Shulchan Aruch: Chapter 508 - Acts that Prepare for Roasting that are Forbidden on a Festival

SECTION 508 Acts that Prepare for Roasting that are Forbidden on a Festival (1-3)

1 On a festival, one may not heat cooking tiles until they become white-hot to bake or roast [food] upon them.1 [This restriction applies] even if it would be impossible for [the person] to bake or roast [the food] in another manner. [The rationale is that] heating [the tiles] until they become white-hot seals them,2 (i.e., it hardens and strengthens them).3 Even if [the cooking tiles] have been used before, nevertheless, they are strengthened further whenever they are heated until they become white-hot. Thus, one is improving a utensil on a festival. (See sec. 495[:4, which discusses the reason for this restriction.])

2 One who roasts fish on a grill may not cut a piece of paper4 to soak in water and place under the fish on the grill so that the fish will not burn. Similarly, one should not break off a shard of earthenware5 to place under the fish. In the same vein, one may not break open a reed to place its sap under the fish.6 Also, one may not break open a reed to fashion something resembling a spit to roast with, for any [activity] that involves making or fixing an article that one uses [for a beneficial purpose] is comparable to fashioning a utensil on the festival.⁵

3 It is permitted to crack open a nut while it is [wrapped] in a piece of cloth.7 There is no concern that [the cloth] might tear,8 for even if [the cloth] did tear, [the person would be] damaging [the cloth. Hence, he] would be exempt according to Scriptural Law unless he is tearing [the cloth] with the intent of resewing [it].9