Kelim, Chapter Twenty-Four, Mishnah Seven
There are three different types of writing tablets:
1) That of papyrus is susceptible to midras uncleanness;
2) That which had a receptacle for wax is susceptible to corpse uncleanness;
3) And that which is smooth is free from all uncleanness.
Section one: Evidently, a scribe would lean on the papyrus while he was writing. This is sufficient for it to be susceptible to midras impurity, for things that are leaned upon are susceptible to midras.
Section two: Some tablets were set up so that they would be covered with wax and the writing would be written in the wax. If such a tablet has a receptacle to hold the wax before it is spread on the surface of the table, it is susceptible to impurity.
Section three: A smooth writing tablet, one that has no receptacle, is not susceptible to impurity. It is considered a simple wooden vessel.