Commento su Ohalot 12:1

נסר שהוא נתון על פי תנור חדש – that was not heated/kindled, that is not susceptible to receive ritual defilement, And we are speaking of an oven that stands in the courtyard or in the garden.

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. The oven referred to here is new and its manufacturing has not been completed. Therefore it is not susceptible to impurity (see Kelim 5:1). The board on top of it overshadows a source of impurity. However, the board acts as an ohel and prevents the impurity from going above or, if the source of impurity is above, from going below.

בפותח טפח – that the board is wide and extends outward from the oven a handbreadth around it from all its sides.

In the case of an old oven, they become unclean. Rabbi Yohanan ben Nuri declares them clean. If the oven is one that has been used, it is susceptible to impurity. In this case the vessels above and below are impure. This is because the board is supported by a vessel that can become impure and in such cases an ohel is not formed (see 4:1). The oven is also impure. According to the Tosefta Rabbi Yohanan ben Nuri holds that the oven is pure, but he agrees that the vessels above and below the board are unclean.

שעל גביו – on top of the board.

[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. In this case one board is placed over the top of two ovens. There is a source of uncleanness in between them. Even if the ovens are new, they are impure because we don't look at the walls of the oven as being the outsides of the ovens. Rather we look at them as being the walls of a tent which do become impure. Rabbi Yohanan ben Nuri still holds that the ovens are pure. Evidently, he does look at the walls of the ovens as being the outsides of the ovens and since the oven does not become impure by contact with impurity on its outside, the ovens are pure.

טהורים – for since the oven is pure, the vessels are protected, and similarly, if defilement is on top of the board and the vessels are underneath the board.

ובישן – as for example, that the oven was heated that is susceptible to receive ritual defilement.

טמא – the oven [is impure]. The board does not protect it. And all the more so, thew vessels that are under the board and that are on the board. And the reason of our Mishnah is that vessels become tents to defile but not to purify, therefore, a new [oven] which is not a vessel protects. An old [oven] which is a vessel does not protect.

ור' יוחנן בן נורי – specifically with an oven. But regarding vessels, he (i.e., Rabbi Yohanan ben Nuri) agrees that they are impure. And this is brought in the Tosefta (Tractate Ohalot, Chapter 13, Halakha 1).

נתון על פי שני תנורים – the head of the board on this oven and the other head of the board on the other oven and there is defilement between the two ovens, and we are speaking of old ovens. But the Rabbis go according to their reasoning and Rabbi Yohanan ben Nuri according to his reasoning. But the Halakha is not according to Rabbi Yohanan ben Nuri.