Keilim, Chapter 10, Mishnah 4. This chapter deals with the seal that protects a vessel and its contents from tumah, and our mishnah turns to the practical question of stoppers: which materials can close the mouth of a barrel, and how exactly the plaster has to be applied around them.
The mishnah opens with the words "Hakadur vehapka'ah shel gomi shenetanan al pi hehavit": a ball of fibers, or a ball wound from reeds and grasses, which someone wedged into a barrel's opening to close it. How does the halachah regard a closure of this kind? The mishnah rules, "Im merach min hatzedadin, lo hitzil": should he have smeared the plaster only at the sides, at the point where the ball presses against the rim, the barrel and what is inside it are not saved from tumah.
The reason is the nature of the ball itself. It is not shalem, not a complete, solid body; it is riddled with openings. Plaster along the edges alone therefore accomplishes nothing, because even with the rim sealed the closure as a whole is not tight.
The mishnah continues: "ad sheyimrach milma'alan umilmatan". Only when he spreads the plaster over the ball from above, or beneath it from below, is the closure effective. When the plaster covers the entire surface of the stopper, above or below, there are no longer any gaps and the barrel counts as properly sealed. The lesson is that the ball can never do the sealing on its own. The plaster must extend across the whole of it, either right over the top or right along the bottom.
"Vechen bematlit shel beged", and the identical halachah governs a rag of cloth laid across a barrel's opening as its covering. To the eye such a rag seems solid, yet it too is woven through with minute holes. Therefore it qualifies as a seal only when the plaster is spread across its face, whether above it or beneath it, alongside the plaster applied at the edges.
The mishnah now sets down the opposite case: "haytah shel neyar o shel or", where what was laid across the barrel's opening was paper or hide. Such materials contain no perforations at all; they are shalem, unbroken surfaces. "Ukesharah bimshichah", and he fastened the covering down with a cord, so that it lies snugly in position. Here the ruling is "im merach min hatzedadin, hitzil": plaster at the edges alone, with nothing spread above or below, already shields the barrel from tumah.
Two features make this closure work. Paper and leather are not susceptible to tumah, and they are not perforated the way fibers and cloth are. Since the covering itself is a complete, tight surface, plaster along the sides is all that is required. One must seal the rim; there is no need to coat the entire covering.
The mishnah thus draws a clear line between two kinds of stoppers. A porous material, whether a ball of reeds or a folded rag, needs the plaster spread over its whole face, above or below, before it can be called a seal. A whole, unperforated material such as paper or leather, tied down firmly, needs only the edges plastered, and the barrel and its contents remain protected.