Pesachim 30B

אין טשין את התנור באליה. ואם טש ⁠— כל הפת כולה אסורה, עד שיסיק את התנור. הא הוסק התנור ⁠— מיהא שרי. תיובתא דרבא בר אהילאי, תיובתא.

one may not smear the inside of an oven with the fat tail [alya] of a sheep, because it is meat. And if one did smear it over the oven, then all of the bread baked in it is prohibited, lest one accidentally eat this bread with milk. However, this applies only until one kindles the oven and burns off this fat. The Talmud infers from this baraita that if the oven was kindled afterward then it is permitted in any case. This would seem to present a conclusive refutation of the statement of Rava bar Ahilai who maintains that the bread cooked in this oven is prohibited forever, even after the oven is rekindled. The Talmud concludes: Indeed, this is a conclusive refutation of his opinion.

אמר ליה רבינא לרב אשי: וכי מאחר דאיתותב רבא בר אהילאי, אמאי קאמר רב: קדירות בפסח ישברו? אמר ליה: התם תנור של מתכת, הכא בקדירה של חרס.

Ravina said to Rav Ashi: Since the statement of Rava bar Ahilai was refuted, why did Rav say that pots that were used for leavened bread on Passover should be broken? Presumably, their status could be remedied in the same way as was that of this oven, by heating them on a fire until the flavor of the leavened bread absorbed in them was removed. Rav Ashi said to him: There is a distinction between these two cases. There, it is referring to a metal oven, which can be cleansed through an additional kindling, while here, it is referring to an earthenware pot, where additional kindling is insufficient, as the earthenware has the capacity to absorb more of the flavor of the leavened bread and it cannot be purged by fire.