Black gunk on Barber

[LI Tom]

I found a 1907 Barber Quarter,that has this black ,hard tar-cement stuff across most of the portrait on the obverse.I scrubed with soapy water and toothbrush,picked with a toothpick and some came off.The rest is really on there.Any suggestions.Please don't say don't clean,because this blob just ruins the coin.Thanks in advance for any ideas.

 

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[SHERMANVILLE ILLINOIS]

Plastic bag, coin, blob of katsup; about 24 hours.

 

[fiatboy]

Soak the heck out of it. Soap, vinegar, vodka---whatever, just make sure you rinse it with water afterwards. If you really want to get that stuff off fast, soak it in ammonia for about 20 seconds and then gently rub the gunk off. Repeat if necessary. Ammonia works on anything but copper.