Does the ball need to be iron? Can we just take a 3d printed sphere, paint it with glue ,roll it around in a bucket of rust and then smash?
Does the second rusty ball really need to be a rusty ball since it is covered in foil?
Please try this at home. You all have rusty iron ball laying around, right?
It wouldn't be weird for someone in the south of France to have a rusty pétanque ball.
Hammer faces would work (def wear goggles!!!). Hit the face of the disposable one with a mixture of hydrogen peroxide, vinegar and salt and wait an hour.
Joining the thread to see where iron balls can be sourced.
Some vendor on AliExpress? Those Lucky Iron Fish?
This might work with old iron dumbbells (or modern iron kettle balls if left unpainted). Your gym might hate you for it though.
I'd guess exact form wouldn't matter. Wrap a hammer in foil and beat up something rusty.