Numbers 19:15

And every open vessel,.... An earthen one, as the Targum of Jonathan; and so Jarchi interprets it; and Maimonides r observes, that this is only to be understood of an earthen vessel:

which hath no covering bound upon it; a linen or a woollen cloth wrapped and tied about it:

[is] unclean; the air of the house getting into it by its being uncovered.

r In Misn. Cholin, c. 1. sect. 6.