The question has been raised of what the difference is and when to use them when making a sculpture.
First things first.. you may not need either. If your sculpt is not big.. or thick, you may not need either of these things.
Some reasons to use them might be: You are trying to capture a look, you need a visual guide, your sculpt is large, you don’t want to waste so much clay (my usual motivation) and more
Armature: Think of this as a skeleton. In a human it is your bones, and the thing that makes you stand up and have structure.
Core/Bulk: This is the stuff that gives you size. Think of this as fat, intestines, organs, and all the stuff that keeps our flesh off our bones.
So when I say a BIG sculpture, what I mean is anything that is probably going to be more than an inch across in total. Or half an inch in any given spot. (We were talking about polymer clay, but this applies for air dried clay too) Disadvantages to going thicker than that, are uneven cooking/drying, and the possibility of your sculpture cracking.
What can you use to bulk out your sculpture? ALMOST ANYTHING. 😀 This is the awesome thing about sculpting. The only restrictions you will run into is how your clay is going to cure.. (ie you wont want to use a styrofoam bulk, for polymer sculpts) The other thing to consider is how well your clay is going to stick to whatever bulk you put into your sculpt. So something smooth sided is less desirable, or you will need to coat it with something so you get the clay to stick to it. The BEST stuff is material that is thin but you can add to it incrementally, like wool, cloth, newspaper, string, tinfoil, duct tape .. etc etc.
Armatures are generally made of wire and epoxy glue. So depending on how large your sculpture is, will determine the thickness of the wire. For something really big you might even consider using iron pipe inside of it. For smaller, a thin bendable wire. For mid sized projects you will want to use regular copper wire, with perhaps another wire wrapped round it for extra sturdiness. Whatever you use, remember that stuff has to stick to it. You will have to add something for the clay, or bulk to grip onto. HERE is a very good tutorial on armature making.
My favorite bulking out material right now is paper mache. I started experimenting with it and it makes an excellent easy to sculpt (when wet) and easy to carve out when dry.My paper mache is nothing more than newspapers and brown paper bags soaked and then tossed into the blender with water. Here is my recipe. Then for shaping it I use a plain old wood rasp to get my roughed in shape when it is dry. Here is a link to your paper mache core and how to use them
If you have more questions .. you know what to do