Removing The Black Crud On My 1845 Large Cent.

Sure looks as though it has been "harshly cleaned" at some point in its life.

Don't know how to get the gunk off...since you don't have a ton of money tied up in the coin you do have some latitude to experiment.

Good luck...let us know if you find anything that works!

Although you didn't specify it appears to be an 1845 Copper Large Cent. In that condition only about an $12 to $25 coin. Since you've already tryed Acetone, brushes, Olive Oils, etc. I don't see why you haven't just put it in Tarn-X. If you don't have any, check your car for a removable lid on the battery. If no car, try someone else's car. A removable lid indicates a Lead-Acid battery and the acid is Sulfuric. Using a turkey type baseter, withdraw some of the acid, emerse your coin in that and presto, all nice and cleaned. If anyone is sandblasting a brick building in your area ask them to turn it on your coin. Usually a wire wheel on a bench type grinder will also help remove that stuff.
As to what it is. That could be anything. It is not the usual Oxydation symptoms. The normal Oxygenation would be to turn it Reddish, then brownish, then possibly green. Happens when any Copper is exposed to the elements such as Oxygen, Carbon Dioxide, Moisture. What you have appears to be excessive Oxydation but difficult to tell from a photo. As long as you've already distroyed most of the coin value you may as well try acids. By the way the brush lines are usually from the original plates of Copper used back then when they would brush the metals to make them brighter.

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As another possibility, soak the coin in water for a few days. Then freeze it in a shot glass full of water, and allow it to thaw.

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Distilled water or it doesn't matter?

I'd use distilled water, to avoid transferring something to the coin. Even though the technique is designed to remove water-soluble stuff from the coin, any time you touch a coin with water it should be distilled.

I have tried the peroxide method on some artifacts(old cut nails) I found with my metal detector. I heated up the hydrogen peroxide in the microwave & dropped the nails in. When the bubbling stopped I took the nails out & repeated the process several times. I don't know how it works with coins, but when I was done you could see the hammer marks on the nails from when they were made.

You can always try WD-40.

When you use the denatured alcohol, keep it in for about 18 hours per side; no rubbing. Some/most of the gunk should lift off on its own. After 2-3 days in this bath, you might want to use a little bit of pressure with a cotton swap and/or toothpick. Keep us informed.

Hardware stores sell denatured alcohol for about $6 for the smallest can.