An acquaintance gave me a plastic baggie of foreign coins his father had accumulated and which he didn't want. Among them were four intriguing coins, all with British kings on the front, but the reverses were obscured by what I assumed was glue and either paper or fabric, as if they had been pasted to a board.
I soaked them in olive oil, and after two weeks the crud was starting to come off, so I took acetone to them, and that cleaned them up completely.
One was a Canadian copper penny from 1918 that is the size of a quarter, and the other three were all British -- a 1944 half penny, a 1929 florin and a 1938 shilling, the latter two 50% silver. Such a nice surprise.
I doubt worth anything, but I'm glad to have them, and proud that I took the trouble to get the gunk off.