Not cleaning is generally a good rule of thumb. BUT, that said, it's not a catch all.
One thing I've found after many, many years experience in dealing with finds is that one tends to develop an "eye" for recognizing items that will lend themselves to a bit of cleaning. Some, just plain will NOT improve! But, some, can and do. Telling the difference is an art, at best!
Ok, so with all that said, how does one obtain this knack for telling? By mistakes. Now you've had one. Just learn from it and move on.
There is one thing I've often wondered though. I wonder if coins like this might lend itself to a painting, plating, or some such? I mean after the coin is already trashed, not much to lose, right? I wonder if one could take a coin like this to be restored? It's already fried and would never be worth anything. (Unless, maybe, it was gold plated?
)
Whatcha think experts? Might a piece of history be somewhat preserved by something like this?
The knowledge of finding a coin like that is quite a bit the reward. Especially since you can still tell what it is, still readable. Maybe an enhanced tarnish campaign would help?
Nice save!
HH!