@_thegeoff How are you going to remove the print from within afterwards? Also am curious why would you want the mould to be out of concrete -- it's neither flexible (like silicone) or easy to form (like casting sand).
@robryk The print has been coated in a release agent - petroleum jelly, which I'm assure by my colleagues works. And it needs to be concrete as it's going to be a large blob of molten glass inflated by compressed air, so needs both heat resistance and a lot of structural integrity.
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@_thegeoff Ah, and it's obviously convex (because otherwise how would you extract the final product).
@robryk It's a nasty set of compound curves, took me a few hours simply to work out where the seam line should run...and it's very not linear...