The tanur discussed in Mishnah refers to an oven, generally earthenware, that is conically shaped and open at both the top and bottom. The twelfth perek opens by discussing a plank of wood covering the top of the tanur and overhanging somewhat. The Mishnah teaches that if the tanur had never been used, the plank can act as an ohel. In other words if tumat ha’met were placed under the overhang, then keilim above the plank would be tahor. Likewise if the tumah were placed above the plank, then everything beneath it would be tahor.
If however the tanur was used, the Mishnah records a debate. The Chachamim understand that despite the fact that the plank can act as an ohel to spread tumah, it cannot prevent tumah from passing through it. The Bartenura explains that according to the Chachamim, all the keilim both below and above the plank would be tameh. This is because “keilim can act as an ohel to [spread] tumah but not to [maintain] tahara”. R’ Yochanan ben Nuri however disagrees. The Bartenura explains that while he agrees with the above stated principle regarding keilim, a tanur is different. We shall attempt to understand why.
Before we can address this question, we need to look more closely at the tanur. While the susceptibility to tumah of klei cheres is mentioned broadly (Vayikra 11:33) the tanur is singled out (Vayikra 11:35). Indeed there are number of characteristics that separate a tanur from other keilim. It is attached to the ground, its base is open, to name a few. R’ Wolf explains that there are two ways one can understand this phenomenon. Either the tanur is a subcategory of kli cheres (Rashi) or that it is a unique category on its own (Ramban).
R’ Wolf (Mincha Tehora p213) uses this distinction to explain our Mishnah. The Gra explains that R’ Yochanan ben Nuri understands that despite an old tanur being susceptible to tumah it is still not defined as a kli. Its susceptibility is a gezeirat ha’katuv. Whether or not the plank on the tanur can act as a separation depends on whether it is a kli and not whether it is susceptible to tumah. Contrast this case with, e.g. stoneware. Despite the fact that it is not susceptible to tumah, stoneware cannot act as an ohel to affect a separation because it is a kli. Consequently in our case, since R’ Yochanan ben Nuri understands that even an old tanur is not a kli, the plank can act as an ohel to prevent tumah from passing through it. Based on this understanding it would appear that R’ Yochanan ben Nori understands that a tanur is separate and unique category. Despite not being defined even as a kli, the Torah decrees that it can become tameh.
There are two ways to understand how the Chachamim argue with R’ Yochanan ben Nuri. Either they differ on the fundamental point – an old tanur is defined as a kli. Alternatively, they argue that even if a tanur is not a kli, since it is susceptible to tumah it cannot act as an ohel to separate tumah.
The Gra however explicitly states that the Chachamim understand that an old tanur is defined as a kli. R’ Wolf is nevertheless still bothered how using the tanur, i.e. lighting a fire inside (hasaka) can turn this conical cylinder into a kli.
R’ Wolf suggests two alternatives. Either the Chachamim also agree with R’ Yochanan ben Nuri that the tanur is a unique category. Despite not sharing many of the attributes common to other keilim, the Torah defines it as a kli. This however is only once this conical cylinder acquires a shem tanur – the status of tanur – and this only occurs once hasaka has occurred.
Alternatively, according to the Chachimim, a tanur fits comfortably into the world of keilim as a subcategory of klei cheres. Why then is hasaka significant? He cites the Mishnah Achrona who explains that prior to hasaka the oven – the kli – has not been completed and much like other keilim, it is not susceptible to tumah prior to being completed.
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