Doesn't have to be at all. I was curious about this plaster as it is said to have a slow and steady set. We have a couple of moulds to be making where these properties would be desirable; a 12' diameter hemispherical dome and 6'square cross-vault (lots of casts required).
I am not trying to reinvent the wheel, just thought if life can be made easier, then why not. Sure, we will be coring-out both moulds to within a dag paper of the profile/rule, but a bit of spare time never goes amiss.