Potter's clay (also called potter's earth) is a clay free of iron, suitable for making pottery or for modeling.
Most common clay has impurities, often in the form of iron oxide, sand, roots, and other debris. Iron oxide, when added to a clay appears as dark-colored grains in the matrix of the clay which itself changed little or not at all by this addition.
Potter's clay also plays as an idiomatic expression to refer to a person or people created and shaped by God (example: I am the potter’s clay; We are but potter’s clay).
A potter was a common occupation in the Biblical world (Jer 18:3-4). While the very rich could afford vessels of silver, gold, carved stone, or even of glass by Roman times, most people used common fired clay vessels for everyday use and storage (2 Tim 2:20). - Crivoice.org.
Are you beginning to create pottery? If you are starting out on your own, here are some tips on how to begin , shared by pottery expert, Beth Peterson, of About.com.