Siman 508

We learned in a Mishna (33a): One may not whiten tiles to roast on them. These are what we call "dachukes," and they would roast upon them, and they would whiten them in the fire and through this they would be strengthened, and it is "fixing a vessel." And in the Gemara there, it implies explicitly that this is only with new ones and not with old ones, for the old ones have already been strengthened. And so ruled the Rashba and our teacher Yeruham (see Beit Yosef). However, the Rambam, the Tur, and the Shulchan Aruch wrote this without qualification, from which it is explained that even old ones are forbidden, because at every moment they are strengthened further. And our teachers hold that there are two reasons in the Gemara, see there. And according to the reason that they are strengthened—it is forbidden even with old ones. And it is a wonder regarding our teacher, the Beit Yosef, in section 1 who wrote: One may not whiten stones to roast or bake upon them, because it hardens them. Thus far his words. Why did he change from "tiles" to "stones"? And regarding "stones," it seems that hardening does not apply to them (Magen Avraham, subsection 1). And in the Rambam, some also have the version "stones," and so it is in the Commentary on the Mishnayot of the Rambam. And presumably it was clear to them that there is also hardening in stones, and therefore according to the law it is forbidden. (And it is a wonder that the Rambam in the Commentary on the Mishna wrote only "new ones," and so too in the Rosh. And it requires further study regarding the Tur and the Shulchan Aruch.)

We further learned in the Mishnah (32b): One may not break a shard, and one may not cut paper, to roast salted fish upon it. Rashi explained: When one roasts fish on a grate, which they call a "gradilla," and it is made like the steps of a ladder (Aruch) – they cut reeds or stalks of grain, or break shards or cut paper, and soak them in water. And they arrange the paper on top of the grate, because the metal becomes hot and burns the fish. Thus far his words. And in a Baraita (34a) it is taught: One may not split a reed to roast salted fish upon it, but one may crack a nut in a cloth, and we are not concerned lest it tear. Rashi explained: For they place the strips of the reed under the salted fish on the grate, which makes a vessel, for anything that one makes to use – it is the fixing of a vessel. Thus far his words. And the Rambam explained that they split the reed to make a kind of spit. And "splitting" is a term for breaking, and the explanation of "cracking a nut in a cloth" appears to be that they wrap the nuts in a cloth and break them. And even though it is possible that through the breaking the cloth will tear – we have no concern with it, for he is a destroyer, and this is not like tearing in order to sew (Rashi).