Andrew,
You do have my sympathies.
On a more positive note while the coin currently looks ugly as hell it does not appear to be any worse off structurally (at least as a result of your efforts), just aesthetically. Think of it as a
work in progress. Wandering into the middle of a DIY project never looks
good.
You have lost a chocolaty
patina - but one that was very likely
applied, assisted etc. and not natural
toning. You have also "gained" an ugly colouring to all the areas that already/previously showed signs of serious
pitting from BD.
On a
cheap coin I would proceed as normal for BD. Manually remove all the newly exposed green and white stuff through brushing (soft toothbrush, harder toothbrush than soft (really soft)
brass brush, and then picking away the green stuff from the deeper holes. Then more GG soak and bake dry. However, I would also be prepared for the fact that there was a chance (hard to evaluate without coin in hand and under microscope) that the entire core of the coin was so toasted by BD that it would not survive and would just reveal more and more until it lost structural integrity totally in the cleaning attempts (ie crumbled).
Obviously your coin is not/not cheap and you do not want to proceed in this way!!But on the other hand you can't just coat or wax or repatinate it and forget it. It is in such an ugly state because it
still has BD. You could have it looking like your original but even better - ie no green BD spots - if you wished by wiping as much white off then re-patinating it with a dye product then sealing with
Ren wax. It would look great. But it would
still have BD and that would chew through it and one day it would totally crumble.
I got a nice VF/gF Claudian as that was like this. Showed some BD on back which I thought could clean out quickly. Then I discovered that almost the entire back
had active BD. It
had been dyed and waxed and what I was seeing was simply the most aggressive and advanced BD spots poking through the dye and wax. When I stripped the dye and wax off in soaks the back looked like a pizza that
had been left on the sidewalk for a week. As I cleaned it I watched the back go from gF to F to aF to
aVG etc. It is
still not BD free to this day. But at least I know I have arrested the worst of the BD and will have it stable one day. I was also not angry as I
had got it for a song from a very honest dealer who knew it
had BD and realized it might be in a bad way.
So what are you to do?
I would suggest:
1) Keep it safe and dry (not sealed in plastic and preferably with silica bags nearby) and wait for more advice. I think it definitely needs to have more conservation treatement before any final preservation or aesthetic treatment. I just
hope someone can suggest something gentler than what I outlined for a cheaper coin above. But there may not be any magic bullet.
2) Have a word with your dealer. Hard to tell how much "
caveat emptor" plays here. Your coin may have looked ok but it clearly was sick on the inside. I assume that the
price you paid took into account the way the coin looked - ie it was
pitted - but was likely based on the assumption that was structurally sound. That was clearly not the case. Who knew what when? Someone has clearly treated it before for BD. Maybe they thought it was cured and tried to "pretty it up". Maybe they knew it
still had BD and tried to hide it. In any event it is important that you point out that you did not ruin or worsen the coin, you took action against BD that revealed the true sorry state of the coin.
Shawn