Keilim, Chapter 20, Mishnah 3. This mishnah examines a keli built of more than one piece, where one of those pieces is a material that can receive tum'ah and the other is a material that cannot. The question in every case is the same: are the two pieces treated as a single keli, or as two independent objects that merely happen to be touching?
The Two Principles Behind the Mishnah
The first principle: a keli made of several components is judged as one unit. If tum'ah reaches one component, the entire keli becomes tamei. That holds true only where the components are legally joined, a chibur. Where the pieces are not considered joined, tum'ah that strikes one of them stops there, and the other pieces remain tahor.
The second principle concerns the materials themselves. A flat metal utensil, with no receptacle at all, is still capable of becoming tamei. A flat wooden utensil is not; wood receives tum'ah only when it forms a container. However, when a flat piece of wood is fixed to a metal keli and serves that keli's function, the wood becomes a yad, a handle, and it takes on tum'ah together with the metal. The mishnah now applies these rules to a series of practical cases.
A Stick Used as an Axe Handle
"Makel she'asa'o beit yad lekardom": a person slipped an ordinary stick into the opening of an axe head so that it should serve as its handle, and he never secured it there in any way. About such a tool the mishnah teaches "chibur letum'ah bish'at melachah". The two pieces count as legally attached with respect to tum'ah, but only for as long as the axe is being wielded. Should tum'ah come into contact with the metal head at such a moment, head and handle contract it together, as one single keli. Wood shaped without any cavity has no susceptibility of its own, and yet here the stick is drawn in, because while the axe is chopping the stick functions as the yad of the blade.
When the axe is idle, the picture changes, even though the stick is still sitting in the socket. The two are physically in contact but are no longer regarded as one entity. Tum'ah that touches the blade renders the blade alone tamei. And tum'ah that touches the stick leaves it entirely tahor, since a flat wooden object with no receptacle has no capacity to receive tum'ah by itself.
The Dyostad
The mishnah turns to the dyostad, an implement used in producing thread. It consisted of an upright wooden shaft pierced by two holes, one above the other. Rods were set into those holes, and the spun thread was wound around the rods.
"Hadyostad chibur letum'ah bish'at melachah": here too, the components count as joined for tum'ah only while the implement is being worked. The wooden shaft and the metal rods are pieces of one keli, so tum'ah reaching one piece spreads to them all, but only during use. Once the work stops, the pieces are viewed as separate entities even if the rods are still lodged in the shaft, and tum'ah that reaches one does not necessarily carry to the others. Part of the reason is the nature of the implement: a dyostad was ordinarily put together for the job and taken apart afterward, so its parts are joined precisely when it is working.
Setting the Dyostad into a Post
"Keva'o bechilonas": the owner may plant the implement in a wooden pillar so that it will stand firm. In that situation, tum'ah reaching any single component spreads through the implement in its entirety. What sets this apart from the previous ruling is that no work needs to be underway; the halachah holds even while the dyostad sits idle. By mounting it the owner has given it the status of kavua, permanently established, and from then on its components are regarded as attached around the clock.
"Ve'eino chibur lo": the post itself, however, is not joined to the dyostad. The post contributes nothing to the implement's function; it merely holds it in position. Consequently, tum'ah affecting the dyostad does not travel into the post. The post stands as an independent flat wooden object, and such an object cannot receive tum'ah at all. So the dyostad may be tamei while the post remains tahor.
A Thick Post Serving as the Shaft Itself
Until now the wooden shaft was a slender stick, with the post serving only as a base beneath it. The mishnah now considers a case where a person took a heavy wooden post and made the post itself into the body of the implement: "Asa lo dyostad." He bored the holes directly into the thick post and set the rods into them, so that the middle section of the post now holds the rods.
Should tum'ah come upon the rods, the mishnah rules "ein tamei ela tzorko": susceptibility is limited to the wood that the implement has need of, meaning the material immediately around the rods, while all the rest of the pillar is left tahor. Why so? Because the pillar is doing the work of a yad for the dyostad: it grips the rods and allows them to do their job. Its bulk, however, goes far beyond anything the implement requires. Whatever genuinely serves the dyostad qualifies as a yad and contracts tum'ah; the surplus timber serves nothing and stays beyond tum'ah's reach.
A Chair Fixed to a Post
"Kisei shekeva'o bechilonas, tamei": a chair that a person fastened to a wooden post is still susceptible to tum'ah, specifically tum'at midras, because it remains fully usable for sitting.
"Ve'eino chibur lo": where tum'ah is concerned, chair and pillar are not one item, and so the pillar, being flat wood, stays clean of tum'ah. Nothing about the pillar helps the chair do what a chair is meant to do, so it does not qualify as the chair's yad, and the chair's tum'ah never travels into it.
A Seat Carved into the Post
"Asa bo kisei": suppose that instead he carved a sitting place into the pillar itself. The ruling is "ein tamei ela mekomo": susceptibility is confined to the spot where the seat is, while everything else in the pillar remains outside it. A seat is a seat, and so it takes tum'at midras. The remaining wood of the pillar is flat and cavityless, which never receives tum'ah, and it cannot be called the seat's yad, since it contributes nothing to the seat's use as a place to sit.
A Chair Attached to an Olive Press Beam
"Keva'o bekurat beit habad, tamei": where a person secured a chair onto the pressing beam in an olive press, tum'at midras still applies to it. Realistically nobody sits there anymore, since whoever took a seat in that spot would obstruct the pressing, and yet the chair has lost none of its identity as a chair, and so midras attaches to it.
"Ve'ein chibur lah": the chair is not joined to the beam, and the beam, again a flat wooden object, does not become tamei.
A Seat Carved into the Press Beam
"Asa beroshah kisei, tahor": should he instead fashion a seat out of the beam's upper end, that beam, together with the seat cut into it, is tahor. The mishnah supplies the explanation: "mipnei she'omrin lo, amod vena'aseh et melachtenu", for whoever settles himself there is told to rise so that the pressing can go forward.
The underlying rule is that a seat is subject to tum'at midras only if it can be sat upon while the object continues to serve its primary purpose. Where sitting interferes with that purpose, it is not a seat for the laws of midras. Here, someone seated on the carved top prevents the beam from crushing olives, so he will inevitably be told to stand. For that reason the carved seat is not treated as a kisei with regard to midras, and the beam in its entirety remains tahor.