What is the rationale for "thermal mass"?
Maybe this is an ignorant question. In fact, it is an ignorant question, since I don't know the answer. But I'm wondering.
I've so often read that it helps to add thermal mass to your oven that I've never really questioned why. But now I wonder - what is the reason?
The most common rationale for adding thermal mass (usually by means of a pizza stone) seems to be that it keeps the temperature stable. In other words, when you open the door to remove or rotate the loaf or pizza, the oven temperature will not drop much.
If you are not opening the oven door much, though, or not baking many loaves or pizzas, how important is that?
Let's say I bake a pizza on a metal baking sheet. My oven heats pretty evenly, so I don't need to keep futzing with it during baking, so I just take it out after 8 minutes or so, quickly. If it takes the oven another minute to get back up to 550, before I put the next one in, what's the big deal? If you're going to be baking a lot of pizzas, maybe it matters, but with 1, 2 or 3, so what if you have to let it reheat for a minute?
And if keeping the oven temperature stable is the main issue, I can do that without a $50 pizza stone. I can just put some bricks on the other rack, right? More mass, and much cheaper.
Now maybe another rationale has to do with the stone itself. If you put a pizza onto a metal baking sheet, I imagine the sheet would cool down a bit more than would a stone, since it's thin and metal is a good conductor of heat. But if it's on the lower rack, near the heating element, and is good and flat, no concavities or convexities, and heats evenly, would it really make that much difference?