I plan on making a mineralized substrate for my tank
You can bake natural soil in the oven to mineralize it. But, I haven't done that, and I don't know how long it takes or what temperature to use. There is no reason to bake the clay, since it doesn't contain any organic nitrogen, as far as I know.
I would bake the clay to harden it faster, just as a clay pot or cup, and break it up and grind it into a powder to mix in the soil (cant really grind up clay in putty form). I just don't know how long to bake it though. As far as soil, what type?
55 Gallon Planted
3 GBR, 2 Bolivian Rams, 1 Juvenile Super Red BN Plecos, 9 Oto Catfish, 9 Bloodfin Tetras
Rotala Indica, Cardamine, Crypt Spiralis, Pygmy Chain Swords, Dwarf Sag, Lutea Crypt, Wendtti Bronze and Red, and Floating Frogbit