How does tumah work in the sale of an oven between a householder and a baker?

Question

The mishnah at Keilim 5:5 describes the susceptibility to tumah of an attachment to a pottery oven (translation from Sefaria):

‮מוסף התנור: של בעלי בתים טהור; ושל נחתומין טמא, מפני שהוא סומך עליו את השפוד. רבי יוחנן הסנדלר אומר, מפני שהוא אופה בו כשהוא נדחק.

The additional piece of a householder's oven is clean, but that of bakers is unclean because he rests the roasting spit on it. Rabbi Yohanan Hasandlar said: because one bakes on it when pressed [for space]. See here page 27

This distinction between a householder and a professional/shopkeeper comes up in several other places as well, arising from their different uses for utensils.

If a householder sold an oven to a baker, is the "additional piece" suddenly susceptible to tumah as soon as it's acquired by the baker, or do we say that since it was insusceptible to tumah after it was completed as a utensil, it will stay insusceptible? If a baker sold an oven to a householder, does it lose its susceptibility as soon as the householder acquires it? Does it matter if it was tahor or tamei upon acquisition?

Answer 1

This is an explicit Mishna in the second half of Keilim 25:9:

כָּל הַכֵּלִים יוֹרְדִין לִידֵי טֻמְאָתָן בְּמַחֲשָׁבָה, וְאֵינָן עוֹלִים מִידֵי טֻמְאָתָן אֶלָּא בְשִׁנּוּי מַעֲשֶׂה, שֶׁהַמַּעֲשֶׂה מְבַטֵּל מִיַּד הַמַּעֲשֶׂה וּמִיַּד מַחֲשָׁבָה, וּמַחֲשָׁבָה אֵינָהּ מְבַטֶּלֶת לֹא מִיַּד מַעֲשֶׂה וְלֹא מִיַּד מַחֲשָׁבָה:‏

All vessels become susceptible to uncleanness by intention, but they cannot be rendered insusceptible except by a change-effecting act, for an act annuls an earlier act as well as an earlier intention, but an intention annuls neither an earlier act nor an earlier intention.

Once the baker rendered the addition susceptible to uncleanness (in this case with an action) there is no way to remove this susceptibility to uncleanness unless one does an action.

In this case, one would have to damage the addition to a point where it would be useless, after which it loses its susceptibility to uncleanness and then one could rebuild it.

This is also codified by the Rambam in Keilim 8:10:

כָּל הַכֵּלִים יוֹרְדִים לְטֻמְאָה בְּמַחְשָׁבָה וְאֵין עוֹלִין מִידֵי טֻמְאָה אֶלָּא בְּשִׁנּוּי מַעֲשֶׂה. וְהַמַּעֲשֶׂה מְבַטֵּל מִיַּד הַמַּעֲשֶׂה וּמִיַּד הַמַּחְשָׁבָה. וְהַמַּחְשָׁבָה אֵינָהּ מְבַטֶּלֶת לֹא מִיַּד הַמַּעֲשֶׂה וְלֹא מִיַּד הַמַּחְשָׁבָה.‏

Answer 2

The general rule for Kelim is that as soon as the Kli is completed and its use is chosen (through intention or actual use), which parts of it can receive Tumah are set. They can only lose that status by being broken or (for some materials) being immersed. See, for instance, Kelim 2:1, 11:1, 15:1, 30:1.

Additional "useless" parts of the object can become Mekabel Tumah if one intends to use them for a set purpose (or actually does so), but nothing can lose its status of Mekabel Tumah until the Kli is broken enough that it is useless for its primary purpose (or it is separated from the Kli). Additional destruction may be necessary if destroying the Kli does not make the part useless on its own.

A "baker's oven" was built with the intention that part of it will be used for resting the roasting spit. Therefore that part is a useful part of the Kli and can become Tamei. It cannot lose its "Mekabel Tumah" status until it is broken off the oven or the oven is broken (and possibly more).

A "householder's oven" was not built in that way, so that part is not directly susceptible to impurity. If a baker bought it and started using that part in the way it's used for a baker's oven, it would become Mekabel Tumah because it has been made useful. If he didn't use it, it would remain insusceptible.