[Note: The below is for information purposes only, as is everything on this site. The decision to act upon any of it or not is the personal decision of the reader and any details regarding the observance of any halakhah – especially those laws which are intricate, complicated, and/or severe – should be discussed with a competent rav.]
Ovens have been used by Jews since ancient times – and since ancient times they have been the topic of halakhic discussion and inquiry. However, much of the discussion surrounding their use found in the Talmud is referring to ovens as they once were: relatively small and made of earthenware (heres).
The halakhot related to the Talmudic oven – also called a tanur – rather than being properly assessed, have been simply extended by most posekim and have been projected onto modern ovens with little if any consideration being given to the obvious differences between them and their Middle Eastern counterparts. However, to act as if they are completely analogous or even mostly similar simply because they both are called “ovens” is a mistake. What follows assumes that the small ancient ovens of Talmudic times and our large home appliance ovens of today are similar in that they are both types of ovens – that is all.
The Big Question of Separate Ovens
The biggest question related to kashruth and the use of modern ovens is whether it is a requirement to have separate ovens for meat and dairy or whether it is permitted halakhically to use the same oven for both. Further, if it is permitted to use one oven for both meat and dairy foods, under what circumstances is this possible and what precautions (if any) must be taken when doing so?
The entire discussion (read, cause for concern) hinges predominantly on two basic assumptions:
 That steam or condensation (zei`ah) presents a halakhic concern at all, and
 That modern metals such as aluminum or stainless steel are boleya or polet (capable of absorbing and giving taste).
As will be clearly seen, neither of these assumptions has a strong foundation in the sources of Hazal and neither presents any concern. Even if steam were genuinely a problem, it would only be an issue when in an enclosed, small space (cf. Arokh HaShulhan 92:55), something which is not the case for modern ovens which are vented and tend to be much larger than their ancient cousins.
While it is true that the Rama (Rabbi Mosheh Issereles) in the Shulhan Arukh (92:8) takes a strict position regarding steam (zei`ah), relying on a teshuvah of the Rosh (20:26) [who quotes a Mishnah in Masekhet Makhshirim (2:2)], the fact is that the sugyah in the Gemara which discusses the entire issue of an oven and kosher and non-kosher foods being cooked in it (cf. b.Pesahim 76a-b and y.Terumot 10:2), does not even mention zei`ah at all. Instead, it concludes that reihah (aroma) is lav milta (“not a halakhic issue”). Further, it only mentions a case where kosher and non-kosher meat were cooked in the same space, simultaneously, and in close proximity to one another. The conclusion of Hazal is that even if two pieces of meat, one kosher and the other not, were being cooked next to each other in the same oven at the same time, it would only be prohibited mi-de-rabbanan (rabbinically), but only at the outset. They further conclude that if such a situation does arise, then the kosher meat is permitted to be eaten bedi`avad.
“Steam is never mentioned and aroma is considered a non-issue.”
The implication of this sugyah and its practical conclusion is that – as many modern posekim explain – it is permitted to cook or bake dairy and meat foods in the same oven, although not simultaneously, and it is not necessary to perform libun kal (i.e. heating it to a high temperature or “self-cleaning”) between each use, as long as it remains relatively clean. Even if the oven has been used to cook non-kosher food, it may be used to cook kosher food as long as it is likewise relatively clean (i.e. no visible grease or pieces of non-kosher food that will almost certainly spoil your kosher meal). This is the pesak of several noted Sefardi hakhamim.
The Talmud Yerushalmi
The Talmud Yerushalmi (cited above) records Rav Levi – the one whose opinion the halakhah follows with regard to reihah – as concluding that since it is not prohibited for the smell of kosher and non-kosher meat to mingle in the same oven while being roasted, it is therefore permissible to do so even le-khatehilah (“at the outset”). This, of course, is not the halakhah (cf. Rambam, Hilkhot Ma’akhalot Asurot 9:22), but it nevertheless gives us further insight into the very reasonable view of Rav Levi regarding kashrut and use of [ancient] ovens.
