I don't understand your sun statement, cooking rocks does not involve heat. Cooking rocks refers to placeing them in a tub, in the dark for an extended curing process.
During this time you dunk and swish the rock in a series of buckets with clean water every week, and place them in new clean salt water. This goes on for a couple of months, and it gives the rock a chance to shead all of the deatris it has accumulated.
Accually heating it up just kills the stuff inside, it does not accually remove anything.
HTH,
Whiskey