 years in (I was gonna be a rock star don't you know). The "oxide" I want is Silver Sulfide (really black) not Silver Oxide (mostly black but more dark gray).
(However, I did find thru the research that mixing Silver Powder & 90% H2O2 makes a great rocket propellant - don't try this at home kids.)
So, liver of sulfur is probably a better answer, here's
A description of the Egg method is on this
page. I've reproduced it here:
"While your eggs are boiling put your silver pieces in a freezer zip-lock bag. Don't overlap them. Take your boiled eggs out, let them cool for a few minutes and put them in the bag with shells on. Take a spoon and crack the eggshell, then just mash up the eggs. I do that by laying the bag on it's side and place a towel over it (the eggs are hot inside) and mash the eggs up, as if you were making cracker crumbs. When the eggs are warm the oxidation starts immediately. I then just roll the bag and let it sit for a few minutes, check, reroll if needed and rinse. It really goes fast this way!"
The other option is Jax Silver Blackener but be aware that altho I wasn't able to find the MSDS online (unlike nearly everything else Jax makes) the warning label says it is a skin irritant so take proper precautions.
Now, off to find that 90% H2O2 and a tool to grind up one of these coins. :D (If you hear a boom near Santa Clara, CA; that'll be me.)