[footfootfoot]

How funny, I just made a plaster mold to pour some lead ducks for fairing boat lines. (relax, it's only a model)

I learned lost wax bronze casting and mold making in school. You can pour lead or pewter into a plaster mold, but you sort of did it backwards, Make your master positive out of something like wood or wax then make a plaster mold of your master positive. If there are no undercuts the positive should pop out once the plaster sets up. You can use your carved plaster as a positive if you seal it really well and use silicone or something as a release agent. When you've made tour plaster mold let it dry thoroughly. an hour or so in a low oven should do it. You can now pour the pewter into the plaster mold.

Another way to get a really detailed mold for one-offs is to get some dental alginate to make your mold. You can pour several wax positives with it before it dries up or falls apart. Then you can pour plaster over the wax positives, encasing them fully but allowing for a sprue and a vent, You melt the wax out of them (outside, low campfire coals be sure to add sand to the plaster to give it strength) Then while still somewhat hot you pour the metal.