Many do use porcelain as
Many do use porcelain as slumping molds. There is a product called “Universal Mold Coat” by Boyce Lundstrom (may he RIP) that many have had success with. Kiln wash won’t stick to the shiny surface on most porcelain. Here is another post on this site with a little info: https://fusedglass.wpengine.com/node/1007. You can do a search online for universal mold coat and you will find more info.
I have the product, but have yet to use it, so I can’t give you any first hand experience.