Purple-Days said:....... The explanation was evaporative cooling, the escaping water vapor was cooling the air under the crust and it wasn't hot enough to bake properly.
Good analogy.
In the vapor tube bowl, the tiny, tiny areas of the ground material where water is still evaporating will remain at about 212 until any remaining water boils away. Then, the vaporization of the desired compounds from the completely dried material begins. That's the way I figure it, anyway.