Kelim, Chapter Three, Mishnah Seven
1) A cauldron which was lined with mortar or with potter’s clay:
a) That which touches the mortar is unclean;
b) But that which touches the potter’s clay is clean.
2) A kettle which was punctured and the hole was stopped with pitch:
a) Rabbi Yose rules that it is clean since it cannot hold hot water as cold.
3) The same ruling he also gave concerning vessels made of pitch.
a) Copper vessels which were lined with pitch the lining is clean,
b) But if they are used for wine, it is unclean.
Section one: The mishnah presents an earthenware cauldron which was lined with either of two materials: mortar or potter’s clay. The mortar is thick and is a permanent coating. Therefore, if the kettle is defiled the coating is as well and it will subsequently affect food or liquid as well, as if they touched the kettle.
But the potter’s clay is not permanent and is thinner than the mortar. Therefore the lining is not affected by the impurity of the kettle.
Section two: According to Rabbi Yose the pitch used to stop up the hole is not sufficient to make the kettle useful again. Since it cannot be used for its main purpose, to hold hot water, it is not considered to be a vessel and it cannot contract impurity. This is true even though it can be used to hold cold water.
Section three: Rabbi Yose issued the same ruling with regard to all vessels coated with pitch. If the vessel was meant to be used with hot water, the pitch cannot be made impure by the impurity of the vessel.
This is illustrated with regard to copper vessels. If the copper vessel was designed to be used with hot liquids, the impurity of the vessel is not transmitted to the coating. But if the vessel is used to hold wine which is cold, the vessel can transmit impurity to the lining because the lining is sufficient for holding wine.