All depends -- if its wet clay you'll need a different material than a block of dry clay.

For wet and shiny clay -- you may want to NOT use bumpmaps, and use specular maps with fairly high values.

For dry clay unmodelled you'll want a random bumpmap (click on the little arrow beside the bumpmap and click on something random, I'd personally use noise (I believe it's called noise) with a very high frequency.) Also you'll NOT want much of a specularity -- if any at all...

For the color for both, I'd recommend a grey with a little hint of brown -- maybe more if its painted pottery but that's all dependant on you.