Shabbat

let them not become unclean through their [flat or convex] backs.  Why did we learn, Earthen vessels and nether vessels  are alike in regard to their uncleanness: they become defiled and defile [other objects] through their air space;  they become unclean through their outside,  but they cannot be defiled through their backs;  and their breaking renders them clean.  Thus, only earthen and nether vessels are alike in regard to their uncleanness, but not other things? — I will tell you: since they can be repaired when broken,  they were assimilated to metal utensils. If so, let them revert to their former uncleanness, like metal utensils? For we learnt: Metal vessels, both flat and hollow,  are subject to defilement. If broken, they become clean; if remade into utensils, they revert to their former uncleanness. Whereas in respect to glass vessels we learnt: Wooden, skin, bone and glass utensils, if flat, they are clean;  if hollow, they are unclean;  if broken, they become clean; if remade into vessels, they are liable to defilement from then onwards. [Thus] only from then onwards, but not retrospectively? — The uncleanness of glass utensils is Rabbinical, and [the resuscitation of] former uncleanness is [also] Rabbinical: now, in the case of that which is unclean by Scriptural law, the Rabbis have imposed [retrospective] uncleanness upon it, but upon that which is unclean by Rabbinical law the Rabbis have imposed no [retrospective] uncleanness. Yet at least let their flat utensils be unclean, since flat metal utensils are [susceptible to uncleanness] by Scriptural law!-The Rabbis made a distinction in their case, so that terumah and sacred food should not be burnt on their account.