When I tested quarry tiles for baking pizza, I used 6 tiles in a 2x3 grid. But I have 12 tiles, so I decided to see if there was any benefit to stacking the tiles on top of each other.
The theory is that a thicker stone holds heat better, which is why many bakers seek out the thickest baking stones they can find. I was pretty happy with the way the pizza baked on my single layer of quarry tiles, but wondered if a double layer would be better.
I preheated the oven for an hour as I had before for the other tests. At the 45-minute mark, the tiles were at 525 degrees and at one hour, it was at 549 degrees, which is slightly lower than they were when I tested the single layer at the same interval.
After exactly 8 minutes of baking, the pizza was done, with a nicely browned underside. The top was just slightly less baked than the previous pizza. Not enough say it wasn't done enough, but the difference was enough to be noticeable. I wasn't sure what to attribute that to, and I also wondered why the stone wasn't any hotter than 549 degrees after an hour in the oven.
It took me just a little while to figure out the answer to both of those questions.
The difference wasn't the thickness of the double layer—it was the oven going through its heat cycles. You see, ovens don't actually hold a steady temperature. They heat up and and cool down, over and over, cycling by as much as 50 degrees from the high point to the low point. Since I was timing my baking precisely, I assumed that my oven would be at the same point in its cycle at the same time for each pizza, but I didn't account for the fact that heating a different mass of tiles would cause the oven to heat at a different rate.
Not only does the air temperature fluctuate inside the oven as it cycles on an off, but you're also dealing with convection currents. When the heating element is on, the air is moving more than when the heating element is off. A pizza in an oven that's at 500° with the heating element at full blast can actually brown faster than a pizza in an oven that's at 550° with the heating element off, since the air is much stiller in the latter case. These convection currents are also what helps things bake more quickly in a convection oven than in a traditional one.
When you're cooking something for a long time, the oven's temperature fluctuations don't make as much of a difference, since it balances out over the long cooking time. But when you're cooking something for just 8 minutes, those temperature fluctuations and convection currents make more of a difference.
Given that it's just the top of the pizza that cooked differently and not the base, it's pretty safe to assume that it's due mostly, if not completely to oven cycles. The quarry tiles—or baking stones—in the oven somewhat mitigate the effects of the oven cycles on the bottom of your pizza, since the stones hold heat which is then transferred directly to the dough.
In the end, there was very little difference between the pizzas baked on the single or double layers of quarry tiles, and the little difference could easily be attributed to the oven rather than the tiles. Since the bottom crusts were nearly identical, I'd call this a tie.
Since the results were so close, I don't see any benefit in stacking the tiles—the effort of arranging 12 tiles instead of six just isn't worth it.
To mitigate my annoyance with stacking tiles, I bought a 3/4 sheet pan to hold them. That worked to allow me to take them out all at once, but they still shifted around enough so that if I was placing the tile-filled pan in the oven, I needed to adjust the tiles once the pan was in place.
The benefit to using the pan was that it caught drips and spills that fell between the tiles. But the real downside was the ghastly scraping noise it made whenever I put the pan in or took it out. If I ever go back to using quarry tiles, I'll opt to use six tiles and arrange them directly on the rack.
But how will these tiles fare compared to stones products that are actually made with pizza in mind? Don't worry, we're getting there.