The Gemara (Pesachim 30b) says that a clay oven, which is regularly filled with coals, may be kashered for Pesach by filling it
with coals. We are not concerned that it might crack, since this is its normal use. However, Chazal made a gezeira that clay pots and dishes may not be kashered in this manner. Since they are not ordinarily filled with coals, we are concerned that doing so might cause them to crack. Chazal were concerned that one will wish to spare them from breaking, and might not kasher them properly. The accepted opinion is that even if one claims that they are not concerned about cracking their cheres pots, they
still may not kasher with libun. However, if one already cooked food in the pot, Pri Migadim M.Z. 451:31 writes that in regards to eating the food, one can rely on Ra’avad (Tamim Dayim) who allowed kashering in cases where the owner was not concerned that the pot would crack. However, if the owner had been nervous, then even the food may not be eaten [1].
There are two opinions in Rishonim as to why Chazal were gozeir. Rashi (Pesachim 30b) explains that if we were to permit someone to kasher pottery by filling with coals inside and out, because of the potential loss, one might be moreh heter (rule leniently) and allow themselves to kasher by placing coals on the outside of the kli, without filling the inside with coals. However, most Rishonim [2] explain that the concern is that one will feel compelled to remove the kli cheres from the fire
too quickly, before it has a chance to have an adequate libun. A nafka mina between these two approaches is whether one may
kasher klei cheres in a kivshon (potter’s kiln). Pri Chadash, based on Rashi’s explanation, says that one may not kasher cheres dishes even in a kivshon. However, Shulchan Aruch (O.C. 451:1) rules that klei cheres may be kashered by returning them to a potter’s kiln. We are not concerned that one may remove the dishes too soon, since the intense heat allows them to be kashered immediately.
Self Cleaning Oven
Although self-cleaning ovens (approx. 850˚F) are hot enough for libun chamur, they are not hot enough to be a kivshon (approx. 1700˚ F). For a self-cleaning oven to effectively kasher, the kli must remain in the oven for the duration of the cycle. Therefore, the concern that one may feel compelled to end the cycle early still applies, and one may not kasher ceramic dishes in a self-cleaning oven. However, special ceramics which are made to withstand 850˚F can be kashered in a self-clean oven.
Slight Damage
Pri Migadim (M.Z. 451:11) points out that even a properly performed libun will cause some amount of wear on the kli. This is considered normal. We are only concerned about out-of-the-ordinary or excessive damage, but a טעומ דספה is not a concern. For example, one may kasher keilim in a self-clean oven even though this will cause them to lose their shine. One may kasher the lip of a pot with a torch, even though this will cause discoloration around the lip.
Safety
If kashering keilim with libun or hagalah might compromise one’s personal safety, or of those around you, Rav Belsky thought then this too can be included in the gezeira shema chayis alla, based on the principle “chamira sakanta meisura” (precautions for safety are even greater than for issur). For example, one may not perform hagalah in an area where they might get sprayed with boiling water, unless they have taken all the necessary precautions. Otherwise, the kashering might not be valid even bi’dieved. If there is a concern that overloading the equipment can cause a fire, or explosion, then aside from common sense reasons not to kasher this way, even if one wants to be a “hero”, the kashering will not be valid.
Checking with Probes
What if a kli can only be safely kashered until a certain temperature, but above that temperature one runs the risk of ruining the kli; would one be trusted to kasher at exactly the right temperature? Mishnah Berurah (Shar Hatziyun 451:196) writes that even though boiling water will crack glass, many poskim permitted kashering glass with hot water kebol’o kach polto. Just as one was not afraid to place hot water in the glass when they used it, so too that same degree of heat would not be a concern when kashering. This will take great vigilance, to make sure that the temperature is not too low and not too high. In these types of cases, where delicate machinery require a precision kashering in a narrow range, mashgichim should double check with their own temperature probes to make sure the temperatures are not too low and not too high.
Example, a mashgiach was recently sent to kasher a convection oven/dryer (375˚F for two hours). Though the temperature gauge was set to 375˚F (the maximum setting), because the dryer was empty, the internal temperature climbed to nearly 600˚F, and the machine over-heated (needed major repair). It turns out, the gauge was calibrated to reach 375˚F when there is product flowing through the dryer, and the operators did not realize that the gauge does not monitor the actual temperature inside the dryer. Similarly, there are many stories of ovens that were much cooler than the assumed set point. It should be standard practice that mashgichim bring temperature probes or infrared temperature guns when they are kashering to double check the accuracy of the gauges that are on the machines.
by Rabbi Eli Gersten
RC Recorder of OU Psak and Policy
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[1] See Pri Migadim (M.Z. 451:2)
[2] Rosh Pesachim 2:7, Rashba and Tur