Hey George, Scott here
Definitely no on the glaze. Glazes might look nice, but they have very little resistance to thermal shock.
Thickness really depends on density, specific heat and conductivity. A good stone will have sufficient thermal mass to store enough heat to bake at least a couple pizzas. It should also have sufficient mass and conductivity to deliver a large enough dose of heat to achieve a short bake time at typical home oven temperatures (less than 5 minutes). From a baking perspective, the thicker the stone, the better. Logistically speaking, though, very thick, heavy stones can be very costly to ship and can stress oven shelves, so you can't go too crazy.
I bake with a 45 lb. 16.5" x 20" x 1.25" soapstone slab, and, although I've never pushed it, from a hearth perspective, I'm pretty certain that, with a full pre-heat, I could bake three 4 minute 16" pies with the oven off. With soapstone's conductivity and density, though, this is probably a bit overkill. If I could get 1" (or maybe even 3/4") soapstone, I'm relatively certain it would perform well in a 550 deg. oven.
As you're aware, a lot hinges on density. Here's some density specs (g/cm^3):
Cordierite (dense) 2.6
Cordierite (porous) 1.9
Second to density is conductivity. Here's the pertinent specs for that (W/mK)
Cordierite (dense) 3
Cordierite (porous) 1.5
I would venture to say that to have any chance at creating a great stone, you're going to have to hit a target density somewhere above 2 g/cm^3. Below that and the air content will be so great that the stone just won't be able to conduct heat all that well, regardless of the conductivity of the materials you use to make it. The water in wet clay is the kiss of death to dense ceramics, so I'd use as little water as possible in your formula and try some form of tamping. You won't have the multi-ton presses that they most likely use for dense cordierite, so I'm not sure how successful you'll be, but I would give tamping a shot.If
you can get a target density of above 2 and use enough alumina to bring the stone into the realm of 10 W/mK conductivity, then I'm relatively certain you can get away with 3/4" thickness. Any lower conductivity than that, then I think you'll need an inch. Measuring conductivity could get tricky. Precise measurements require very expensive equipment, but I'm thinking there has to be a way to get a very rough sketch. For instance, if you were able to ascertain that your material was somewhere between 7 and 13 (a pretty wide range, in my opinion) then I think that should be more than sufficient. I googled DIY thermal conductivity measuring and didn't come up with much. Perhaps we can make an offering to the Rube Goldberg gods?
Off the top of my head, I'm picturing a pot of boiling water set on top of the stone, a stopwatch and an IR thermometer taking readings on the bottom of the stone. Or... you could bake pizza. If it bakes in less that 4 minutes with a 550 pre-heat, you're set.
It's probably a bit psychological, but, when launching a pizza, I feel like I have a bigger target with a square stone. That being said, round stones, if they can take the heat, can be used in round grills. Round stones can also be combined with steel lazy susans for easier rotation of the pizza during baking. I think it's a toss up. If a square stone is easier to make, I'd go with that.
I always tell people to buy the largest square stone that their oven can handle, allowing space on the sides for air flow, but NOT in the front and the back. Bigger stones allow for bigger pizzas, and, from a NY perspective, bigger is better. As far as accommodating the most potential buyers? Well, since a good number of home ovens can't handle stones larger than 16", then that's what I'd go with, but if there's any chance you could offer two sizes, then I'd do a 16" and an 18" stone.
Out of everything you've mentioned, the color aspect is the most interesting. Right now, as far as I know, there aren't many colored retail stone options and the ones that are available are uselessly thin. Even soapstone, if sufficient quality, should have a high percentage of talc, making it fairly light gray. Not only would a very dark stone heat up faster, but, for gas oven owners with a separate broiling compartment, a very dark stone could be the best choice for a false ceiling. So far, I've been recommending quarry tiles in this scenario, but something dark would be superior at emitting radiative energy and brown the top of the pie faster. How black can go? This is probably a single color black, right, such as very very dark blue? Any chance for a deep dark 4 color black?
Btw, in the interests of full disclosure, I think what you're doing is incredibly exciting and fun and I think there's a chance you might find a small niche market for it (such as for ceilings), but, as a hearth, I don't think you're likely to create something superior to steel plate, which is relatively inexpensive and widely available. In other words, I'm kind of hoping that your motivation lies more in tinkering/having fun than making real money here. The forum has done a little testing on absorption and it's impact on undercrusts, and the results were fairly conclusive that most pizza stone materials don't absorb water and, even for the ones that do, the difference is fairly negligible. Around the time of the testing, I was very confident about the conclusiveness of the results, but, after some time, a tiny amount of doubt has crept in. Don't get me wrong, I'm still doing back flips over the superior baking properties of completely non absorptive steel plate, but, maybe, just maybe, there is a slightly porous ceramic material out there that crisps up the bottom of the crust a tiny bit better while still being not so porous that it's thermally weak. Maybe. I wouldn't get your hopes up.