Umm.... I'm pretty sure the oven itself overall weighs about that much, if not more. So that is quite a bit of thermal mass. The baking steel is just a cooking surface that you place on one of the oven racks to preheat and cook. I see no danger in doing this, but I'm curious to hear why you think it might be unsafe.

But now I'm gonna go get my morning coffee from a bikini barista. bouncingtits
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"Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars." - Martin Luther King, Jr.