Keilim, Chapter 10, Mishnah 6. This chapter is occupied with the law of tzamid patil: an earthenware vessel is sealed shut with a tight covering, and by virtue of that seal its contents remain pure even in a place where tumah is present. Our mishnah continues to examine the practical question that runs through the chapter: what counts as a genuine seal? A hole in the side of a barrel, a stopper made of vine wood, a wooden board laid across the mouth of an oven, and in each case the Mishnah asks whether the closure is truly closed.
The Mishnah begins: "Chavit shenikvah usetamuha shemarim, hitziluha" - an earthenware barrel of wine developed a hole, a hole that would ordinarily have to be stopped up, and the sediment settled in the wine drifted into the opening and clogged it. The ruling is that the sediment shields the barrel, and no further plugging is required. This follows the position of the Chachamim taught in the previous mishnah, that a seal need not extend beyond the rim of the opening and spread onto the wall of the vessel around it. As long as the substance is lodged in the breach itself, the breach counts as stopped, and everything inside the barrel stays pure.
"Pekakah bizmorah" - this time nothing clogged the breach on its own; a length of grapevine wood was forced into it as a plug. Of such a case the Mishnah rules "ad sheyemareach min hatzedadim": the barrel is not treated as sealed until plaster is smeared along the rim of the breach, at the points where the wood presses against the earthenware. Vine wood driven into an opening is no closure by itself; only the plaster at the margins turns it into a genuine seal.
"Hayu shtayim, ad sheyemareach min hatzedadim uvein zemorah lachaverta" - suppose the breach was so large that a single piece of vine could not fill it and two were required. Plaster along the outer rim does not suffice here. He is obligated to apply plaster in the line between the one piece and its fellow as well. Where two lengths of wood lean against each other, they never meet with enough pressure to count as shut, so that inner seam demands plaster no less than the outer margins do.
The Mishnah now moves from a barrel to an oven: "Neser shehunach al pi hatanur, im mirach min hatzedadim, hitzil" - a wooden plank was set down across the opening of a clay oven. Should plaster be applied at the margins, at the points of contact between plank and oven, the plank protects whatever lies inside from tumah. What makes a plain plank suitable for this role? A flat wooden board has no receptivity to tumah whatsoever, and it is therefore fit to serve as a cover closing off the oven's mouth. As with every cover discussed in this chapter, though, it accomplishes nothing until plaster has been laid around the opening.
"Hayu shenayim, ad sheyemareach min hatzedadim uvein neser lachavero" - if two boards were laid side by side to cover the oven's mouth, the covering is not considered sealed until he plasters both around the outer edges and along the line where the one board meets the other. The reasoning is exactly as with the two pieces of vine: at the point where the two boards touch, they are not pressed tightly against one another, and an untreated seam is not a seal.
The Mishnah closes with an exception: "Asa'an besinin o beshugmin, eino tzarich limareach min ha'emtza" - if he joined the boards to one another with the sticky shavings of a certain bush, or with shavings of reeds, substances that adhere and can serve as a kind of glue, he does not need plaster in the middle. The two boards have effectively been fastened into one piece, the seam between them is genuinely closed, and plaster around the outer edges of the oven's mouth is all that this covering requires in order to guard what lies inside.