Mishnah Oholot 12:1: If a board was placed over the mouth of a new oven and it extends from all sides by a handbreadth's space, if there was impurity underneath it, vessels that are on top of it remain pure. If there was impurity on top of it, vessels beneath it remain pure. If it was an old [oven], they are impure. Rabbi Yochanan ben Nuri declares them pure. If it was placed across the mouth of two ovens and there was impurity between them, they are impure, but Rabbi Yochanan ben Nuri declares them pure.

נֶסֶר שֶׁהוּא נָתוּן עַל פִּי תַנּוּר חָדָשׁ וְעוֹדֵף מִכָּל צְדָדָיו בְּפוֹתֵחַ טֶפַח, טֻמְאָה תַחְתָּיו, כֵּלִים שֶׁעַל גַּבָּיו טְהוֹרִים. טֻמְאָה עַל גַּבָּיו, כֵּלִים שֶׁתַּחְתָּיו טְהוֹרִים. וּבְיָשָׁן, טָמֵא. רַבִּי יוֹחָנָן בֶּן נוּרִי מְטַהֵר. נָתוּן עַל פִּי שְׁנֵי תַנּוּרִים, טֻמְאָה בֵינֵיהֶם, הֵם טְמֵאִים. רַבִּי יוֹחָנָן בֶּן נוּרִי מְטַהֵר:

Mishnah Oholot 12:2: If an oven screen [which is not able to become impure] was placed over an oven which was sealed, if there was impurity beneath it or on top of it, everything is impure. But opposite the airspace of the oven is pure. If the impurity was opposite the airspace of the oven, everything opposite it up to the firmament is impure.

סְרֵידָה שֶׁהִיא נְתוּנָה עַל פִּי הַתַּנּוּר מֻקָּף צָמִיד פָּתִיל, טֻמְאָה תַחְתָּיו אוֹ עַל גַּבָּיו, הַכֹּל טָמֵא. כְּנֶגֶד אֲוִירוֹ שֶׁל תַּנּוּר, טָהוֹר. טֻמְאָה כְנֶגֶד אֲוִירוֹ שֶׁל תַּנּוּר, כְּנֶגְדּוֹ עַד הָרָקִיעַ טָמֵא: