Separate Ovens for Fleishigs and Milchigs
1. Nowadays, we use ovens for many foods, both fleishigs and milchigs. Lechatchiloh, it is best to avoid the many shailos that arise when the same oven is used for fleishigs and milchigs by having in the home separate ovens for fleishigs and milchigs.
2. Microwave. One should not use the same microwave for fleishigs and milchigs; otherwise, it is almost impossible to avoid problems. Even when food is double wrapped in plastic wrap, the plastic usually ruptures in the middle and the food does not end up totallly covered the whole time it is in the microwave. This will lead to the food and/or the microwave becoming ossur. If this does happen, one should ask a poseik if it is possible to kasher it, and if so, how.
Oven with Two Compartments
3. Some people have an oven with two compartments, one designated for fleishigs and the other one for milchigs. This still creates kashrus concerns since the two compartments sometimes have vents in back, causing steam from both of them to join in one place. When using them simultaneously, the steam and aromas from the fleishigs and milchigs join together and the oven's contents can become ossur. Also, sometimes, when the lower compartment is in use, the upper compartment gets hot, leading to shailos of bliyos [absorbed matter] due to the rule of "cham miktzoso cham kulo" [bliyos in a hot sheet of metal spread through the entire sheet].
4. Oven with a hechsher. When a two-compartment oven has a hechsher, it certifies that the compartments are totally separate, taking away the concern of fleishige and milchige bliyos mixing. It also certifies that the lower one does not heat the upper one.
5. After using an oven for some time, the rubber that hermetically seals the oven's door can wear away. Also, sometimes, opening the bottom door causes lots of steam to rise to the upper compartment. These conditions can lead to shailos of steam from the compartments mixing even if the oven has a hechsher. If this occurs, one must be careful about the kashrus of the two compartments.
