Negaim, Chapter 11, Mishnah 9. This perek is devoted to Nigei Begadim, the tzara'as that appears on cloth. Scripture does not confine this tumah to a completed garment: alongside a beged of wool and a beged of linen, the pasuk also names the "shesi" and the "eirev", the warp threads and the weft threads, as things that can contract the nega. Our mishnah defines the point in the manufacturing process at which such threads first become susceptible, and how much thread has to be there before the tumah can take hold.
Shesi and eirev of wool
The mishnah opens: "Hashesi veha'eirev mitam'in benega'im miyad", the warp and the weft become susceptible to nega'im immediately. The Rav explains that this refers to warp and weft of wool, where the two are recognizably different from one another: the shesi (the long threads that run lengthwise on the loom) is thin, while the eirev (the threads that cross them) is very thick. Because one can tell at a glance which thread is destined for which role, the threads are already "shesi" and "eirev" as soon as they are spun, and the Tanna Kamma holds that they are susceptible to tumas nega'im from that moment, even before they have been woven into any beged.
"Rebbi Yehudah omer: hashesi misheyisholek, veha'eirev miyad." Rebbi Yehudah agrees regarding the weft but not the warp. The warp becomes susceptible only misheyisholek, once it has been boiled in hot water as part of the whitening process. The Rav notes that there are sefarim with the reading misheyishlof, meaning from when he draws the thread out and removes it from the pot in which it was boiled in order to whiten it.
Rebbi Yehudah anchors his position in that very pasuk, which sets a beged of wool and a beged of linen beside "shesi" and "eirev". A beged, he reasons, is liable to nega'im only after its production has been finished; the same standard must apply to warp and weft, each of which is susceptible only once its own production is finished. And evidently, weft thread is a finished product the instant it leaves the spindle, whereas warp thread is not regarded as done until it has gone through whitening.
The Tiferes Yisrael explains why the two differ. The eirev threads are pressed in between the threads of the shesi, and the warp threads stand packed tightly together, so many of them and so close that they cover the weft on both faces of the cloth. Since the weft will not actually be seen once the garment is woven, people are not so particular about how white it is; its work is therefore complete as soon as it is spun, and it is susceptible miyad. The warp, which will be visible, is not finished until it has been whitened.
The Tanna Kamma, who holds that both are susceptible immediately, reads the verse differently. The word "o" in "o bishesi" interrupts and begins a new subject: the beged requires that its manufacture be complete, but shesi and eirev become tamei in nega'im as soon as they are spun and identifiable as warp and weft.
Spun flax
"Veha'onen shel pishtim, misheyislaben." The Rav explains that onen shel pishtim means bundles of spun flax. Here the mishnah draws no distinction at all between warp and weft, because in flax there is no recognizable difference between the two as there is in wool. (In wool the difference may lie in the number of strands twisted together, or perhaps in using coarser wool from one animal for the weft and finer wool for the warp.) Since the two cannot be told apart in flax, the mishnah simply gives one rule for the bundles: they become susceptible once they have been whitened. The Rav describes the process: the bundles are placed in an oven so that they warm from the hot air inside it and whiten there. The heat itself does the whitening, drawing off the impurities in the fibers.
How much thread on the bobbin
The mishnah then asks: "Kamah yehei bapekhas?" The pekhas is the piece of wood, what we would call a bobbin, described by the Rav as the wood onto which women wind the spun thread from the kush, the spindle. The thread is spun and then wound off onto this bobbin. The question is how much thread must be wound on it for the thread to be susceptible to tumas nega'im should it turn adamdam or yerakrak, the discolorations of Nigei Begadim.
The mishnah answers that the bobbin must carry enough thread to produce a swatch of "shalosh al shalosh", three etzba'os in each direction, and the full wording continues: "shesi va'eirev, afilu kulo shesi, afilu kulo eirev." That measure holds whether the wound thread is a mixture of warp and weft, or nothing but warp, or nothing but weft. No weaving needs to have taken place; warp on its own, or weft on its own, is mekabel tumas nega'im so long as this amount is present.
Cut threads and knots
"Haysah pesikos, einan mitam'os benega'im." According to the Rav, the mishnah is speaking of a bobbin holding severed lengths of thread, spun at separate times and broken off rather than forming a single unbroken strand. The fact that these lengths lie coiled about one another on the bobbin does not make them a chibbur, so they cannot be combined to reach the shiur. Should a knot bind one piece to the next, however, that does count as a chibbur.
"Rebbi Yehudah omer: afilu pesikah achas ukesharah, einah mitam'ah." Rebbi Yehudah holds that even if there was only one break in the entire bobbin's worth of thread and it was tied back together, it does not become tamei; the knot does not create a chibbur. The Tiferes Yisrael spells out his reasoning: once the thread has been severed, even in a single spot, even if the owner goes back and reties it, it no longer counts as an intact natural thread, and therefore it is not mekabel tumah in nega'im.
The Rav concludes, "ve'ein halachah keRebbi Yehudah." The halachah follows the chachamim: tying the threads together is a sufficient chibbur, and the thread is susceptible to tumas nega'im.