Having got a number of successful casts out of the moulds, I began fettling them when they were leather hard to achieve an even finish getting rid of any unnecessary slip such as along the edges and where the pour hole has been. Once they were bone dry, I gave them a light sanding to get rid of any surface marks and imperfections before putting them into the kiln for a bisque firing.
Below you can see the successful bisque fired forms.
I now plan to sand the the forms down again with wet and dry so their surfaces are nice and smooth as there are quite a few imperfections on the surfaces which I would like to smooth out. The porcelain still needs a further firing to 1280 degrees. Firing it to 1000 degrees first should help minimise warping at the higher temperature. I am also considering firing it to 1260 degrees which should still give the sheen that porcelain gets when it is high-fired, but the slightly lower temperature should help reduce warping further.