Keilim, Chapter 9, Mishnah 7. Our chapter has been occupied with sealed earthenware vessels and with the protection that such a seal provides. An earthenware vessel that is closed off with a tzamid patil, a covering fastened tightly all around, shields whatever is inside it from tumah. The question our Mishnah now takes up is what happens when that seal is no longer perfect. If a crack or a break opens up, how large must it be before we say the seal has been defeated and the contents are once again exposed to tumah?
A plate resting on the mouth of the oven
The Mishnah presents a specific arrangement: "Sereidah shenetunah al pi hatanur, u'mukefes tzamid patil." A tanur, an earthenware oven, stands open at the top. An earthenware plate has been placed over that opening, and it has been plastered down onto the rim of the oven so that it is fastened all the way around. The plate is now functioning as a cover, and because the fastening is complete with no gap anywhere, the oven qualifies as a sealed vessel and everything inside it is shielded from tumah.
When the split forms at the join
"Nisdak min hatanur lasereidah": the fissure opens right along the line where the plate meets the rim of the oven, so that the sealing itself has been compromised. A narrow gap has formed there. How wide must such a gap become before we declare the seal ineffective, with the result that whatever sits inside the oven is once again able to receive tumah?
The Tanna Kamma rules: "shiuro kimlo pi mardea shelo nichnas." The measure is the width of the tip of a mardea, an ox-goad, the pointed implement used to prod an ox and steer it in the direction the farmer wants. The opening must be as wide as that tip, but the Tanna Kamma does not require that the goad actually be able to pass through. Once the gap matches the width of the goad's tip, even if the goad cannot literally enter, the seal has been breached and whatever is in the oven can now contract tumah.
"Rabbi Yehuda omer: nichnas." Rabbi Yehuda takes the lenient side. So long as the point of the mardea cannot actually be pushed inside, the oven keeps its standing as a properly sealed vessel. A gap that merely equals the goad's thickness, with no room for it to be inserted, has in his view not yet undone the seal, and the food within remains guarded from tumah.
A crack in the plate itself
The Mishnah now takes up the second case, and here the two Tannaim trade places. "Nisdekah hasereidah, shiuro kimlo pi mardea nichnas." This time the break is not along the plaster but in the covering plate itself. For such a case the Tanna Kamma sets the measure at a hole into which the point of the mardea can truly be inserted. "Rabbi Yehuda omer: shelo nichnas." Rabbi Yehuda maintains that a gap the mere thickness of the goad's point already suffices, even though the goad cannot be pushed in, and the oven has thereby lost its seal.
Notice what emerges from this inversion. Both Tannaim clearly possessed a tradition that in these cases there are two distinct measures at work, one being the width of the ox-goad's tip and the other being an opening wide enough for the goad to enter. Neither one invented a standard. Their disagreement is only over which of the two received measures belongs to which of the two cases.
A curved opening
The Mishnah ends by clarifying what it means for the point of the mardea to be able to enter. "Haya agol, ein ro'in oto aroch." Picture a break, whether at the plaster or in the covering, that opened up in a rounded, bending shape. Total up the missing space and it holds the necessary amount, yet a goad is a straight implement while this gap curves, so nothing can be threaded through it. Do we treat the hole as though it had opened in a straight line and credit it with the amount its area contains?
The answer is no. A bent gap is not reassessed as if it were straight. "Kimlo pi mardea nichnas" is to be understood literally: there must be a hole through which the goad's point genuinely passes. A break that contains an equivalent quantity of missing space, but whose shape refuses to admit the goad, is not reckoned a rupture of the seal, and the vessel goes on protecting its contents from tumah.