Halakhah sobre Job 21:35

Knives should be scoured in the first vessel and then they are permitted. The first vessel was where water was boiled on the fire, even if it is not now on the fire but is still boiling. Before the scouring, they must be cleaned well with a whetstone or a millstone to remove all rust before they are kashered. Therefore, if there are crevices such that they can't be adequately cleaning, it is not sufficient to scour them (the meaning of [the word "hagalah" - scouring] is spitting, because the objects spit out the prohibition in them, and it comes from the language of "Their cow calves and does not cast her calf" [(Job 21:10)]) alone, and they need to be made white-hot where there are crevices. Rem"a: The sheath of the knives cannot be fixed by scouring, and it is forbidden to put the knife in it on Passover (Mahari"l).