I've decided to make my first sculpted puppet. I've worked with the materials for makeup purposes before, but this will be my first attempt at a puppet one. The issue I've come to in my planning is a core. What the heck do I use? I was considering a stone cast of my arm, but then there would be just a thumb hole instead of a full sized opening for the lower part of the mouth, not to mention the issues with the undercuts in casting my arm in stone. What do you guys use?

I tried casting my arm in plaster once  for this very purpose...however, not enough petroleum jelly, and  hairs  firmly embedded in the  cast, had to painfully cut the hair off arm, but got a beautiful casting of plaster arm, complete with hair!  Did not work for the puppet casting for me, but did sell in a garage  sale for a  reasonably good price.

Yeah don't think your arm would be a good choice. I think as a rule builders do not use a core but simply slush cast.  I assume you are wanting to use expandable latex foam for this though. What about no core. Cast it solid then dig out what you need to create the correct cavity.

Oh, I've got the materials and the knowledge to cast my arm without losing hair. Got my face cast with a full, very long beard and only lost 6 hairs. But I agree it might not work. Yeah, I'm not looking to slush cast mainly because I need strength to support magnets and a moderately heavy mask/hat piece. And I don't intend to use latex, the degradation factor is not one i like, and latex doesn't have the right look for the puppet. Was planning on silicone. Maybe foam silicone. Expensive, but worth it. How big a pain would it be to carve it out and have it be comfortable to perform with?

Oh silicone. Don't know how easy it is going to be to dig that out. How big is this going to be? What are you using as a core for the original or is it solid clay?  If you are using a core for the clay original, I think I would use that. Hard to know what to suggest when we can't know what is in your head for the final puppet.

I would think that silicone could be very heavy for a puppet. If it is for video then you can start and stop but if you are thinking live performance then you might want to figure some way to support the puppet in performance stance.

Well if I were to build a core I would sculpt on top of the core. Only proper way to do it. If I don't end up making a core I'll probably just use a dowel with some wire armature to reduce the weight. As for size, I'm anticipating 5-6 pounds of clay if I go cordless. Pretty big. Silicone would be a slip cast. I'm having someone send me a sample of some foam silicone so I can test out the carving possibilities with it, as well as how much of the translucency is still there. Not going to spend 80 bucks on material if it's not much better than polyfoam. I love learning new materials though, it's like an adventure.