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)?
There are countless details involved in converting ovens and stoves from meat use to dairy, but this is not the forum to delve into this at length. (We will limit the discussion to details relevant to the Alter Rebbe’s treatment of the matter in his laws of Shavuos.)
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.
The Alter Rebbe raises an additional point: Be careful to use dairy implements to remove food from the newly converted oven, and not to use the usual tools which remain meaty from previous use in the oven.
Note that koshering the oven on Yom Tov is problematic: The light libun that is performed on ovens nowadays generally requires the creation a new source of fire, and it is therefore forbidden on Yom Tov. [**]
* According to halachah, complete libun is not necessary to convert a utensil from meat to dairy (since the meat is kosher, unlike the conversion of an oven used for non-kosher food); rather, hagalah or a light form of libun is sufficient. According to the Alter Rebbe, hagalah works even in a case where first meat and later milk were absorbed into the walls of a vessel without a koshering process in between. However, this case is different, because we are dealing with an oven made of earthenware. Such material cannot be purged without a full libun, and the source of the koshering fire must be within the oven itself and it’s only effective if the oven regularly uses a source of fire that is located within.
** As a general rule, it is forbidden to perform hagalah or libun during Yom Tov. That would be considered mesaken kli, equal to fixing a utensil on Yom Tov. This is true even if one wishes to use the oven or utensil on that same day of Yom Tov for food preparation. However, one may perform libun on a metal pan in which he previous cooked meat, so that he can now use it for a dairy product, because it does not appear to be mesaken kli; rather, it appears that he is warming the pan prior to using it for food, which is permitted on Yom Tov. This works only if he places the food into the pan immediately upon removing it from the source of fire used for libun. Once the pan has cooled down, however, if he then reheats it before putting the food in, it becomes clear that his first act was simply an act of libun, which is considered mesaken kli. In addition, this is permitted only when converting a pan from meat to dairy, because the pan requires only a light libun, which is similar to warming a pan for immediate use.
If the pan does not require libun or hagalah according to the strict letter of the law, but only due to a chumra (stringency), he may perform libun or hagalah on Yom Tov, even if he could have done so in advance of the festival. The reason for this is because, according to the law, the pan does not require koshering; therefore, the act cannot be considered mesaken kli, for he cannot rectify a utensil that does not essentially require rectification.
(An excerpt from the Shavuos Day to day halachic guide, by the Badatz).