פירוש על כלים 24:7

שלש פנקסיות הן (three kinds of writing tablets) – that are made to write uppn them, like the shopkeeper on his writing tablet.

There are three different types of writing tablets:
That of papyrus is susceptible to midras uncleanness; 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.

אפיפורין (papyrus) (see Tractate Kelim, Chapter 16, Mishnah 1) – a board that they place upon it the powder of the dust and they write upon it purchases. And it is large and appropriate for sitting.

That which had a receptacle for wax is susceptible to corpse uncleanness; 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.

שיש לה בית קבלו שעוה – a board that the grind its face in wax and mark on the wax with a chisel.

And that which is smooth is free from all uncleanness. A smooth writing tablet, one that has no receptacle, is not susceptible to impurity. It is considered a simple wooden vessel.

וחלקה (smooth one) – that has no wax on it, and they write upon it with ink, and it has no receptacle.