Olden-Day Ovens
6. In the time of the Gemoro, food was baked or roasted on the floor or walls of the oven itself. Thus, to be kashered, ovens needed libun chomur (שו"ע יו"ד סי' צ"ז ס"ב ), i.e., getting heated to the point that sparks fly (שו"ע או"ח סי' תנ"א ס"ד ). This required a high temperature, about 400°C (יד אהרן ).
Modern-Day Ovens
7. In modern-day ovens, food is usually placed on pans and baked or roasted in the oven's heat. Thus, an oven used to cook meat usually is only fleishigs due to the meat's aroma or steam, not contact with the meat itself. Often, there is not even fleishige steam or flavor, e.g., if the food is covered or in a pot with walls, or the like. Thus, it is easier to kasher the oven, as will be explained.
8. The consensus of the poskim is that one may rely on libun kal – a temperature of about 250°C that can be created by running the oven on its maximum heat – especially if it was only used for fleishigs and is considered "heteiro bola" [i.e., the bliyos in the oven are not from ossur food] (מ"ב סי' תנ"א סקכ"ח ), which does not require libun chomur (יד יהודה סי' צ"ז סקי"ג, שו"ת מנח"י ח"ג סי' ס"ו ).
