Keilim, Chapter 20, Mishnah 2. Our mishnah continues to examine kelim, beginning with another leather item, and asks about each one whether it is subject to tum'at midras, the tum'ah that a zav or a niddah transmits to something made for sitting, lying or leaning. The governing principle throughout is simple: a keli receives midras only if it is designated for a person's weight. If people do not ordinarily sit or recline on it, midras does not apply to it, though other forms of tum'ah, such as tum'at met, may still apply.
The bagpipe
"Chemet chalilin tehorah min hamidras": a bagpipe, which is essentially a leather bag fitted with pipes, is tahor from midras. Although it is made of leather, and leather items are frequently designated for sitting, this one is not. People do not make a practice of sitting or lying on a bagpipe, and so the category of midras simply does not reach it.
The builders' bowl: Beit Shammai and Beit Hillel
Next in line is "arevat pisonot", a wide trough that workmen use on a building site for hauling earth and stones. About this keli the schools disagree. "Beit Shammai omrim: midras", the school of Shammai holds that it is susceptible to midras, whereas "u-Beit Hillel omrim", the school of Hillel says, "tamei met", it can take on only the tum'ah of a corpse.
Beit Shammai hold that such a bowl can indeed contract tum'at midras. Their reasoning: builders would fill these bowls with stones and load them onto a wagon to be brought to the work site, and while the wagon traveled the workers would seat themselves on the bowls. Since they are in fact used for sitting, midras applies.
Beit Hillel hold that the bowl can become tamei only through corpse tum'ah, not through midras. True, construction workers do sit on them, but the general public does not, and the status of midras is determined by the practice of most people. Since most people would never treat such a bowl as a seat, it is not in the category of items designated for sitting.
The kneading bowl that cracked
The mishnah now turns to a wooden kneading trough: "Areva mishenei log", a bowl that holds from two log, "ve'ad tisha kabin", up to nine kav. "Shenisdeka", should it split open, "temei'ah midras", it now falls under the tum'ah of midras. That is surprising, since nobody reclines on a vessel meant for dough. The explanation is that a split trough is useless for working dough, and its owner will from then on turn it over and sit on it. Its role has changed, and midras follows the new role.
"Hiniha bageshamim venitpecha", if the owner left the split trough outdoors in the rainy season, the timber drank in the water, expanded, and the split sealed itself, then "temei'ah met": corpse tum'ah can take hold of it, while midras cannot. With the split gone the trough is fit for dough again, and it will not be pressed into service as a seat, so midras drops away and only tum'at met remains.
"Bakadim venisteka, mekabelet midras": if he then left it out in the hot dry wind, the moisture evaporated, the wood shrank, and the crack opened again, the bowl once more receives midras. Cracked again, it cannot serve for kneading, and it returns to being used as a seat.
Where the remnants are stricter than the original
The mishnah then formulates a rule: "Zeh chomer", here is a stricture that applies "bishirei chelei etz mibitechilatan", to what remains of wooden kelim as against those same kelim when they were new. While the trough was intact, midras had no hold on it whatsoever. The moment it split, its purpose was transformed, it turned into a seat, and it is specifically in its damaged state that midras can come upon it.
"Vechomer bishirei chelei netzarim mibitechilatan": having raised the theme of remnants being stricter than the original keli, the mishnah brings a second example, wicker kelim. Wicker vessels are baskets woven from reeds, and here too the remains are stricter than the beginning.
"Shemitechilatan einam mekablim tum'ah", at the outset such kelim are immune to tum'ah, "ad sheyit'chasemu", until the rim along the top has been bound off, since only at that point do we call the item a finished keli. "Nit'chasemu", once that rim is in place, then "af al pi", even if, "shenashru siftoteihen", the rims later drop off, and even if some of the sides come away as well, "kolshehen temei'in": whatever is still standing of the sides leaves the basket open to tum'ah.
The logic is that a keli which has already come into being as a complete keli stays open to tum'ah for as long as it can still do a job. While any piece of the side is upright, something can be held in it, and it counts as a receptacle. Hence the stringency: a fresh basket takes on no tum'ah until it is finished and rimmed, whereas that same basket in its damaged state contracts tum'ah even when nothing but a shred of its side survives.
The thread running through the whole mishnah is that tum'ah follows use. A bagpipe stays tahor from midras because nobody sits on it; a builders' bowl is debated because only builders sit on it; a kneading bowl moves in and out of midras as rain and wind close and reopen its crack; and a wicker basket, once it has been born as a keli, keeps its status as long as anything of it remains useful.