When first making a mould for my bowls, i made a two part mould that felt a seem along the surface that needed fetling, but in some cases it was hard to remove and hide. To counteract this, i remade the moulds with a plug. The plug when removed would allow the object free without a vaccum but only leave a seam along he flat base of the foot ring.
The pictures then show the outcomes of my slip casting. This left the inside needing to be turned with normal slip casting porcelain. I found that watering the porcelain as well as covering the porcelain when in the mould, it allowed the porcelain to pour out cleanly and prevent turning. After being turned i sadly put my finger through the base of one plate as it was so thin.