Knives That Cannot Be Used for Shechitah

For the more general knife ideas, see The Shechitah Knife.

Invalid Knives

Detached From Its Source

1. Anything sharp attached to the ground or a body may not be used for shechitah until it is detached.
2. If it was detached and then reattached, one still should not use it regardless of whether he plans to leave it there (which would render the meat prohibited due to the Gemara's uncertainty) or not (which would be invalid on a rabbinic level). If one's intention is unformulated, we can determine based on context.
3. An animal's jaw with sharp teeth may not be used, as the gaps between teeth are the same as nicks in a knife, but a singly tooth attached to a jaw is fine. Even a single nail on a severed foot would be ok. Some argue that if it's still attached to a whole limb, such as a whole jaw or whole animal foot, that would be considered "attached" to its source of growth.

Pegimot

1. A knife with a pegima (nick) may not be used. Bediavad, if one used it and claims he didn't use that part of the blade, the meat is kosher, but if he didn't realize it was there until afterwards, he has no claim.
2. Ideally even if one covers the pegimah, he might accidentally use the part with the pegimah.
3. If the blade has two edges, one must make sure both are free of pegimot.
4. On Yom Tov or in pressing circumstances, one can cover the pegima and shecht carefully, avoiding the pegimah, as long as there are two neck widths of length left of the knife from the pegimah in either direction and one shechts in a sawing motion. Some say the length is measured from the end of the cloth and not the pegimah. All this applies for two pegimot with enough blade length in between, as well.

Glowing Hot Knife

1. A knife that is heated till it glows (incandescent) may not be used, as the heat from the sides of the blade burns the simanim before the blade finishes severing them, resulting in a piercing instead of a shechitah. The meat would be prohibited on a Torah level. Some are lenient bediavad if the shochet was aware and maneuvered the knife in a way that avoids the sides touching the simanim, but the halacha follows the strict view. Some say the minimum temperature of the metal is Yad Soledet Bo.

Non-Kosher Knives

See Koshering a Kitchen for more details.

1. Due to the temperature of the animal neck and potentially also to the pressure of the knife, one may not use a non-Jew's knife before kashering it or sanding down a layer from both sides of the knife. Hagala is sufficient unless there are holes in the blade, which would then require libun.
2. Although shochtim bring their own knives to the slaughterhouse, there is still an issue sterilizing their knives in the hot water "baths" the USDA requires slaughterhouse equipment be dipped into.

Ben Yomo

1. If it was definitely used for non-kosher in the past twenty-four hours (ben yomo) but not kashered, a layer must be sliced off (kelipah) from the neck, due to the potential for non-kosher substances on the blade to rub off and be absorbed in the neck and because generally these things are not absorbed deeper than a kelipah.
2. Whereas if the blade was properly cleaned of surface substances with neitzah, one would think this halacha does not apply, some argue that even if so, there's still a concern for absorption of flavor facilitated by the heat of the animal neck and the pressure of the blade. Moreover, there may still be hard to see non-kosher substances still present on the blade.
3. Without kelipah, cooking the meat would require measuring if the kelipah is less than one sixtieth of the rest of the meat and then kelipah.
4. In circumstances where kelipah is required, one may not shecht with intention to afterwards do kelipah; if the kelipah would just be a stringency, then it is ok.
5. Washing the knife is insufficient to remove the adhering non-kosher substances.

Eino Ben Yomo

1. If the animal was shechted with non-kosher knife that was not used for non-kosher food in the past twenty-four hours (eino ben yomo); although the knife should have ideally been kashered, the meat is kosher. The same is true if one is unsure if it was used in the past twenty-four hours. If there is any visible non-kosher substance on the blade, as knives are often not perfectly clean, then the rules of an actual ben yomo must be followed. Even if one thinks it was clean, there's room to be strict, unless one made sure to do a thorough job or it is a pressing situation.
2. Regardless, one is not allowed to use an eino ben yomo lechatechilah unless he kashers it first. Bediavad with neitzah ten times (and no less) the meat is permissible, but, ultimately, in all circumstance, a scrupulous person should strive to do kelipah regardless.

