Can Someone Explain This Acetone Phenomenon To Me?

A little late to the thread but I always use a shot glass or small water glass for acetone treatment. I cover the glass with aluminum foil and secure it with a rubber band. I use just enough acetone to cover the coin and will repeat treatment with fresh acetone if necessary. There is very little evaporation and the foil cover never pops off.

I pour the used acetone (small amounts only) onto my concrete patio and ignite it with a gas match. Burns off quickly with no smoke and no stain, and no leftover used solvent storage problem.