Refractory Casting Techniques: Exercise ball mold

[Emo]

Hi, I’m building a wood fired oven and plan to mould an 85cm dia exercise ball. Rather than perlite or vermiculite cement I’ve been advised to consider “refractory casting techniques” which will have better longevity and thermal conductivity/heat retention. This is something I would like to do, but does anyone have any advice on what mix/materials can be used to mould over a ball?

[UtahBeehiver]

Do a search in Other Oven Types or cast ovens. Or look at David S threads, he is our cast expert. Gym balls are not your best option.