I normally have a separate oven that we uses for all dairy. Unfortunately it has broken and can’t be fixed before Shavuos. I was wondering if I could use the oven that I normally use strictly for Fleishigs and use it to make dairy at this time (covered if necessary)?
The Alter Rebbe states that those who are accustomed to bake dairy products for Shavuos must be careful when koshering their meat ovens (in which meaty juices and the like occasionally drip or splatter, and these juices are then absorbed into the oven walls)—the oven must be heated to the point that sparks fly from it and that the coals used for koshering are passed along all of its surfaces.
Nowadays, ovens are made of enamel, and there are divergent views about the halachic status of this material. Food is usually baked in trays nowadays, and it is not baked directly on the inner surface of the oven as in former times. Accordingly, the oven walls generally absorb the steam of the food rather than the food itself. Therefore, many authorities are lenient and allow a light form of libun. Practically speaking, a modern oven is heated to its highest heat and left running at that temperature for a considerable length of time; this method can be relied upon for converting the oven from meat use to dairy. If the oven has a floor that separates the source of heat, the floor should be removed, if possible, so that the heat enters directly into the heart of the oven.
Some authorities are stringent about this type of koshering as they consider enamel a form of earthenware. Others are concerned that the oven door is glass, and glass cannot be koshered. Nevertheless, if the oven has a self-cleaning mode, we can rely on this self-cleaning process to kosher the oven without concern.
(An excerpt from the Shavuos Day to day halachic guide, by the Badatz).