In class tonight we played an impromptu round of Top that Chumrah. For the last several weeks I've been bringing in objects to the Hilchot Shabbat class to illustrate questions and/or solutions we had come up with for several different questions about cooking food on Shabbat.
Tonight I brought in a black knitted scarf to return to one of the class members. Naturally everyone wanted to know what sort of bishul (cooking) b'shabbat question I had that I could illustrate with a knitted scarf.
I explained that in his comments on siman 318 of Orach Chayim the Magen Avraham says that as a penalty for intentionally illegally cooking on Shabbat (Bishul Shabbat b'issur b'mazid) the pot in which the food was cooked is considered treif until it is kashered by libun gamur (high intensity heat, most commonly a blowtorch or the self clean cycle of an oven.)
I pointed out that if this scarf were wet and was draped over a radiator on Shabbat, it would technically be a violation of the laws of cooking on Shabbat. I asked if this meant that if that happened, would it mean I could no longer drape the wet scarf over the radiator even on a weekday, since it would involve cooking something treif.
One classmate piped up and said that the scarf wasn't the pot, the scarf was the food(*). The radiator was the pot, and therefore the radiator would have to be kashered. Another person said even the radiator wasn't really the pot, the real pot was the main unit of the boiler, and thus the hot water tank would have to be kashered. I mentioned that in my house we don't have a separate hot water heater - we have a coil that runs through our oil furnace. It was immediately suggested that I would have to use a blowtorch to kasher the oil furnace. One person said he'd really like to see that, but after due consideration agreed that he would be better off having a camera in the basement and watching remotely.
(*) Under that interpretation the scarf cannot be used by any Jew ever again, although one may sell it or give it to a non-Jew.