PMC can be re-hydrated at any stage of it's being dried. It will,
obviously take the drier pieces longer to re-hydrate than the pieces
that are less dry.
All forms of PMC are re-hydrated with water. PMC does not absorb the
water immediately, like a sponge or paper towel would. The best
thing to do is cut the PMC into very small pieces. The smaller the
pieces the faster the water will be absorbed. Some people even put
the PMC through a pepper mill so it comes out as more of a powder.
Place the PMC onto a sheet of saran wrap. Add enough water to cover
the PMC, wrap it in the saran wrap, and set it aside to give it time
to absorb the water. Check in about an hour to see how it is doing.
The PMC should be softening up. Work the water into it using a small
plastic or metal trowel tool. If the PMC seems a little dry then add
a little more water to it. If the PMC is a little wet, then let it
dry out some. Work it back and forth this way until all the lumps
are out, and it gets back to a consistency that is workable for your