The ceramic pieces will be slip cast in black porcelain. In order for the pieces to be turned and made smooth on the inside, I leave them in the mould for 30 minutes, up to and hour and a half.
I also found covering the top of the mould while the clay was inside, the clay didn’t form a skin and left the inside cleaner. Another tip was watering down the slip allowed it to run out smoother.
The form below was something I turned in plaster and would be slip cast in porcelain. The image shows the CNC machine, cutting the top smooth and flat. The intention is these will be fired to 1280 and keep the bowls perfectly round and allowed the bisked ceramic to shrink up the sides.