Kelim, Chapter Two, Mishnah One
After the introductory chapter, our mishnah begins to deal with the purity and impurity of vessels.
1) Vessels of wood, vessels of leather, vessels of bone or vessels of glass:
a) If they are simple they are clean
b) If they form a receptacle they are unclean.
2) If they were broken they become clean again.
3) If one remade them into vessels they are susceptible to impurity henceforth.
4) Earthen vessels and vessels of sodium carbonate are equal in respect of impurity: they contract and convey impurity through their air-space; they convey impurity through the outside but they do not become impure through their backs; and when broken they become clean.
Section one: Vessels made of wood, leather, bone or glass can come in two forms: simple or with a receptacle. They are simple if they are not made to accept anything. Simple vessels cannot become defiled. They are susceptible to impurity only if they have a receptacle. This rule seems to be derived from Leviticus 11:32.
Section two: If the vessel was impure when it was whole and then was broken, it now automatically becomes pure.
Section three: Once it is fixed, it is now susceptible again to impurity. However, the vessel does not return to its old impurity. In other words, the slate is wiped clean.
Section four: The rules regarding earthenware and sodium carbonate vessels are different. As I explained in the introduction, they can be made impure only by something entering their air space, even if the defiling agent does not touch them. They also convey impurity to food or other vessels that enter their air space, even if they do not come into contact with them. This is derived from Leviticus 11:33.
They also convey impurity through their outsides. So if they are impure, and a piece of food touches the outside of the vessel, the food is impure. But if something impure touches the outside of the vessel (their backs) they are not defiled.
Earthenware vessels cannot be made pure by entering a mikveh. The only way they can become pure is by being broken (see Leviticus 6:21, 11:33, 15:12).