Baking Bread and Roasting Meat
As for the discussion of bread being baked in the same oven as meat is being cooked, Hazal forbade the eating of such a loaf of bread with kutah (a porridge containing dairy that was popular in the Middle East and Persia) because of concerns due to basar be-halav – b.Pesahim 76b, see there. However, this is again talking about bread and meat being in the oven simultaneously, not one after the other (see also Hilkhot Ma’akhalot Asurot 9:22 – “…bread baked together with meat…”).
A Foundational False Equation
As is common in the methodology that underlies the Ashkenazi approach to halakhah, there is a false equation in the standard Haredi/Hasidic position between the laws of hekhsher/tumah va-taharah with the laws of kashrut. While there is some overlap, they are definitely separate, as anyone who comparatively reads Hilkhot Tumat Okhalin and Hilkhot Ma’akhalot Asurot will be able to easily ascertain. Thus, the reason why the Rosh had to bring a proof for his new proposal from m.Makhshirim regarding the condensation in a bath house being tamei is because the whole idea of steam is not considered an issue within the scope of kashrut and is never mentioned until the erroneous equation of these two areas of halakhah at a much later time in Jewish history. Hazal never make any such claim in the sources that they left to us.
There were, however, those among the Ashkenazim who understood this separation properly, such as the prominent student of Rabbi Hayim of Volozhin z”l, Rabbi Ya`akov ben Aharon Karliner z”l, who authored a collection of responsa known as Mishkenot Ya`akov wherein he states the following:
TEXT [MISHKENOT YA`AKOV 1:34]
“…rather, it is certainly the opinion of Rashi z”l that [condensation – zei`ah] is not considered a liquid at all, even in the matter of a prohibition, but [it is taken into account] only in matters of tumah and hekhsher just as we previously learned…rather, it is certain that condensation from steam does not ascend [from one substance] and descend [to be absorbed in another substance, thus becoming a concern] in matters related [to kashrut], and neither of them [i.e. steam nor condensation] is given the status of a liquid except in matters of tumah…”
We can see from here that there were indeed those in the Ashkenazi-Lithuanian camp who understood that kashrut, and particularly the laws of basar be-halav, is not a matter of “purity,” but is based on real principles of forbidden food mixtures, etc. In other words, whereas steam and condensation can certainly cause people (and objects) to contract ritual impurity (tumah), they are not considered to be a form of forbidden food mixtures which could even lead to a prohibition of eating them from the standpoint of the halakhah – it is simply not an issue (unless you are a kohen or planning to visit the Temple complex).
Pottery vs. Glass and Metal
As for the kelim which are subject to beliyuth (absorption), the Gemara is largely discussing klei heres, which is porous pottery/adobe (cf. Rambam, Hilkhot Ma’akhalot Asurot 17:1f). Although metal and glass are discussed in the context of a Jew obtaining used dishes and utensils from a non-Jew, they are not given central attention as is earthenware. And even if it were, it would be a discussion of the rough casted metals of antiquity, such as iron or copper. Some today hold that modern metals (such as stainless steel or aluminum) are not boleya or polet and thus not subject to these laws anyhow. I would agree. Add to this that the Gemara explicitly states that beliyut is an observable phenomenon – not an imperceptible “state” (cf. b.Avodah Zarah 33b-34a) – and one may reasonably conclude that our metal utensils today, as long as they are completely cleaned and scoured of visible, palpable residue between uses, may be used for both meat and dairy foods.
Another “halakhah” that many will be happy to never have to worry about again is the supposed need to be careful of the steam from pots on your stove-top rising and becoming condensation on the kitchen utensils hanging on hooks above it, as if there was such a thing as “meat steam” or “dairy steam.” The reality is that it’s no problem at all, as we have seen. So go ahead and hang that fancy kitchenware above your stove and never give it a second thought.