Midrash sur Les Nombres 19:15

2) I might think (that food) which is sealed air-tight in an earthen vessel and placed in the atmosphere of a stove becomes tamei; it is, therefore, written "of all the food," and not "all of the food," to exclude the above. I might think (that food) which is sealed air-tight in a "rinsing" vessel (i.e., one of wood or metal) placed in the midst of the oven is clean, and that this follows a fortiori, viz.: If an earthen vessel, which does not rescue itself (from tumah) without an air-tight seal in the tent of the dead, rescues therein what is sealed air-tight within it, then a "rinsing" vessel, which rescues itself without an air-tight seal in the atmosphere of an earthen vessel (see Vayikra 11:1) above) — how much more so should it rescue what is sealed air-tight within it! It is, therefore, written "the food," to include (as tamei) what is sealed air-tight in a "rinsing" vessel placed in the midst of the oven. An a fortiori argument that a rinsing vessel should not rescue itself (from tumah) without an air-tight seal in the atmosphere of an earthen vessel: If an earthen vessel, which rescues what is sealed air-tight in it in the tent of the dead, does not rescue itself therein without an air-tight seal, then a "rinsing" vessel, which does not rescue what is sealed air-tight within it in the atmosphere of an earthen vessel, how much more so should it not rescue itself therein without an air-tight seal! It is, therefore, written (in negation of the above): "the food." Food acquires tumah in the atmosphere of an earthen vessel, but (other) vessels do not. An a fortiori argument that an earthen vessel does not rescue what is sealed air-tight in it in the tent of the dead: If a "rinsing" vessel, which rescues itself without an air-tight seal in the atmosphere of an earthen vessel does not rescue what is sealed air-tight within it in the atmosphere of an earthen vessel, then an earthen vessel, which does not rescue itself without an air-tight seal in the tent of the dead, how much more should it not rescue what is sealed air-tight in it in the tent of the dead! It is, therefore, written (Bamidbar 19:15): "And every open (earthen) vessel, over which there is no air-tight seal is tamei." But if it does have an air-tight seal over it, it is clean.