Reference for Shabbat 249:9
ורבנן נמי הכתיב יותץ ההוא לאידך גיסא דסלקא דעתך אמינא כיון דחבריה בארעא כגופא דארעא דמי קמ"ל
Raba demurred: If so, instead of disputing about the fragments of an oven, let them dispute about the fragments of utensils in general? Rather said Raba: They dispute about the fragments of the following oven. For we learnt: If he sets it [the oven] over the mouth of a pit or a cellar and places a stone there, — R. Judah said: If one can heat it from underneath and it is [thereby] heated above, it is unclean; if not, it is clean. But the Sages maintain: Since it can in any wise be heated, it is unclean.<span class="x" onmousemove="('comment',' The reference is to an oven. In ancient days this consisted merely of walls, without a separate bottom, and was set upon the ground and plastered thereto. Now, here the oven is set over the walls of a pit, not actually on the ground, and a stone is placed between the oven and the pit as a wedge. R. Judah maintains that if the oven is so placed, e.g., its walls almost correspond to those of the pit, that if a fire is made beneath the oven, in the pit's atmosphere, the oven itself is heated (sufficiently for its work), it is an 'oven' in the technical sense (as stated below) and is susceptible to defilement. But if the fire must be placed in the atmosphere of the oven, it is not an 'oven' and cannot be defiled. (Rashi). ');"><sup>7</sup></span>