Keilim, Chapter 20, Mishnah 6. This mishnah takes up the case of a bedsheet. A sheet is normally set aside for lying down upon, and an object made for lying, sitting or leaning is subject to tum'at midras, the tum'ah transmitted by a zav or others in that category when they press their weight upon it. Our mishnah asks what happens when the owner takes such a sheet and redirects it to an entirely different purpose.
The mishnah states: "sadin shehu tamei midras va'asao vilon." A sheet that was fit to receive tum'at midras, and its owner converted it into a curtain to be hung in some place, "tahor min hamidras, aval tamei tum'at met." It is now clean of midras, since nobody lies upon it any longer; its function has changed. But it can still contract the tum'ah of a corpse, because it has not stopped being a keli. It is no longer a sheet serving a bed, yet it is a curtain serving a purpose, and a curtain is a type of utensil that receives tum'ah. It was a functional keli before the change, and it remains a functional keli afterward.
From what moment does the change take effect?
Now the mishnah raises a further question: "Me'eimatai hi taharato?" At what stage does the sheet actually shed its status as something susceptible to midras?
"Beit Shammai omrim, misheitaper." Beit Shammai hold that the turning point is when the sheet is trimmed down to the size required for a curtain. A person will also fasten loops to it so that it can be suspended in a doorway, as a curtain needs to be. Once the sheet has been physically reworked from bedding into a curtain, from that moment onward it is no longer subject to tum'at midras.
"Beit Hillel omrim, misheyikasher." Beit Hillel say that cutting it to size is not necessary. It is enough that the loops be tied onto it so that it can be hung. Even if the sheet remains exactly the size it was, the attaching of the loops shows that it now carries a different designation.
"Rabbi Akiva omer, misheyikava." According to Rabbi Akiva, the decisive point is when the owner installs the sheet in the spot where he intends his curtain to serve, even though he neither cut it down nor tied any loops to it. This is the most lenient of the three positions: hanging it in place is all that is required. The object itself need undergo no alteration whatsoever, no trimming and no loops, and even so its susceptibility to tum'at midras falls away.