Hi Kim
How big is your glass globe and do you have one or two of them?

It's about 8 inch diameter, I only have one.

Hi Kim.
Here is one way I use for balls from 8 to 18 inches. You can even use your basketball.
I use a chunk of styrofoam with a hole in it that fits tight around the 'waist' of the globe forming a base at dead center. Image the planet Saturn. I wet the ball then cover it with thin plastic (handiwrap stuff, 18" wide rolls are at the dollar stores)then slide the styrofoam ring down pulling the plastic tight on the ball.
You can cover the half ball with fiberglass tape and a thin coating of something like Quickwall ( I use Greatwall sometimes, it has fibers) or your own Portland recipe.
Allow it to cure for a day then pull the globe out the bottom. The plastic and the WET glass will make this easy.
Store your half ball in a plastic bag and make the other.
In a day or two you can join the two halves with the same recipe. If you can do this fast enough it can be almost unnoticeable that's why using two glass balls and making both halves at once is even better. Both cure at the same rate.
If you want a completely hidden seam you can now topcoat the entire ball in another coating of mud.

If you want to do the ball in one thick coat and keep it round be careful that slump doesn't give your ball a fat waist.:)I check the thickness with a small nail to make sure the ball is evenly coated.
If this isn't clear I can post you a picture of the 8 inch rig I use.

Does the styrofoam act as a guide so both sides will be even, and also to hold the plastic wrap? With my luck, the styrofoam will break when I slide it on, so I'm wondering if a thick piece of elastic will work.

What I can't picture is how to join the two halves. They'll be moving around while I'm layering on my portland mix.
Yes, I'd love to see your picture, maybe it will make things more clear.

Hi Kim
Use blue styrofoam 1.5 inches thick.
Cut a square about 12 inches then cut the hole in the center just slightly smaller than you globe.
Cut a slot from the circle to the edge of the styrofoam.
This will allow it to expand as you slid it on.
Now tape over the slot squeezing the styro tight on the ball. The very best tape it TUCK tape, the red stuff. Sticks like crazy and concrete does not stick to it.

This will give you a tight fit and pull your plastic tightly into place on the globe.

This picture was taken a while ago when I posted a tutorial on how I did my perlite pink ball.
It's made quite a few 8 in balls and half balls since. Even a happy face cookie jar complete with lid.:)

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Okay, I'm getting a clearer picture, thanks. Now how are the two halves joined together?

Joining the two parts.
I have no picture.
You make another styrofoam ring, or recut the one you just used, this time to fit tight on the outside of your crete half ball.
You put the ring on the ball so 3/4 inch of the ring is showing about the edge of you ball. Apply your admix enhanced mud to the edge of your damp ball. It can be thin 1/8 to 1/4. Now take the other half and press it down into the half with the mud. A little twist helps.
Now you have Saturn.:)
The styrofoam ring holds the wet mud in place so it can't squeeze out and the two halves together.
This can be made almost invisible if you join the balls with the identical recipe and you do it within a day or two.
If not, wet the ball and apply a sticky admix enhanced slurry coat to the outside of the complete ball.
I do this to keep the costs of my pigments and fancy aggregates to a minimum.
I can make a strong inner layer covered with softer lightweight outer shell.

Okay, I think that makes sense, sounds scary! Tricky, at least. I can see where two balls would be best, I'll hunt for another one the same size. Thanks so much!!

Hi Kim
It's easy, just take your time.
BTW if you want to make a straight sided trough of any height styrofoam works well for that.
I'll give you a for instance.
Take 2 thickness of blue styrofoam 2 feet by 3 feet and glue them together for at least three inches thickness. more is better. You could use 3 layers of the 1.5 inch thick stuff. I use thicker stuff for a 5 inch thickness but 3 inches will do.

Now cut out your shape of you trough. It could be oval or even like those giant clam shells in shape. Or like my first one a kidney shape.

Now you have a rectangle with a big hole cut in it.
Lay it on a sheet of plastic on your bench.
Mix up your tufa and cover the inside for the bottom of your trough and work it up the sides right to the top of your 3 inches, keeping your walls a uniform thickness. This is so low that it won't slump much.
Wait a few hours then carefully wiggle your styrofoam form UP two inches. It will stay there held by the tufa.
Build up you wall, wait, move the styrofoam up, add to your wall. Keep doing this until your trough is as high as you would like.
You can brush the lower layers as they are revealed under the mold. Make sure you keep the work damp.
This is a cheap way to make a straight sided trough or bowl of any shape or size. If you don't like the layered look, when done cover the outside with a coat of the mud of your choice.

You can make a straight sided container of any shape doing this.
I did a BIG straight sided vase this way, 12"x9" oval 26 inches high using 12 different colors of tufa.
It was certainly different looking.:)

I used admix and my mold was 5 inches thick so it only took two days to build it that high.