I'll start with a few castings done on AA.. Was into skulls at the time these were done.. A different type ceramics, but still by definition, ceramics.. Starting with two coats of Drywall mud.. One brushed on, the second dipped, or poured over the mold, including a hollow pour cup.. After they are dry, a layer or two of fiberblanket saturated with silicon dioxide.. Thin mold melts surface before the wax expands.. Now what happens with the mud prevents any cracking.. The drywall mud is still absorbent and sucks up the melting wax negating expansion of the wax.. A couple mm of compressed blanket stops any leakage on metal.. In the second pic, only one of the waxes has fiber blanket coating. Will follow up with the ashtray mold pics..