I went glaze material shopping yesterday in my own back yard so to speak. I live on the Niagara escarpment that is a limestone outcropping that reaches some 400 kilometers from Queenston, New York to Tobermory, Ontario on the Bruce Peninsula. There is also a vein of red clay that runs over a 100 miles from Queenston, NY to a wee village north of Toronto called Terra Cotta. I use the red clay under and over shino glazes for a lusterous persimmon colour. Also a 50/50 mixture of the two materials give a nice shiny glaze that looks like an ash glaze. Doing a gas firing next week so should have something to show y'all.
I get the dolomithic limestone from the gravel company for free and ditches are another good source of the free red clay. The simpler the glaze formula the more complex the materials.