We have reached the final mishnah of the tractate - chapter 5, mishnah 12. The mishnah deals with the halachic requirements that apply to someone who buys a metal utensil used for food from a non-Jew. There are two separate requirements here, and both are derived from the same verses in the book of Bamidbar, which discuss what happened following the war with Midian.
The two requirements for a utensil bought from a non-Jew:
Immersion in a mikvah - any utensil that until now belonged to a non-Jew requires a kind of inauguration when it enters the service of a Jew: a complete immersion in a mikvah, with all of its sides immersed at once. As we will see in the mishnah, this law applies to metal utensils that come into direct contact with human food, and the Sages added glass as well. Glass was not in use as a utensil material in the era of the verses but only in later generations, and since it can be melted down and reshaped like metal, the Sages treated it as metal for many purposes, including the obligation of immersion.
Kashering the utensil - the utensil must be purged of any taste of non-kosher food absorbed in it. We have already established that we hold "taste is like substance," and therefore we are concerned that the absorbed taste will be released back into our food and render it forbidden; for this reason the non-kosher absorptions must be expelled from the utensil.
The guiding principle in kashering is "the way it absorbed is the way it expels" - the manner in which the taste was absorbed is the manner in which it is released. In practical halachah there are several levels here, even more than those listed in the mishnah, but the mishnah focuses on three main levels.
The three levels of kashering:
Cleaning alone: if no heat was involved at all in the use of the utensil, we assume that no taste was absorbed into it, and all it requires is a proper cleaning so that nothing remains on its surface.
Hagalah: a utensil used with water over fire. The water limits its heat to the boiling point - one hundred degrees Celsius (212 Fahrenheit) at sea level - and it does not rise above that. This can be demonstrated: a thin paper cup filled with water placed over a flame - the water will boil and the cup will not burn, because the water absorbs the heat. Therefore such a utensil requires waiting twenty-four hours, a proper cleaning, and then a brief insertion into boiling water - this is hagalah - and whatever was absorbed in it is expelled.
Libun: the word literally means whitening, and it refers to placing the utensil into the fire itself, or as is done today even with a blowtorch, until the metal glows red and throws off sparks from the intensity of the heat, at which point everything absorbed in it is removed. This level is required for a metal utensil used directly in fire without water, such as a barbecue grate placed over an open flame, for which hagalah in water does not suffice and libun with direct fire heat is required.
This is the principle underlying almost the entire mishnah; at the end of it there is one additional point, which we will reach below.
The source of these laws in the verses:
The verses in Bamidbar 31, following the war with Midian, open: "This is the statute of the Torah which Hashem commanded Moshe," and continue: "Only the gold and the silver, the copper, the iron, the tin and the lead." These six metals are the metals that existed in the biblical era (copper is sometimes translated as bronze, since an alloy of copper and tin produces bronze). Some learn from here that only these six metals require immersion of utensils, and according to this approach an aluminum utensil does not require immersion, while others disagree - and that is a discussion in its own right.
"Kol davar asher yavo ba'esh ta'aviru va'esh v'taher" - anything that came directly into fire must pass through fire again in order to become pure. "Pure" here means fit for eating, and it is not a matter of tumah and taharah. This is the source of libun.
"Ach b'mei niddah yischata" - ordinarily "mei niddah" refers to the waters of purification, a mixture of spring water with the ashes of the red heifer used to remove tumah. But the Gemara explains that here the discussion is not about tumah from a corpse, and therefore "mei niddah" means the water in which a niddah purifies herself, that is, a mikvah. This is the source of the obligation to immerse utensils.
"V'chol asher lo yavo ba'esh ta'aviru vamayim" - a utensil that did not come into the fire itself but cooked with water inside it, such as a pot, suffices with passing through water, meaning boiling water - this is hagalah.
The wording of the mishnah:
"Halokeach k'lei tashmish min ovdei kochavim" - one who buys a utensil used for food that comes into direct contact with human food. In truth the law is the same for any non-Jew, and it makes no difference whether he is an idol worshipper or not; even a spoon manufactured in China that was never used with non-kosher food requires immersion.
"Es shedarko l'hatbil - yatbil" - Whatever requires immersion must be immersed. At first glance this sounds circular, but it refers to utensils that were never used with heat, so they contain no absorbed flavors requiring kashering, and immersion alone suffices. Before immersing, one must make sure the surface of the utensil is completely clean, since the water of the mikvah has to touch every part of it at once. Take an ice cream scoop that was never used with anything hot: once it is cleaned, it is immersed in a mikvah and that is enough.
"L'hag'il - yag'il" - Whatever requires hag'alah must undergo hag'alah. This refers to utensils used in cooking, such as pots, and even a ladle that was used inside the pot on the stove while there was water in it. These require hag'alah after being cleaned and immersed.
"L'laben ba'ur - y'laben ba'ur" - Whatever requires libun in fire must be heated in fire. This is a utensil that needs actual libun in fire, and the Mishnah gives examples: the spit - the metal skewer on which meat is roasted over the fire; and the grate - the grill placed directly over the fire, like a barbecue grate. These require libun in fire: they must be heated in fire in order to expel the non-kosher absorptions from them, and hag'alah in boiling water does not suffice.
The law of the knife:
The last part of the Mishnah deals with a different point: a special stringency that applies to a knife, even for cold use, where no flavor is transferred. With an ice cream scoop, even if it scooped non-kosher ice cream, cleaning and immersion suffice, and there is no concern at all about transferred flavor. With a knife, however, because of the great pressure applied while cutting - dochka d'sakina - tiny particles of flavor and residues of cheese or meat fat become embedded in it, even when they are cold, and the knife becomes greasy in a way that is hard to remove.
Therefore rinsing or wiping is not enough. Instead one must perform what halachah calls ne'itzah: thrusting the knife into hard ground ten times in ten different places. The very same principle is at work here: the pressure applied, together with the ground scraping the surface of the knife, removes any flavor or fat embedded in it. And this applies to a smooth knife, not a serrated one; once it has teeth, or grooves and pits in which fat remains, a problem may arise, and it is possible that ne'itzah will not help at all.
The words of the Mishnah: "Shafah - v'hi tehorah" - He scours it, and it is pure. "Shafah" means scraping, and this is the ne'itzah. One who buys from a non-Jew a knife that was used and is not kosher, and wishes to use it for cold foods, should scrape it clean with ten thrusts into the ground, "v'hi tehorah" - and it is fit for cold use.
One who wishes to use it with hot food needs more than this: he must perform hag'alah on it in the usual manner. And if the knife was never used with heat at all and he now wishes to use it with heat, ne'itzah does not help; he must run its surface over a sharpening stone and polish it anew, until no outer layer or peel of absorbed food remains on it. Once it has been kashered in this way, it may be used for other purposes as well.
To summarize: A metal utensil (and likewise glass) purchased from a non-Jew requires two separate procedures: immersion in a mikvah, derived from "ach b'mei niddah yis'chata," and kashering from absorptions according to the rule "k'volo kach polto" - cleaning alone for a utensil never used with heat, hag'alah for a utensil that cooked in water, and libun for a utensil used in the fire itself, such as a spit or a grate. And with a knife, the stringency of ne'itzah was introduced even for cold use, because of the pressure involved in cutting.
And with this, with God's help, we conclude Maseches Avodah Zarah. Our next tractate will be Maseches Avos.