When I went looking for bakestones 2 years ago a local department store was
selling "pizza stones" at half price ($15 or so - not cheap).
I bought two and embarked on a journey through Carol Fields "The Italian
Baker" and those 2 stones have stayed in my oven since I bought them. They
are made in China, fired to a very high temperature so at least partially
vitrified unglazed and made from a fine grained clay about 1/4 inch thick.
The blurb that came with the stones made no mention of leaving them in the
oven but I'm lazy and when I found the stones didn't suffer I just left
them in.
Any unglazed, stone or vitrified ware bakestone should not be affected by
the relatively low speed thermal cycling involved in an oven's clean cycle.
John
John Wright
Yorkshire, England
"That which does not kill us makes us stronger"