Sorry, but Iím not buying your argument Mr. November. (I know Iím likely to get schooled, but maybe this will be a good teaching moment for someone besides me.)
Emissivity is a measure of the percentage of heat radiated by a surface. Aluminum foil with an emissivity value of 0.04 would reflect 96% of the radiant energy. Thatís nice, but the interface between the cooking surface and the pizza skin has little to do with radiation, but has everything to do with conduction. As an example, I think I would scream almost as loud if I put my thumb on a burner covered with aluminum foil, as I would with the burner alone. The foil conducts most of the heat but provides an excellent porosity barrier Ė which was the goal of the test.
Sure we could argue about the insulation properties of a thin air gap between the stone and the foil, or between the foil and the pizza skin, but that misses the point. I think the point is that the porosity barrier created by aluminum foil influenced the cooking of the bottom of the pie hence, porosity matters.
Maybe where there is confusion is that emissivity is also a measure of how much heat is absorbed in the material. In that case, cordierite stores an order of magnitude more heat than aluminum foil. Got that. But if for sake of argument, we assume a small area of the stone holds 10 BTUs of heat, those 10 BTUs are available to cook the pie with, or without, the foil there as foil doesnít store any heat but conducts nearby 10 BTUS very efficiently.