Brian, You might try drying it out a bit in a conventional or toaster
oven. Though I myself have never worked with PMC in this particular
way, I do know that there are people out there who bake it to a
leathery consistency and work it/tool it from there. I have recently
started carving PMC in a hardenened form, though I usually form it in
its original putty-like consistency. Basically, it sounds like you
need to control the drying out process. You might just let it air dry
again. In general, PMC has a sort of expiration date, and it sounds
like yours has definitely expired though is not completely beyond
revival. Good luck. You might also want to post your query on the
pmclay.com site.
Best,
Riva