Sand, salte, and burned earth
Sand, when overheated, does not hold as well. Slate is baked and will mold clean, yet it is true that often it becomes porous, just like burned earth also does, as well as any kind of greasy sand.
at left top margin of folio 087v
I find that one must not hammer at all on sand already very finely pulverised, because it would damage it, and then make the sand would not be able to release the cast cleanly. Still, it needs to be pressed hard, and moisten it sufficiently.