If memory serves me, it is water that cures silicon...similar to polyurethanes....the other possibility is oxygen, but I don't think so....anybody know which it is for certain????

....the addition of heat merely hastens the curing reaction.....in the absence of the appropriate curing agent, the heat won't help....BTW every 10 degrees centigrade roughly doubles the speed of chemical reactions...

in any case, if the plastic covering provides enough of a barrier to the curing agent, then obviously a slow cure results! If it is water that cures it, a little spritz of water will get things going