Sheḥita must be performed with an instrument that cuts cleanly. In principle, one may slaughter even with a detached object such as flint, glass, or wood, provided that it is sharp, smooth, and free of any nick (Ḥullin 15b; SA YD 6:1–2). In practice, however, sheḥita is performed with a metal knife, because metal is strong and can be sharpened to a fine, reliable edge.
Any nick in the knife invalidates the sheḥita, because at the point of the nick, the knife tears the simanim rather than cutting them. A nick is disqualifying only if it “catches”: if a hair drawn along the blade would get caught in the damaged spot, it is a nick that catches (Ḥullin 17b; SA YD 18:2). For this reason, the knife must be examined carefully before sheḥita, on all three sides of its edge. The examination is performed by gently running one’s fingernail back and forth along the full length of the right side of the edge, then along the left side, and finally along the cutting edge itself. If a nick is found, the knife is sharpened again until the edge is completely smooth. Originally, the three sides of the knife were also checked against the flesh of the finger. In recent generations, however, as metalworking improved and knives became sharper, this became difficult to do without risking a cut to the finger. Shoḥatim and inspectors therefore customarily rely on the fingernail examination alone (Ḥullin 17b; SA YD 18:9; Beit David 5).
The knife must be examined again after sheḥita. If a nick is then found, the animal is forbidden, because the blade may have been damaged by the animal’s hide before it reached the simanim, so that the actual sheḥita was performed with a defective knife. To reduce this risk, mud and clay are removed from the animal’s neck before slaughter, and thick wool is shaved away (SA YD 18:12).
When many animals are slaughtered, the knife must be checked between one animal and the next; otherwise, if a nick is discovered only at the end, every animal slaughtered since the last valid inspection becomes forbidden. Large-scale poultry slaughter follows a different practice. There, after the initial inspection, many birds are slaughtered in succession, and the knife is examined afterward. If a nick is found, all the birds slaughtered since the previous inspection are rendered forbidden. This system is used because the loss caused by stopping to inspect the knife before every bird would be greater than the occasional loss incurred when a damaged knife is discovered and an entire group of birds must be rejected (SA and Rema YD 18:11; see Beit David, Yesodei Ha-Bayit 18:6:3). In practice, improved metal production has made this concern far less common: today’s knives are made from high-quality metal, and nicks serious enough to invalidate them are found only rarely.
