And every open vessel that has no cover fastened around it is unclean.
Torah Temimah: Chullin 25a — The air-space of all [other] vessels is pure and their exterior is impure, for the verse says “and every open vessel that has no cover fastened around it is unclean” — this one, when it has no cover, is unclean and when it has a cover is pure; but all other vessels, whether they have a cover or not, become impure.
English: The air-space of all [other] vessels is pure and their exterior is impure… all other vessels, whether they have a cover or not, become impure.
Contrast between earthenware and other vessels.
For vessels other than earthenware, the exterior becomes impure in a tent of a corpse, while the interior air-space does not receive impurity in the same manner.