Keilim, Chapter 22, Mishnah 10. Our chapter has been examining seats of every sort, and asking of each one whether it is the kind of object that can receive tum'ah, and in particular whether it can receive tum'at midras, the tum'ah that attaches to something whose purpose is to be sat or lain upon. This mishnah brings us five cases: a chair used as a toilet, a padded board called a teraskal, a bench in the bathhouse, the planks laid on the bathhouse floor, and a utensil used to perfume clothing. In several of these cases the object is made of two materials joined together, and the interesting question is what happens to each part when the two separate.
The toilet chair
The first item is the aslah. Picture a bowl set below, and above it a seat of leather stretched on a metal frame, so that a person could relieve himself while seated. The mishnah rules: "ha'aslah temei'ah midras u'temei'at meit." Such a chair is susceptible to tum'at midras, since people sit upon it, and it is also susceptible to tum'at meit, the tum'ah that comes from contact with a corpse.
Next the mishnah considers the two components once they no longer hold together. It says "peirshah" - they separated - and then rules on each piece: "ha'or tamei midras," while "veha'barzel tamei tum'at meit." Should the leather seat detach from the iron frame beneath it, the leather by itself continues to be susceptible to tum'at midras, for even alone it is something one can settle down upon, no different from a mat. The frame, though, has lost every possibility of serving as a seat now that the leather is gone, so midras is no longer relevant to it. Yet it has not stopped being a functional metal vessel, and metal vessels do contract the tum'ah of a corpse, which is why that susceptibility stays with it.
The teraskal
We come now to the teraskal. It consisted of a level plank of willow with leather stretched across it; the leather was pulled over the surface and turned in around each of the four edges, producing a low wall along the perimeter. Two uses followed from this construction: small objects could be kept inside the shallow space, and the padded surface served as something soft to sit upon.
Regarding this object the mishnah rules: "hateraskal shechipuyo shel or" - a teraskal whose overlay is of leather - is "tamei midras u'temei meit." Because it is used for sitting, tum'at midras applies to it, and because its turned-up edges make it a container, it is susceptible to the tum'ah of a corpse as well.
And when the two parts come apart? "Peirshah, ha'or tamei midras vehateraskal tahor mikelum." The leather covering, once removed, is still susceptible to tum'at midras, since a person can lie or sit upon it just as he would upon a mat. But the bare wooden board is completely tahor, free of every form of tum'ah. It cannot receive midras, because nobody would choose to sit on a plain board once the leather padding is gone. And it cannot receive ordinary tum'ah either, because without the leather rim folded around its sides it no longer holds anything, and a flat board that is not a receptacle is not susceptible.
The bench in the bathhouse
"Safsal shebamerchatz": a bench in the bathhouse, the sort of bench people sat upon while bathing. This one was built with a seat of stone resting on "u'shtei raglav shel etz," two legs of wood. The ruling is "tamei": the bench is susceptible to tum'ah. The stone seat itself of course cannot become tamei, since stone is not susceptible to tum'ah at all, but the wooden legs can.
This deserves a moment's thought. Ordinarily we say that a part attached to an object that is not susceptible to tum'ah takes on the status of the object it serves, and would therefore be tahor along with it. Here that principle does not apply, because the stone seat is useless without the legs that raise it up and hold it in place. The legs are therefore not a secondary appendage at all; they are the essential component of this bench, and being of wood, they remain susceptible to tum'ah.
The mishnah then changes one detail: "achat shel etz ve'achat shel even, tahor." If one leg was wood and the other stone, the entire bench, wooden leg included, is tahor. Since the bench cannot stand at all if a leg is missing, the stone leg is itself a fundamental part of the structure. The wooden leg is now attached to a bench whose essential parts, seat and stone leg, are not susceptible to tum'ah, and so the wooden leg is treated as a secondary component of a structure that cannot become tamei. It too remains tahor.
The planks on the bathhouse floor
The bathhouses of those days had used and dirty water running along the floor. So that people could walk through without soiling themselves and their clothing, wooden planks were laid down over the floor, and people would sometimes sit down on those planks as well. Our mishnah asks whether such planks are subject to tum'at midras.
The mishnah records: "hanesarim shebamerchatz sheshigman" - bathhouse planks that were planed down and made smooth - and then the dispute, "Rabbi Akiva metamei vachachamim metaharin." A bare flat board would normally lie outside the categories of tum'ah altogether. Here, though, something was done to the wood on purpose: it was smoothed, and smoothing made it pleasant to sit on. Rabbi Akiva holds that this treatment turned the boards into seats, with the result that tum'at midras takes hold of them.
The chachamim rule that the planks are tahor, and the mishnah gives their reason: "she'ein asuyin ela sheyihyu hamayim mehalchin tachteihen," they were made for no purpose other than to let the water run along beneath them. Even though they were smoothed and a person is able to sit on them, their real purpose is to keep people and their garments from being dirtied by the water below, not to serve as seating. Since that is what they were made for, tum'at midras does not apply to them.
The fumigator for clothing
The mishnah closes with the kankilin, a wooden utensil used to perfume garments. Spices were burned underneath, the clothes were arranged above, and the fragrance rose into them. The ruling depends on the utensil's shape.
"Kankilin sheyesh bah beit kibul kesut temei'ah": one that is built like a basket, with a hollow in which the garments are placed, is susceptible to tum'ah, since it is a wooden keli that holds something inside it. "Veha'asuyah kecheveret tehorah": but one built in the form of a beehive, open at its ends, where the clothing is simply spread out over the top and never placed inside, is tahor. It has no receptacle, and a wooden utensil that holds nothing cannot become tamei.
Running through all five cases is a single thread: the mishnah asks not what an object is made of alone, but what it is truly for, and which of its parts the whole thing depends upon. Leather that can still be sat upon stays susceptible; a board that has lost its rim is free of tum'ah entirely; wooden legs that carry a stone seat are the heart of the bench, while a single wooden leg beside a stone one is merely an appendage; planks smoothed for walking over dirty water remain, in the view of the chachamim, exactly what they were made to be; and a fumigator becomes a keli only when it actually holds something within it.