משנה על כלים 5:15

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.