Non-Jewish Workers Cutting

1. If the non-Jews cut the veins in the neck with their own knives right after the shechitah, some say one can be lenient since it's hard to be careful about this, but it still needs to be thoroughly washed.

Terefot

1. Due to residue leftover from shechitah on the knife, a knife that was used to shecht what turned out to be a terefah must be washed clean before it's used again. Hagala and neitzah are not necessary, since one time is not enough for the fatty substances to adhere to the blade so well, but a thorough washing and drying are necessary. After a number of terefot in a row, then neitzah would be in order, without which, further shechitot would require a kelipah to be removed. If they were alternating kosher and taref, then the neitzah would be recommended lechtechilah, but bediavad washing or drying the knife after each one would be enough to say that the meat only needs to be washed well.

Bloody Knives

1. In general, one may shecht consecutive animals without cleaning the knife or being concerned for the blood to transfer flavor to each new animal, since there is so much blood exiting the neck, so we are not concerned that any blood will enter. In the absence of bleeding from the neck, then kelipah is recommended. Regardless, it is praiseworthy to wash any raw animal neck one gets from a fresh shechitah.
2. A knife used for many consecutive shechitot has much blood absorbed into it and must be kashered via hagala or, at least, irui before cutting hot food with it; otherwise, kelipah or possibly even netilat makom would be required.
3. Dipping the knife into a pot of hot soup prior to hagala would require shishim against the kelipah of the knife and whatever substances are stuck to the blade. The rules of Ben Yomo and Eino Ben Yomo also inform circumstances such as these.
4. Even cutting something cold warrants washing the knife well first.
5. See the end of Yoreh Deah Siman 10 for more details.

Knives Used for Idolatry

1. One may not benefit from tools used to worship an idol, therefore, the knife used to slaughter animals for idolatrous sacrifices is prohibited from benefit. That said, killing a healthy animal is arguably more detrimental than it is beneficial, as the animal is no longer alive to grow or work. A dead animal is only good for its meat and entails a necessary loss. Similar arguments could be made for birds. As such, shechting a healthy animal or bird with a knife used for idolatrous activities is permissible, because causing a loss is not considered benefit. However, a sick animal may not be shechted with that knife, as that would be considered an improvement. Some argue the same is true for a very violent or high maintenance animal that comes at a loss to its owner or if the price of meat is higher than the price of the animal it comes from.
2. If the knife was used to cut wood which was then used for idolatry, then it's permissible. It's only considered a knife of Meshamshei Avodah Zarah when the idol is finished with the knife or sacrifices are slaughtered with it.
3. A sick animal slaughtered with a knife of Meshamshei Avodah Zarah is entirely prohibited from benefit until one throws away enough meat the value of which is equal to the cost of shechting with it. If there aren't any more knives avaialble, then the cost of shechting must go up.

Sources

Category	Topics
General Laws of Kashrut	Bishul Akum - Checking for Bugs - Gelatin - Kosher Food without Kosher Supervision - Kosher Food Packaging for Deliveries - Kosher Food Left with a Non-Jew - Koshering a Kitchen - Kashering for Pesach - Kosher in the Workplace - Medications - Pat Palter - Selling Non-Kosher Foods - Serving Guests - Sharp Foods - Shechitah (Kosher Slaughter) - Tevilat Keilim - Tzaar Baalei Chayim - Yashan
Meat and Milk	Dairy Bread - Eating Dairy and Meat at the Same Table - Kosher Cheese - Kosher Milk (Chalav Yisrael) - Milk and Meat in the Kitchen - Non-Dairy Milk - Waiting between Meat and Milk
Principles of Kashrut	Items That Cannot Be Nullified - Transferring Taste - Nullification - Zeh Vzeh Gorem - Trusting Others for Kashrut
Shechitah	Shechitah_(Kosher_Slaughter) - Who_Can_Be_a_Shochet - The_Shechitah_Knife - Modern_Day_Industrial_Shechitah - Glatt Kosher Meat - Kashering Meat