Miszna do Jeremiasza 2:22

Vessels of wood, vessels of leather, vessels of bone or vessels of glass: If they are simple they are clean If they form a receptacle they are unclean. If they were broken they become clean again. If one remade them into vessels they are susceptible to impurity henceforth. 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.