The stove bases of householders which collapsed: [If it was] less than three handbreadths, it is tamei, since one heats it from below and the pot up above will cook, [but if it was] more than that, it is tahor. [If] one inserted a stone or a pebble it is tahor. [If] he plastered it with clay, it will be susceptible to tumah from then on. [And] this was the response of R’ Yehudah regarding the oven that was placed over the mouth of a pit, or over the mouth of a cistern.