Hi Kim
You can find the 10 inch light fixtures in second hand stores for a dollar or less so you might try covering one of those. Just tape up the hole and slap on the mud.
It will be hollow so lighter as well.

I use these globes right up to the 18 inch size. After that they get reall expensive. i cast in two pieces and reuse the globes over and over.

This comes from personal experience with outdated spray foam. It has an expiration date on it for a reason. I found some even a few months past it in the basement and they were no good.Just dribbled out and didn't do anything except create a mess to clean up. Give it a try but it may not even be any good for you. :(

My experience with spray foam is that one can produced a lot less foam than I expected. (maybe 2 cups)
Your styrofoam idea will work. I have done a couple of things that way and the concrete or tufa stick easily to the styrofoam. I haven't had anything made long enough to know how sturdy it all is over time or if it is easy to knock chunks off the sides. It seems really sturdy....
Laura

If I recall correctly, someone applied concrete or tufa over a sprayed-foam armature, but when the finished piece was set out in the sun at a later date, the cement/tufa cracked. The poster drew the conclusion that the heat from the sun apparently activated the foam somehow and created some expansion.

My faulty memory can't bring up all the details.

Sue

I feel like the foam can wouldn't produce enough pressure to expand the balloon. It's very delicate stuff when it is wet. I think Peak used the foam for armatures, but not sure. It is an intreging idea, though. Kim, how about blowing up a ballon, covering it with dry wall or duct tape and covering that with tufa?

Jo

I tried the blow up the balloon and cover it like for papermaiche`masking tape over the entire balloon. In my case the balloon actually lost some of it's support and sort of deflated, but I was able to 'paint on bonding agent mixed with a little sand cement for a slurry' first to create a rough surface for the mix to further stick.
I made a couple pieces of fruit with the balloon. The apple in the picis covering a balloon. It is very lightweight in comparison to the pear in the pic which is over a hollowed concrete egg that got covered with the pear shaping.
Dena

Here is a link that might be useful: Apple and pear behind the fence

tufaenough: I can't picture what those globes are like--plastic with a hole for the lightbulb and a lip to hold it to the ceiling fixture? Did you cut the lip off? Making a sphere in two sections, how did you connect the two pieces?
Rockhewer: my can is dated on the bottom 3-16-03 but it doesn't say "use by" anywhere on the can. I almost bought two more cans at a yard sale today but they weren't dated at all. I may experiment with what I have since I have no other use for it.
I got Sherri Hunter's Creating with Concrete from the library (woo hoo!!), and it has a section on using styrofoam/polystyrene and it sounds like it's more for inner support that is removed later. Hardware cloth or chicken wire is wrapped around the support then the tufa is spread on. I'll be studying this book a lot more before I try anything else.
I agree, a balloon would likely deflate a bit before the tufa dried.
My eyes are getting trained to look at things for molds--the plastic L'Eggs containers for fruit? Didn't luck into any jello molds at yard sales today. I've got to make a fish for DH! :-)

Hi Kim
Not all have a neck, so just have a hole.
I've seen the smaller ones in almost every second hand store both glass and plastic.
I make a strong mix using fibers and coat half the ball twice then join them using a very strong mortar mix.
This gives me a completely round ball I can finish any way I wish.

you can actually cover less than half the globe and then join it to a half globe of a smaller size to make very nice pots that are not globes at all. with a hole at the top you then attach a partial globe in reverse giving you a pretty classic pot.

Basically you can cover any thing up to half the globe and by using the different sized pieces you can make some very fancy pots that look like the were thrown on a potters wheel. I like to join 2 pieces that only cover a third of the globe creating very squat pots.
Then a nice colored slurry coat say in dark blue with bits of yellow and red and you have a beautiful thing.
In my view anyway.

Here is a link that might be useful: globes galore

Kim,
If you are planning to leave the armature inside, consider using wadded newspaper tapped into the shape you want then cover the shape with a plastic bag, tape and a slurry mix initially. When you are applying our final coat of mix you can check the thickness overall by having a pencil with tape on it to poke in at various places. The armature/form here is not for strength.
Dena

I used a round gourd to cover with tufa for my pumpkin. However, my first attempt had a major flaw. After I applied the tufa, the gourd absorbed all the water and expanded quite a bit, cracking the uncured tufa. My second try was to drill a hole in the bottom of the gourd, fully soak it for several hours, them apply the tufa. The gourd, this time, shrunk away from the tufa as it cured, and I was successful in making my pumpkin. Learn as you go. I made a nice sphere using a basketball. I left a small hole near the inflation area, deflated it, removed it, (after it had set up real hard!) and patched the hole. This was a good way for me to do it. The firm basketball works great, but don't work in the sun, it will expand, and cause problems, and don't forget, deflate and leave in or remove it later. .

The can of expanding foam had definitely expired...the foam barely came out and the trigger on the nozzle was very hard to press, I got only 1/2 cup out of it. Oh well, at least now I can dispose of it.
I like the basketball idea, but I'd sure be nervous deflating and removing it! I'll be looking for something I can use and leave inside. I'm not to the stage of being able to join two halves anyway. Thanks for all the input!

This was a good topic about spheres and how to build them. I've had this idea about those vinyl play balls that are cheap but don't have an inflator/deflator built in.

I like the idea of reusing something as many times as possible.So My thought was about putting something like an auto tire repair stem on the vinyl play ball. You can get them for inner tubes that have a flat for adhering to the tube.

My main concern would be how to make the two dissimilar materials stick together (vinyl vs rubber) and be strong enough to take even the small air pressure needed to reinflate it. Anyone have a magic glue/sealant that may work for this?

Plumbers Goop? Works for leaky water pipes, is flexable and tough. I think it would be perfect. I love GOOP!
Jo

For those of you who didn't read the thread,scan down from this one to "I made a 16 ball with Quickwall".Using scrim or detail mesh (here in Canada,eh?),you apply your material over a deflatable ball,then pull it out from the opening.I've tried this material with my own mix(sticky,sticky,sticky!)in a bowl and got VERY thin walls that are strong!Did the drop test on concrete floor from about 4 feet-no cracks!Like others,next up is an exercise ball!
Cheers from here

I tried filling a beach ball with the foam and the foam wouldn't expand, I'm thinking that it was because of the foam not being exposed to the air?
bonnie

Rick, That's what I meant when wanting to put something on those play balls. They come in such a variety of sizes, it'd be nice to have an assortment to work with. They have such nice thick skins to stand up to the 'crete or 'tufa.

For your fish, you could always make a plaster cast of a real, fishmarket fish and use that for your mold.

Dena just looked at your whole photo album. you have been workin! I especially like that huge faux rock. How did you start it/ did you use some kind of mold or start small and kept adding to it> Would love to tackle one of those. Also is it very heavy when complete/
Thanks
Claire

tufaenough

Can you offer advice how to join two hypertufa hollow half spheres?
Can I make a slurry of portland cement and sand and slap on top of hypertufa?
Thks
Pat