That sounds like it's already just about the lightest materials you could use! The only thing lighter I could think of would be if it's hollow. What are you building up the outer surface with?
Here's a solution on my blog I came up with a couple years ago that worked really well - Empty Head etc...
Oh, something I just thought of - it's not all just about how heavy the head is - the strength of the armature is another important factor. Notice I used 3 strands of wire in the neck for the one I linked to above. If your head seems too heavy - especially considering the materials you're using, maybe the armature needs to be stronger.
What is the main material that the outside of the head will be made of? Sometimes working backwards from what you want it to look like might help stir up an answer.
I usually use tin foil to get the head form.
15" tall is a huge puppet. Like Strider said, be sure you've got enough wire in the armature. For something like that, go with 1/8" wire rather than 1/16". Wire stiffness is largely dependent on diameter; simply braiding 3 wires together doesn't have the same effect.
The strategy you use for making a lightweight head depends somewhat on what the surface treatment will be. I've had good luck with gluing neck wires into a piece of square K&S, then gluing that connector into a piece of blue or pink insulation foam. The surface treatment will need to be cold materials like paper clay, latex, or epoxy clay. The foam core would melt and release very toxic gasses if you try to cure a skin of polymer clay in the oven.
Thank you everyone for your awesome suggestions! I ended switching from balsa and cork to balsa base and shaping the head in foam. It's definitely lighter, but we'll see how it handles being covered in magic sculpt. I'll post the results! Also, Strider - when I saw that you are the Darkmatters blog author, my heart skipped a beat! Your blog has helped me so much throughout the years! Especially when I made my thesis, "The Plague Bearers"!
Yeah, that's me.
Guess maybe I should make the connection more clear - it's kind of weird having two different names. I kind of wanted to remain Strider in here for continuity's sake.
The way I skinned that head was to roll out a very thin sheet of epoxy putty to coat it with - I used a pasta machine and if I remember right I placed the expoxy putty between two pieces of waxed paper so it didn't stick to the rollers. You could use a rolling pin or just something flat - maybe press it out with a big wooden spoon or something, or under a piece of wood, but sandwiched between sheets of waxed paper. I went about as thin as I could get it - 1/8th of an inch or less, and after it hardened I roughed up the surface with a big file to give it something for the next layer to grip.
Then I sculpted onto that with more epoxy putty. It was essential to get that solid hard shell in place before trying to sculpt the head though - I would never have been able to sculpt directly onto the wires.
** edit **
Just watched The Plaguebearers. Very nice! Interesting mix of stopmotion and 2d, with the eyes (or just eyelids?) drawn on. I wasn't expecting the big reveal at all!
How about paper mache? While I have not tried to make a build-up head with paper mache, I have had success casting paper mache from a mold. I sculpted my head in clay and made a plaster mold. Using vaseline as a release agent, I took strips of paper mache and pressed them into each mold half, allowing each layer to dry. It will take about three layers. When removed, simply glue each half together using carpenters glue, with the armature wire inside. The result is extremely strong and light weight. See an example here:
Paper mache is a good option! Another way to go is to use a urethane resin filled with lightweight microballoon filler. That makes a very lightweight head.
Okay, I've found this WONDERFUL two part epoxy "putty" that is amazingly low density but very strong when cured. It's called "Free Form Air Epoxy Putty" by Smooth-On...
It has the initial consistency when mixing of that very low density Crayola brand "modeling paste" for kids, or if you're familiar, "light weight" wood Spackle. It takes a good 24 hours to really set, but oh man, when set it's strong. It also will give a good finish surface if you press it into a silicon mold, very smooth and shiny if desired. It drills, sands and takes paint well. It's lighter than balsa wood, I think.
You'd still have to put normal epoxy putty around wires, but you can use this to bulk up your shapes to ridiculous extremes with almost no worries that you'll over-burden your wire armature.
An excellent find Jim, thanks for sharing!!
It's expensive - $25 for a small kit - but I imagine there are times when nothing else will do. Though now you've got me thinking about blending microballoon filler into regular epoxy putty...
Thing is... it's a pretty large volume of material for the money and would last a long time without spoiling. Compare its price to something like the heavier density Apoxy Sculpt material, it's much cheaper.
It's rather sticky and clumpy when mixing (use gloves), but the results are fantastic.
You can hand mold it into approximate shape, then used sanding and filing tools to finish, or you can make a mold and press it into the mold for a more polished final product. It's the first new material in some time that's causing me to re-evaluate methods of bulking up large areas at low density that need more strength than say urethane solid foam would provide...