Mishná sobre Keilim 10:11

If [lying over the hatch] there were vessels made of dung, vessels of stone, or vessels of [unbaked] earth, everything [in the upper story] remains clean. If it was a vessel known to be clean for holy things or for [the water of] purification, everything remains clean, since everyone is trusted with [regard to matters of] purification. For clean vessels and earthenware vessels that are [known to be] clean protect with the walls of ‘tents'.

Vessels [that are] between the rims of the olive-basket and the rims of the cellar, even to the depths, remain clean. If there is uncleanness in the cellar, the house becomes unclean. If there is uncleanness in the house, vessels in the walls of the cellar remain clean, if the place where they are has a content of one cubic handbreadth; if not, they become unclean. If the walls of the cellar are wider [apart] than those of the house, in either case the vessels remain clean.

A board placed over the mouth of a new oven, overlapping it on all sides to the extent of a handbreadth, If there is uncleanness beneath [the board], vessels above it remain clean; If there is uncleanness above it, vessels beneath it remain clean. In the case of an old oven, they become unclean. Rabbi Yohanan ben Nuri declares them clean. [If the board] is placed over the mouth of two ovens, if there is uncleanness between them, they become unclean. Rabbi Yohanan ben Nuri declares them clean.