ABS slush went stale!

I take all my printed mishaps and throw it in a can with acetone to smelt it down to slush, which I use either for glueing parts together or coat the bed with to make prints stick. Now, I havn't used the printer much lately (since it's a Solidoodle and doesn't work) but today I did, and was about to put yet more waste into one of the cans and everything had just hardened to a concave shape with airholes everywhere. Very monster-looking.
I tried pouring in more acetone to "re-smelt"(?) but it didn't work, it just made the concave air-holemonster a bit more white... (filament was green BTW, lime if you're a woman).
I'll check back on it after some time to see if anything more happens with it.

So! You who make and use slushies, ever experienced this? What causes it?