Keilim, Chapter 19, Mishnah 5. This mishnah continues the discussion of the mizran, the length of woven material that is wound around the frame of the bed and stretched across it so that the frame holds together and the mattress rests upon it. Here the Mishnah examines what happens when such a mizran is wound around a bed that is already tamei, and then what happens once the mizran is unwound and taken off. The answer turns out to depend entirely on how severe the bed's tum'ah was.
Our first situation is a bed bearing tum'at midras, the tum'ah a bed acquires when someone who is himself a source of tum'ah presses his weight upon it. The Mishnah's language: "mittah shehaytah teme'ah midras" - the bed already held midras tum'ah - "vecharach lah mizran" - and a mizran was wrapped around its frame - "kulah teme'ah midras": the entire assembly bears tum'at midras. A mizran that has been wound onto a bed is no longer an independent object; it functions as a part of the bed, and a part is graded according to the utensil it serves. Bed and mizran together are now one single tamei article.
Next the two are pulled apart: "pereshah" - the mizran was unwound and taken off - "hi teme'ah midras" - the bed still holds its tum'at midras - "vehamizran maga midras" - while the mizran retains only the lighter status of an object that came in contact with midras. Why so? Detached, it is no longer a part of the bed, so it can no longer be graded as the bed is graded. Contact, however, certainly took place, and a bed bearing tum'at midras ranks as an av hatum'ah, a primary source. Whatever touches an av hatum'ah descends to the level of a rishon, a first degree of tum'ah, and that is exactly the status the removed mizran now carries.
The second situation is a bed that had picked up a tum'ah lasting seven days: "haytah teme'ah tum'at shiv'ah" - it was tamei with a seven day tum'ah - "vecharach lah mizran, kulah teme'ah tum'at shiv'ah". Imagine a corpse that had been resting on that bed. A corpse is an av hatum'ah, and through such contact the bed took on tum'ah for a seven day period. Afterwards the bed underwent its purification, and later someone wrapped a mizran onto the frame. Since the mizran now counts as a part of the bed, the seven day tum'ah covers the two of them alike.
And when they part: "pereshah" - the mizran comes off - "hi teme'ah tum'at shiv'ah" - the bed retains the full seven days - "vehamizran tamei tum'at erev" - while the mizran is left tamei only until nightfall. The reasoning repeats itself. Separated from the frame, the mizran can no longer be graded as a part of it. Yet it did touch the bed, and a bed rendered tamei by a corpse holds the rank of av hatum'ah, so the mizran drops to the level of a rishon, whose tum'ah expires in the evening.
The third situation moves to a bed that never reached the rank of av hatum'ah at all, but stands merely as a rishon: "haytah teme'ah tum'at erev" - the bed was tamei until evening - "vecharach lah mizran, kulah teme'ah tum'at erev". Say the bed had touched a sheretz, one of those creatures whose carcass transmits tum'ah. Such contact makes the bed a rishon, tamei until nightfall. A mizran was then wrapped around the frame, and because it now serves as a part of the bed, bed and mizran alike are tamei until evening.
Here the outcome diverges: "pereshah" - the mizran is unwound - "hi teme'ah tum'at erev" - the bed stays tamei until evening - "vehamizran tahor": the mizran is entirely pure. Once detached it cannot share the bed's grade, and on this occasion the contact it had with the bed achieves nothing whatsoever. Behind this stands a rule worth fixing in memory: a rishon cannot transmit tum'ah to utensils. It can defile food and drink, but a keli lies beyond its reach. Since this bed never rose above the level of rishon, and the mizran is a keli, the mizran emerges tahor.
Taken together, the three cases resolve into one simple picture. While the mizran remains wound onto the frame it is nothing other than a part of the bed, and it bears whatever tum'ah the bed bears. Once it has been stripped off, a single question remains: what can this bed transmit by touch? Where the bed holds the rank of a primary source of tum'ah, whether through midras or through a corpse, the mizran becomes a rishon and is tamei until evening. Where the bed itself stands no higher than a rishon, the mizran comes away tahor.