Keilim, Chapter 28, Mishnah 3. Our chapter deals with pieces of cloth and leather and the question of when such an item still counts as a usable article, which can receive tumah, and when it has been ruined and set aside from use, which leaves it tahor. This mishnah applies that question to bandages, and it presents two different kinds.
The first is the isplonit, a plaster spread with fatty, oily substances: oil, butter, wax and the like. The second is the melugma, a poultice smeared with a paste of flour and figs, which is not greasy at all. The difference between the two substances, and the difference between the two backings on which they are spread, will determine the halachah.
The greasy plaster
The mishnah begins: "Ha'oseh isplonit, bein bebeged bein be'or, tehorah": one who makes a greasy plaster, whether upon cloth or upon leather, it is tahor. The reasoning is straightforward. The fatty ointment soaks into the backing and stains it. Whatever this piece of cloth or leather was once good for, it is good for nothing else now. Since it can no longer serve any other purpose, it is no longer regarded as a utensil at all, and an item that is not a utensil does not receive tumah. This is true equally of cloth and of leather, because grease ruins them both.
The flour and fig poultice
Concerning the second type the mishnah rules: "Melugma bebeged, tehorah, ube'or, temei'ah." Here the two backings part ways. If the flour and fig paste was spread on cloth, the piece is tahor, because such a paste clings to the weave permanently and leaves the cloth unfit for any further use; the cloth is finished as a utensil. But if the paste was spread on leather, the piece remains susceptible to tumah, because leather does not absorb it. The paste comes away from the smooth surface of the leather, the leather is as good as it was, and it therefore keeps its status as a usable article that can become tamei.
The view of Rashbag
On the case of the paste spread over a garment, a second opinion is recorded. Rabban Shimon ben Gamliel holds "af bebeged temei'ah": a garment treated this way, too, remains capable of contracting tumah, and he attaches his reasoning in three words, "mipnei shehi nin'eret", that the mixture gets shaken loose. His point is that a paste of flour and figs eventually dries out on the fabric; once dry it turns brittle, crumbles, and a shake is enough to remove it, leaving the garment clean and perfectly serviceable again. A cloth that was never actually spoiled never forfeited its standing as an article of use, and so it goes on being susceptible to tumah like any other garment.
So this short mishnah turns on one clear principle: tumah attaches to what is still usable. Grease finishes off both cloth and leather; a dry flour paste, according to the first opinion, finishes off cloth but not leather, while Rabban Shimon ben Gamliel holds that a paste which can simply be shaken away has not finished off anything at all.