Kathy, Some things you can do the lazy way but putting reinforcement on a sphere can be a lot of work. Depending on the size of the supporting ball/mold, you might have to cut a gazillion small pieces so they will lay flat. The lazy way would be to buy a Quikcrete product called "Quikwall". It comes in a 50 pound bag and is fiberglass reinforced. Or if you have access to fibers you can add them to your SAND/TOPPING mix. I use drywall tape for reinforcement and mortar mix.
I'm pretty sure that plantman puts a layer of mixture on the ball then adds the drywall tape then another layer of mixture.
There are some great threads on this forum about making spheres if you search. Hope this helps just coming out of migraineland. Billie

How big of a sphere are you making? I attended a demonstration in which the speaker created spheres using mixing bowls. He built up tufa inside 2 identical bowls and covered them in plastic to cure for 24 hours. The next day, he removed the tufa forms from the bowls, painted water around their top edges, and added some fresh tufa mix to the top edge of the bottom tufa form. He then inverted the other onto it, and smoothed the fresh mix around the middle so that there would not be a line. He then rewrapped in plastic for curing.

I'm just getting ready to try out his technique this week. It seems pretty straight forward. He has had a thriving tufa business for years selling finished pieces.

I have made them from 6" ball, basketball, soccer ball to the large Wall Mart ball in the toy section. Believe it or not I still use the exercise ball to stretch muscles! So far the way I've been doing it is successful but after reading about layers I thought maybe they needed reinforcement to last the winter.

Billie I did try the fiberglass stuff and that is why I got frustrated, didn't have the pieces small enough. The fibers sound like a good idea. I'll search for them.

Claysoil, Last year I started the bowl idea and the bowl wasn't completely round so it looked like I had a flat on both ends. I did use a glass globe, only flat on one side and broke it off after it dried and liked the look of it. I would think if they were to big they could be kind of heavy but then you don't have to worry about an animal knocking them over and in my case rolling down the hill!
Kathy

Kathy, Get the Quikwall, it's got the fiberglass already in it. The mesh fibers can be harder to come by and if not added correctly they can clump or make your project look like a hairball.
I was also wondering where he got round plastic bowls. The only one I have that's round on the bottom is a copper one AND I'm not using that for cement mixes! Maybe he is filling in the flat spot after unmolding. Billie