From Digitalegypt:
Ostraca
"Potsherds and stone flakes bearing writing and/or depictions. They are very common in New Kingdom Thebes and then again in the Ptolemaic, Roman and Coptic Period. The sherds and stone flakes were used as cheaper substitute for the expensive papyrus. Writings on ostraca are often short notes, accounts or literary excerpts in some cases possibly written by pupils."

Figured Ostraca
"The largest group of such material comprises thousands of sketches, mainly in black, or in black and red, on medium to large limestone flakes from the workplace and village of Ramesside Period craftsmen working on the tomb of the king at Thebes (Deir el-Medina)."

"The subject matter varies greatly, and, without record of context or in instances of secondary deposition, function is often difficult to ascertain; often figured ostraca are interpreted as evidence for the training of artists, as in Deir el-Medina and the Ramesseum, but this is not made explicit by context or by accompanying writing."