Decided as long as it was raining today I'd take an hour drive down to a town in Massachusetts that has a pottery supply business, and buy some silica sand. It's getting hard to find, and expensive for the real stuff because of health concerns.
They had the pure stuff in #60 mesh. I had read a recommendation for #70 from Stewart Marshall, but 60 was all they had. So I bought 200 lbs of it and trucked it back. Brought plastic bins with me to keep it dry.
I'm going to use Ironman's mix of 85% sand, 7.5% Bentonite, 3.5% coal dust (I might have found a source for some coal I can pulverize -- but can I substitute charcoal dust?), and 4% water.
Ironman's video:
(Ironman, if you read this, is this greensand composition by weight or volume. I'm assuming weight. Thanks!)