The slurry contained insufficient Davan deflocculant. During casting it gelled excessively in the mold. Even shaking the piece while pouring out the slip was not enough to loosen it up and get a good drain. If slurry rheology is not right the quality of ware produced is affected like this.
A method of forming ceramics where a deflocculated (low water content) slurry is poured into absorbent plaster molds, forming a layer against mold walls, then poured out.