Keilim, Chapter 10, Mishnah 2. This chapter deals with the sealed earthenware vessel, the tzamid patil, whose tight seal protects whatever is inside it from becoming tamei. Our Mishnah asks a practical question: which substances actually qualify as a seal? The Mishnah answers in three stages: materials that work, a material that does not work at all, and a material that would work but which the Chachamim forbade using.
The materials that form a valid seal
The Mishnah opens: "Bameh makifin", with what do we seal off the mouth of a vessel? The list follows.
"Besid uvegypsis": sid is lime used as plaster, and gypsis is a second sort of plaster, gypsum. "Bezefet uvesha'avah": pitch, and beeswax. "Betit uvetzo'ah": wet earth or clay, and fresh dung while it is still damp. "Bechomer uvecharsit": mortar, and a paste made by grinding earthenware shards down into powder.
The Mishnah then generalizes: "uvechol davar hamitmare'ach", and with anything else that can be spread or smeared into place to form a seal. Any glue or paste that people ordinarily use for sealing falls under this heading. The common denominator is clear from the examples: these are substances that are applied while soft and then cling to the vessel itself.
A material that cannot serve as a seal
Now the Mishnah turns to what does not qualify: "Ein makifin lo beva'atz velo be'oferet", one does not seal with tin or with lead. And it gives the reason: "mipnei shehu patil ve'eino tzamid". Tin and lead are not smeared on and do not adhere to the vessel; they merely sit over the opening as a snug covering. A metal lid may fit tightly, but it does not bond with the vessel the way plaster, tar, wax or clay paste do. Every substance in the first list qualifies precisely because it sticks fast to the vessel and becomes one with it.
Materials that would work but are forbidden
The Mishnah continues with a different category: substances that would technically form a proper seal, yet may not be used. "Ein makifin lo bidveilah shemeinah", one does not seal the mouth of a vessel with a rich, sticky mass of pressed figs. "Velo bevatzek shenilosh bemei peirot", nor with dough that was kneaded with fruit juice.
Why leave these out? Because in the condition described they are incapable of contracting tumah at all. Produce that has been picked has no susceptibility to impurity until it undergoes hechsher, meaning that one of the seven liquids has come into contact with it. Juice pressed from fruit does not appear on that list of seven. It follows that the sticky mass of figs, and dough moistened with grape juice, never received hechsher and stand entirely tahor. Judged by the strict law, they should therefore have been perfectly serviceable for closing off a vessel and holding tumah out.
But the Chachamim had their reason: "shelo yevi'einu lidei pesul", so that a person should not end up bringing the contents of his vessel to be disqualified. Consider what can happen with a seal made of food. Should that food later be wetted by one of the seven liquids, it immediately becomes susceptible to tumah. If a corpse is then present in the same tent, the seal itself contracts tumah, and a seal that is itself tamei cannot shield anything. The contents of the vessel would then become tamei from that corpse as well. The Chachamim were concerned that a person would not notice what had happened: the food seal would quietly become susceptible, then become tamei, the seal would no longer be doing its job, and the owner would go on assuming that everything inside was protected. They therefore decreed that one may not use foods or such substances as a shield for a vessel against tumah.
After the fact
The closing clause of the Mishnah is "ve'im hekif, hitzil". Where a person went ahead and closed off the opening using such food, food that was never rendered susceptible to impurity, the closure counts, and whatever lies inside is spared from tumah, provided that the food itself has remained tahor.
The distinction is precise. The Chachamim issued a decree against doing this in the first place, but they did not go further and declare a vessel sealed this way unprotected. Therefore one should not use food as a shield against tumah; yet if he already did, and the food seal itself has not become tamei, then after the fact it does seal the vessel and shields everything inside it from impurity.