Full Idea
A 'piece' of clay is a portion of clay which comes into existence when all of its parts come to be stuck to each other, and cease to be stuck to any clay which is not a part of the portion.
Gist of Idea
A 'piece' of clay begins when its parts stick together, separately from other clay
Source
Allan Gibbard (Contingent Identity [1975], I)
Book Reference
-: 'Journal of Symbolic Logic' [-], p.189
A Reaction
The sort of gormlessly elementary things that philosophers find themselves having to say, but this is a good basic assertion for a discussion of statue and clay, and I can't think of an objection to it.