At least a piece by Ai Wei Wei is replaceable, somewhat. Unlike the piece Ai dropped as a statement. Destroying the irreplaceable for political commentary is no more commendable than destroying current work is.
When does it crack? What shape is the object you are casting? Long, narrow stems will crack where they attach to the bowl of a goblet. Plates and bowls tend to crack across the bottom.
A few possible reasons:
The differences in thickness of different parts of the object are great enough that they dry at very different rates.
You are dumping the extra slip before the casting is thick enough.
You are letting your casting get too dry in the mold. You should pull it out when it is leather hard.
Do you use a defloculant? If your slip is sludgy instead of smooth and creamy the differences in viscosity can cause problems.