A: It depends a lot on the temperature at which it’s been stored for the last five years. I’ve got some rubber that’s older than that, which has been kept reasonably cool (basement temp) that still works fine. It’s a little bit stiffer than when it was new, so it’s a little harder to pack a mold initially, but the finished molds are just fine. I’d probably be more careful in packing the mold, too, as you might find the rubber won’t flow as far to fill details. What happens isn’t so much that it will no longer vulcanize, but that over time, it starts to partially vulcanize on its own. So it’s stiffer to begin with, and more viscous when it gets hot.

It’s easy to check this before you commit to something serious. Just pack some into a mold frame, with or without a model, and vulcanize a test mold, nothing fancy, just to see if it flows well enough to properly fill the frame, and whether you can cut it apart and get a mold that isn’t warped. If the stuff is too far gone, you might find that though the test mold looks OK before you cut it, the rubber would have stresses in it that tend to dish the two halves after you cut it apart, so then it’s hard to close up properly. If it does that, then I’d suggest you toss it. If your test mold can be cut decently, the rubber has flowed well enough to work with a model, and the halves of the test mold match up and close again they way they should, then I’d say your rubber is likely still usable. For very intricate molds needing complex cutting, you might want to use newer rubber, but like I said, why not just try it? An hour from now, you can have your answer.