Slip as Ink

Ive been thinking about Classical Greek "painting" on ceramics as a form of drawing. 

Did these painters also work in full color on murals or specialize on painting on clay with slip ? 

Let us consider for a moment there was NO drawing paper.

Yes NO Paper !

And imported papyrus would have been too expensive for rendering anything other than finished works? 

The sketch pad for artists would have been wax tablets spare clay tiles or fragments of broken ceramics or possibly strips of linen from discarded clothing ?

So perhaps we should view those masterworks of painting as also masterworks of drawing with colored clay slip being used instead of paint or rather I should say clay being used as the binder instead of gum or glue or plaster ?