Reference for Kelim 7:1
הַקַּלָתוּת שֶׁל בַּעֲלֵי בָתִים שֶׁנִּפְחֲתָה פָחוֹת מִשְּׁלֹשָׁה טְפָחִים, טְמֵאָה, שֶׁהוּא מַסִּיק מִלְּמַטָּן וּקְדֵרָה בְשֵׁלָה מִלְמַעְלָן. יָתֵר מִכָּאן, טְהוֹרָה. נָתַן אֶבֶן אוֹ צְרוֹר, טְהוֹרָה. מֵרְחָהּ בְּטִיט, מְקַבֶּלֶת טֻמְאָה מִכָּאן וּלְהַבָּא. זוֹ הָיְתָה תְשׁוּבַת רַבִּי יְהוּדָה בְתַנּוּר שֶׁנְּתָנוֹ עַל פִּי הַבּוֹר אוֹ עַל פִּי הַדּוּת:
The fire-basket of a householder which was decreased in size to less than three <em>tefachim</em> is susceptible to impurity, because when it is heated from below a pot above would still boil. If [it was decreased in size] further, it is not susceptible to impurity. If [subsequently] a stone or gravel was put into it, it is still not susceptible to impurity. If it was plastered over with clay, it may contract impurity from that point onward. This was Rabbi Yehudah's reply regarding the oven that was placed over the mouth of a cistern or over that of a cellar.