Kelim, Chapter Ten, Mishnah Two
Our mishnah discusses what materials can be used to seal the cover of an earthenware vessel such that it will protect it from becoming impure.
1) How may it be tightly covered?
a) With lime or gypsum, pitch or wax, mud or excrement, crude clay or potter’s clay, or any substance that is used for plastering.
2) One may not make a tightly fitting cover with tin or with lead because though it is a covering, it is not tightly fitting.
3) One may not make a tightly fitting cover with swollen fig-cakes or with dough that was kneaded with fruit juice, since it might cause it to become unfit.
a) If he did make a tightly fitting cover [from such material] it protects.
Section one: All of the materials listed here will form a tight seal around the lid’s edges. Thus all of these materials cause the vessel to be protected from impurity if the vessel is found in a building with a corpse in it.
Section two: Soft metals such as tin or lead do not offer as tight of a seal and therefore they do not protect the vessel. They are a “covering” but they are not “tightly fitting.”
Section three: These soft, pliable food products will, at least temporarily, offer a tightly fitting seal. Therefore, if one makes a seal out of them, they do protect the vessel from impurity. However, since they will dry up and no longer be a tight seal, the mishnah says that one should not use them.