And every open vessel that has no cover fastened around it is unclean.
Torah Temimah: Chullin 25a — The air-space of an earthenware vessel is impure and its exterior is pure, for the verse says “and every open vessel that has no cover fastened around it” — which is a vessel whose impurity precedes its opening? You must say this is an earthenware vessel. And only when it has no cover fastened around it is it impure; if it has a cover fastened around it, it is pure.
English: The air-space of an earthenware vessel is impure and its exterior is pure… only when it has no cover fastened around it is it impure; if it has a cover fastened around it, it is pure.
Distinction between interior and exterior of earthenware.
An earthenware vessel becomes impure via its interior air-space when open in a tent of a corpse; its exterior surface does not contract this impurity in the same way.