They are generally plastic. I have one, about 12 Inches across and the “map” just peels off, its flimsy plastic but the inner ball is strong and sturdy.
I thought about it originally, until I looked at the cost of the stuff. If you buy it in spray cans, then I would think there will be a bunch of lines where the spray streams come together, sort of like fissures, only smaller and all over. If I made a box around it and used the liquid 2 part stuff…, well, the cost would be way too high.
Anyhow, I took that option off the list. I have contacted two plastics outfits about a 36" plastic sphere made from 2 hemispheres. I imagine, just guessing, about $350 to $450 with holes and $275 to $350 without holes, but that's only a guess from a guy who was in plastics for 26+ years, so I may be way low on the price. The material cost for a ball like that would run about $10 and all the rest is overhead plus 150% markup. At least I will know the price by asking.
I am trying to think of a way to make a 36” styrofoam sphere by starting with a sllightly larger styrofoam cube.
Surely it can be done and with a more simple jig than I can currently envision.
The styrofoam cube would be easy enough to build by gluing blocks of the foam together.
Thinking out loud here, the outside of the enclosure doesn’t need to be a sphere, only the inside.
So starting with a cube, it could be quartered, and within each section a radius removed to form a perfect sphere inside when reassembled. Cube on the outside, sphere on the inside.
As for how to radius the interiors, I do not know, but perhaps someone here will
Also someone mentioned covering the ball with spray foam, but what about inflating the ball with spray foam in order to paper mache it? Possibly would eliminate the size fluctuation. Then I guess you could halve the sphere to extract the ball later.
Wouldn’t you have to do some finishing work on the inside of the sphere no matter how it’s made? Why would spray foam be any more of a problem? When you remove that ball, no matter what material your sphere is made of, it will need some work, right? Paint the inside of it, sand it smooth, paint again maybe a time or two more… Small fissures should be no problem to fill in with paint, right?
The nitrogen molacule is larger then O2 and so does not leak as easily plus it is more stable over themal changes (but it need to be dry).
Does it need to ber perfectly round? I mean light bounces pretty well and so as we are not trying to get images (like a telescope) maybe good enough is good enough?
Also how about a rod 18 from tip to “heating element” if the heating element was made with a 36 radius and then it could be turned on its axis to cut each of the quarters then assembled, wha la a good enough 36 interior sphere!
When i built my sphere,directly inspired by OL’s original sphere thread,I could see the problems of paper mache straight away.Long drying times,lack of rigidity without a LOT of layers…so after the first couple of layers of paper mache,I switched to polyester resin and tissue/mat.Much stronger,cures in an hour,you can even add carbon/kevlar to the tissue to go hi-tech .Not as cheap as paper and glue , but not dear either.
With the large diameter,you could even go sandwich construction for more rigidity and less weight,polystyrene sheet,say 5mm thick between an inner and outer skin of GRP.
Coat the ball in washing up liquid at the start to help break it out when finished.
I think Pilotdog 68’s steel (careful)spheres is a great idea