Reference for Kelim 6:4

שְׁתֵּי אֲבָנִים שֶׁעֲשָׂאָם כִּירָה וְנִטְמְאוּ, סָמַךְ לָזוֹ אֶבֶן אַחַת מִכָּאן, וְלָזוֹ אֶבֶן אַחַת מִכָּאן, חֶצְיָהּ שֶׁל זוֹ טְמֵאָה וְחֶצְיָהּ טְהוֹרָה, וְחֶצְיָהּ שֶׁל זוֹ טְמֵאָה וְחֶצְיָהּ טְהוֹרָה. נִטְּלוּ טְהוֹרוֹת, חָזְרוּ אֵלּוּ לְטֻמְאָתָן:

Two stones upon which a stove is made that become impure: if he put a stone near the outer side of the one and another stone near the outer side of the other, [the inner half] of each [of the inner stones] remains impure while [the outer] half of each [of these stones] is pure. If he removed the pure stones, the others return to their impurity.

Jastrow