Thanks I will try the corn starch.
I use metal and glass and one old two-piece plaster mold, all as press molds. I use plain ol' Argo Corn Starch, brushed into all the little parts of the design and so far it has worked great. Any corn starch left on the piece cooks off in the bisque firing. I used a cheap cooking spray on a metal mold when I first started using it. It stayed sticky after many uses, so I just started brushing corn starch over it and keep on using it. Maybe the earthenware is still too porous to use for a mold. I'm not knowledgeable about earthenware, but someone is certain to give you a better answer soon!