משנה על אהלות 13:5

The base of the goldsmiths’ anvil is susceptible to uncleanness, but that of the blacksmiths is clean. A whetting-board which has a receptacle for oil is susceptible to uncleanness, but one that has none is clean. A writing-tablet that has a receptacle for wax is susceptible to uncleanness, but one that has none is clean. A straw mat or a tube of straw: Rabbi Akiva rules it is susceptible to uncleanness; But Rabbi Yohanan ben Nuri rules that is it clean. Rabbi Shimon says: the hollow stalk of colocynth is subject to the same law. A mat of reeds or rushes is clean. A reed-tube that was cut for holding anything remains clean until all the pith has been removed.