Siman 9

It is stated in the Gemara [8a]: If one heated a knife in fire—and the measure of heating is stated in the Yerushalmi [end of Avodah Zarah] as being sufficient for sparks to fly from it—and slaughtered with it, his shechitah is kosher. We do not say that the heat burns the simanim before the completion of the shechitah, rendering it a tereifah, for the sharpness of the knife precedes its heat. And even though there are the sides of the knife, and when it enters into the incision the sides have the potential to burn the simanim, nevertheless even for this we are not concerned, for the place of the shechitah widens; that is, the incision expands from the sides of the knife on this side and that side, and the contact with the simanim is only the sharpness of the knife, which takes hold prior to the heat [Gemara]. The Talmud does not distinguish between whether the knife is very sharp or not, or whether the heating is intense or not, for behold there is also a light heating as explained in Orach Chaim siman 451. Similarly, it does not distinguish between an animal and a bird, even though in a bird the simanim are soft and would easily be singed by the heat; nevertheless, relative to this, the sharpness also takes hold of them easily. One should not ask that according to what was written, that sparks fly from it, would the sparks not burn the simanim? For it can be said that sparks have no substance [as written at the end of the third chapter of Shabbat].

And behold, the Rif, of blessed memory, did not cite this statement, and in its simple sense one can say that since it is a matter that is not common at all, he was not concerned with citing it. There are several laws that he omitted regarding shechitah for this reason, such as fixing a knife to a wheel and the like. And it is obvious that this is not common at all, for one must necessarily check the knife while it is white-hot to see if it has a notch, and who is able to check it when it is white-hot? Moreover, in most cases, when it is heated in fire, it is unlikely that it would not be damaged by some small notch. Furthermore, perhaps he holds that there is a distinction between one type of heating and another, and between one type of sharpening and another, and between an animal and a bird, and therefore he was not concerned with citing them. However, there are some of our teachers who said that he rejected this from the halakhah [the Rosh and the Ran]:

And their reasoning is from that which is stated later [17b], that the knife requires examination for a notch even from its sides. There are those among the Amoraim who disagree regarding this and brought a proof from this statement, that we are not concerned that the sides of the knife will tear the simanim because the place of the shechitah widens, as has been written. And the same applies regarding the concern of a notch; we are also not concerned for this reason, see there. But we hold that it requires examination on the sides, as will be explained in siman 18, and if so, this law is rejected from the halakhah.

However, the Rambam, of blessed memory, in chapter 1, law 22, wrote: If one heated a knife white-hot and slaughtered with it, his slaughtering is kosher; see there. And there are some among the Rishonim who cited his version as "his slaughtering is invalid," but before us the version is "kosher." And such is the opinion of several of our teachers, that this requirement for an examination of the sides of the knife is only ab initio, and that which we validate regarding heating white-hot is ex post facto [Ra'avad]. Furthermore, even though the place of slaughtering is spacious regarding the matter of heating white-hot so that it does not burn it, nevertheless it is not spacious regarding a projection on the knife so that it does not puncture the siman [the Rosh in the name of the Ri]. Furthermore, regarding heating white-hot where he knows that the knife is white-hot, he is careful in this regard not to touch its sides, whereas regarding the examination of the knife, if he does not examine its sides, he will not know to be careful [ibid.]. And the opinion of the Tur tends toward validating, and the opinion of our teacher, the Beit Yosef, tends toward prohibition. And so all the great Acharonim agreed to prohibit even ex post facto, for we do not say the place of slaughtering is spacious; and even though its sharpness precedes its white-heat, there is concern for the sides.

And know that there are some of the great authorities who wrote that the heating of a knife is to the extent that the hand recoils from the sides of the knife [Tevuat Shor], and according to this, we who are stringent in this law, if there was heating such as this and one slaughtered with it, his slaughtering is forbidden. In my humble opinion, it appears that this is an excessive stringency, and from where do we have the authority to interpret this heating as being unlike the heating mentioned in the entire Shas? Furthermore, from this very sugya it is proven that this heating also includes absolute heating, as is explained there [for it compares this to the law of an inflammation and a burn where one heated a spit and struck with it, see there; and regarding a burn, it also considers there a coal and hot ashes, and perforce Rabbi Zera permits even with absolute heating; therefore, it is sufficient that we say that regarding absolute heating we do not rule like him, and delve into this].

Regarding the marks that the craftsmen make on the sides of the knife, there is no concern for them since they are far from the edge. However, when it is protruding, one should be stringent [Tevuot Shor]. And ab initio, it is proper that there should be no mark at all on the knife, so that the knife shall be smooth on all its sides.