It may seem like harsh treatment but I used a strong mix of salt, vinegar and lemon juice which removed the crap fairly quickly. The process was monitored fairly closely to ensure that once the encrustation was removed, not too much of the surface metal went with it. One can’t just chuck a coin into a jar of acid and leave it for months. There might not be much of a coin left afterwards.
A couple of days in bi-carb of soda solution helped to neutralize the effects of the acid, then the coin was given a rub with olive oil on a soft cloth.
The re-toning process was very low-tech and basically involved keeping the coin in my pocket for a while handling it often, even breathing on it regularly, then leaving it on a window ledge in the sun for a couple of months.
It might seem a bit crazy to go to all that trouble, but the results speak for themselves.
Back in the 90’s I bought a 1923 1/2d from a coin dealer for $300. It was in fairly good nick, but had a black patch across the obverse.
I gave it the same treatment, being extra careful and a few years later it realized $1000 at one of Downies auctions in Melbourne.
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