It's very easy, albeit time consuming, to rehydrate metal clay from
bone dry to perfectly creamy and usable. What I like to do is to use
a tissue blade or other sharp blade to chop the dry clay into the
tiniest bits you can (some folks use a coffee grinder or pepper
mill), transfer it to a mortar and pestle, grind it into powder, dump
the powder onto a glass surface and LIGHTLY spritz it with water
while mixing it with a spatula. When it is all incorporated and will
hold together, put it in a sheet of plastic wrap (I like Saran Wrap)
and knead it with your fingers - through the plastic. If it's still a
little dry, dip your finger in water, wipe it across the clay and
knead it again - through the plastic (otherwise your fingers will
absorb most of the water). Then allow it to sit for at least a minute
or longer, up to overnight, so the binder has time to absorb the
I've gotten the clay back to perfect every time using this method.
No lumps, no bumps.
To extend the working time of new, fresh clay add one drop of
glycerin to the lump and knead (through the plastic wrap). Let sit
for at least an hour and up to overnight. Glycerin retards the
evaporation of water and extends your working time.
Hope this helps. I know you'll get other answers with other methods,
but I like this one best.