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When Kashering an Oven from Fleishig to Milchig is OK

It is a minhag (custom) among Ashkenazim to avoid kashering from milchig (dairy) to fleishig (meat) and back again; one of the reasons for this is that were someone to do this on a regular basis, dilma asi l’mit’ai (perhaps one will make a mistake [and won't remember the current status]). B’dieved (after the fact), if a kli (utensil) was properly kashered from one type to the other, it is considered kosher.

According to most poskim, kashering through libun (purification [through fire], i.e. the most stringent manner of kashering) is excluded from this minhag. Specifically an oven—which is usually preheated, and therefore kashered in a simplified manner for each use—would cause less issues in case of a mix-up. However, some poskim still maintain that kashering keilim through libbun should also be restricted by this minhag.* (Practically speaking, an oven is the most common kli to undergo such switch-offs and are often the source of slip-ups.)

Since not changing the status of a kli from milchig to fleishig (or vice versa) is a minhag and not a clear-cut halachah, there are many exceptions to the rule:

B’sha’as had’chak (in pressing circumstances) it is permitted to make the change.

When kashering for Pesach it is permitted to switch the use of a kli and have it remain the new status year-round.

Twelve months of no dedicated usage: if no fleishigs was cooked in all that time—then a utensil or dish that was formerly fleishig may be converted to milchig, or vice versa.

A second-hand kasherable gift that is the opposite status of the recipient’s needs, may be switched from one type to the other upon receiving it.

A kli which has an indefinite status: if the current user does not know whether it is milchig or fleishig, it may be kashered to be used for the desired status for the long-term.

A dish or cooking utensil that became treif may be switched to the alternate usage upon kashering it.

One way to transform a kli that does not fall within any of these exceptional circumstances is to purposely make it treif in order to kasher it. However, since we are prohibited to mix milk with actual meat, the mixture should be of chicken and milk—for example, pouring hot chicken broth on the milchig kli. In the reverse situation (since most fleishig dishes are used for chicken and meat indiscriminately), the kli would have to sit twenty-four hours unused before pouring hot milk on it—unless it is indeed a utensil used exclusively for fowl.

It is also permitted to switch between milchig and pareve, or fleishig and pareve. So an oven that was fleishig and was subsequently kashered to pareve, may afterwards be made milchig—or the other way around—though this should not be done on a regular basis.

*Actually kashering an oven can be a complicated task, though it is less problematic when switching from milchig to fleishig or fixing one that was involved in a (kosher food) mix-up. For more details, see Halachah #719.

 

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Video #727