There are a few methods you can try for clay that has firmed up a bit much, but
isn't at the dry or too firm leather hard stage.

remove the clay from the plastic and wrape a wet towel around it and then wrap
in a plastic. A large plastic trash bag works well. Leave it overnight and it
should be the way you want it in the morning.

Another method is to slice the clay about 1/2 - 1" thick, dip each slice in
water and put it all back in the bag. This will do a fast job of bringing it
back to a soft stay.


Another easy method it to fill a 5 gallon bucket with water. Put your bag of
clay in the bucket and open the top of the bag to allow the water to move into
the bag and cover the clay. Let sit a few hours or overnight till it is the
consistency you want.

Regards,
June
http://www.angelfire.com/art2/shambhalapottery/