1937 Mercury Dime -- Toning

Toning not nice in this case, but can't do much about it, unfortunately.
If it hasn't had an acetone bath, at least do that much, but don't expect to see any improvement in eye appeal. It's just that if there is any organic junk on the coin, it should be removed anyway.

All toning is destructive to the coins surface. Deciding if the toning increases or decreases the eye appeal is strictly a matter of opinion.

Although most opinions will be together on a thumbs up or down.

the Whitman gunk comes off usually. Sometimes water is sufficient in my experience. Most of my collection was in them and have a black ring on the edge and a black/blue back. Its horrible. Look up how to clean stuff if you don't already know, if you want to bother. It gets tedious in a hurry. I don't blame them, they probably didn't have a clue how to design them to last for 100 years back then, but that closed back idea was no good.

I'd leave it as is. The last thing you want s for this merc to look cleaned.