Help with adjusting the plasticity of my Bone China and Parian slips

[Ancam]

I need some technical help. 
I use Valentine’s casting slips. My moulds are very complicated (38 pieces) and their shapes somewhat restricts the shrinking of the cast. My casts are thin for lighting translucency. My moulds are at two scales, one takes 1.5l the larger 12l.  The larger mould in particular has highlighted some problems: Glacier porcelain slip works well and I can leave the cast in the mould for 12 hours before demoulding. You can tell that there is some plasticity in the cast that allows a bit of flexibility in the body on demoulding without it cracking. Bone China and Parian however are much more tricky to succeed with and often result in multiple cracks that I repair with mixed success. After  just 4 hours the cast will crack all on its own before I’ve even attempted demoulding. And demoulding causes so many cracks as the cast it brittle. What can I add to the Bone China and Parian slips to make them behave more like my Glacier Porcelain slip? Tony on a Digital fire post mentions adding 0.5% bentonite. I need to test but has anyone got any suggestions or input before I start messing with these pre made slips. 
many thanks. 
Anna 

[PeterH]

I await the discussion with great interest.

PS
It's almost certainly irrelevant, but I'll mention an odd-ball method I once read about for reducing the adhesion between the casting and the mould.

They were making a medical device that needed to be X-ray transparent, so the thin body was basically alumina and totally non-plastic. They found that mould release was  more practicable if they first cast a very thin layer of paper fiber into the mould.

It was obviously a high-value item, so mould life may not have been an issue. I don't think that there was much detail in the mould either.

Edited May 12 by PeterH

[Jeff Longtin]

Good Morning Anna. Welcome to the Forum.

Not really able to give you the answers you're looking for. Mixing your own would be the best way to adjust your recipe.

I'm curious about an object that requires a 38 part mold. (Pictures?) As is, you're looking to change the clay to accommodate the mold, have you thought about the opposite, change the mold to accommodate the clay? (That's where I would start. Most production molds are in the 2 - 4 parts range. )

As you don't know the composition of the clays have you tried simply mixing them together? My guess is that any adjustment, to the Bone China clay, will result in reduced translucency, is that acceptable to you?

Most Bone China objects I've seen were not made with complex molds but instead were made with several simple molds and many pieces were then assembled to make a complex form. Is that an option in your situation?

Good luck