Hi George, welcome to the forum. The work you're doing sounds fascinating.
As I'm sure you've found in your research, cordierite can vary pretty widely from manufacturer to manufacturer. From what I've learned, cordierite can be extruded or pressed. I'm not exactly certain which of the popular cordierite brands (old stone, pampered chef, williams sonoma, etc.) are extruded or pressed, but I do know, from studying ceramics in college, that clay molecules are flat and that when you press them, they sandwich together and become stronger- a lot stronger. This is one reason why slipware (clay molecules that dry facing every possible direction) is so much weaker than hand built or thrown pottery.
I have a working theory that extruded cordierite, due to moisture content of the extruded clay, is most likely less dense than pressed. If this is the case, then one could look at the weight/volume/density of commercial stones and determine which are extruded and which are pressed, but I haven't gotten that far with my testing.
The strength obtained by pressing is flexural (bend strength). Flexural strength (and thermal conductivity) are directly related to thermal shock resistance. I've seen videos of pressed high alumina cordierite stones going from red hot (more than 1000 deg. f.) into ice water without any issue. I highly doubt any extruded material could achieve this.
Lack of porosity/air pockets is critical to a good pizza stone. Air is an insulator/slows the transfer of heat/conductivity. When heat doesn't transfer quickly, you will have hot, expanded areas next to cooler unexpanded areas, resulting in structural failure. Air pockets also allow a stone to absorb water, not so much during baking, but more during cleaning and/or moisture from humid air. Water plus heat equals massively expanding steam. Lastly, and most obviously, air pockets decrease strength. Air is the kiss of death for pizza stones *cough* fibrament *cough*
Conductivity is also critical for good stones, especially for the home baker. It's essential, as discussed, for thermal shock resistance, but materials with greater thermal conductivity transfer energy at a faster rate, allowing lower temp home ovens to bake pizzas faster, resulting in pizzeria quality results. From what I've read, cordierite is silica and alumina, and, because alumina is more conductive, it's ideal to maximize the alumina content.
The other side of the heat transfer coin is thermal mass. Dense, highly conductive materials such as steel can make great pizza at thicknesses as low as 1/2". Cordierite, in order to bake pizzeria quality pizza in an average 550 deg. home oven should really be at least 1". Thickness gets tricky because, as you go thicker, the stone will weigh considerably more, which, in turn, can potentially challenge the structural integrity of the oven shelf.
