I remember reading elsewhere on the board that gray/blue clay (bentonite) isn't so great - think there was
video from Ernie and Erica related to this as well. The issue is cracking as it responds more dramatically to heating/cooling cycles. I too have a lot of gray clay derived from shale here and there's some in my mix - I've seen quite a bit of cracking around the barrel and over the manifold as well as around my burn tunnel/feed tube where the perlite insulation was a bit thin and I'm sure it's the gray clay doing it's thing. Overall, if you have
enough of other types of clay, you
should be ok as long as it's mixed out well. Straight gray/blue clay would probably not work out too great.
Here on my property, it's a 1-3" layer of the stuff pretty much right at the surface, then mixed out between gray and reddish clays as you go down another 2 to 4 feet. I've yet to find the bottom of my very heavy clay (something like 90-95% by jar test, and yes, property is WET) but it seems most of the bentonite-esque clays are closer to the surface.
As others said, test bricks will help - make sure to do some thermal shock tests on your bricks as well though. Heat them up to a few hundred degrees and let them cool off a few times so you know how it will react to normal cycling.
