In regards to kashering celim that something has to become so hot that it is the definition of “kash she-nisraf alav”. What’s the geder of this l’maseh? Does it have to burn mamash completely until it becomes like ashes? Or enough to become brown a little and “start” to burn?
Answer:
In practice, this level of heat (which is the required level of heat for libbun kal) can be checked by placing the back of a match (not the side with sulful) on the surface, and checking if it blackens. If it does, this is the level of heat of kash she-nisraf alav.
