If you don't mind buying numerous bowling balls then this is a practical approach. A solid hypertufa sphere around the ball will not go anywhere, it will be at least as strong as a hollow ball. A bowling ball seems unnecessarily heavy, you might want to consider using something like a carved polystyrene ball or a cheap play ball. The ball will only be as strong as the outer layer of hypertufa so not much point having a really strong inner ball, unless maybe you want to use a really thin layer for some reason. Even then the outer layer will be subject to chipping if it is too weak.

It can be a pain to mould hypertufa around a sphere. Read some of the threads here about how people have done this. It might be a good idea to work with hypertufa on an easier project first, just to get the hang of how it handles.

i put hypertufa over a cement ball and so far so good. It's in the garden right now and buried in snow so I'll have to wait and see how it survived the winter.

The rate of expansion and shrinkage would be the issue.
I have however seem plenty of mosaic on bowling balls in a Z5 and them seem to stand up fairly well.
Ice getting into hairline cracks would probably be bad. Covering for the winter might be the way to go.

Thanks for your advice. I have done a few tufa troughs, but never a ball. I picked up several bowling balls at a garage sale, and they always seem to be available for $1. I guess I thought that a solid ball would be stronger than a hollow sphere. I wasn't planning to move them once in place, so it's ok if they are heavy. Glad to hear I won't need to deal with mesh.

I guess I'll just experiment and see what happens. Need to wait for the weather to warm up a bit though.

Thanks

Has anyone done this yet? Pics please if you have...thnx