Illusive Floating Penny Variety

How?

The bucket had been used to mix an augmented mortar in. In an attempt to clean it , someone filled it with water and acid.
The acid must have leached something out of the mortar because the liquid was much like a syrup.
Someone dropped a zincoln in the bucket and the acid found/made a hole in the edge of the plating. Once exposed to the acid, it ate the zink creating a gas bubble inside the plating which would eventually float the coin to the surface. Once there , the bubble would pop and the coin would make its way back down through the syrup to the bottom to start the process all over again.

We are easily amused

I remember that one.
Couldn't remember what you originally said was the cause, but guessed it must have had to do with the zinc oxidizing and the resulting gas causing it to float. Still mesmerizing to watch, even now.