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

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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. 

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I await the discussion with great interest.

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 by PeterH
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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

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  • 5 weeks later...

That could help, but ...  Here's my two cents for what it's worth.  First, the really complex molds with a ton of pieces are great for casting very plastic clays that can stand some bending when demolding.  But this is usually a big no-no with bone china.  Instead as Jeff says  you usually cast /pieces/ that are easy to demold /without/ any flexing of the pieces what so ever.  And you don't leave them in the mold anywhere as long as you mention.  Hour, maybe two, but 12?  No way.  No wonder it's cracking in your molds.

So instead of casting your objects whole, can you remold them to cast in smaller sections and slip stick them together?  This is how bone china is normally worked with because no matter what you add you aren't going to get much plasticity - like ever.  At least not that I'm aware of - and it retains memory.  So if you bend it, it's going to be a PITA to keep it from warping or cracking when you fire it later even if you support it with settles.   Therefor it's really important to be able to remove the pieces from the molds without flexing them.  Sometimes I get a little flex but I do try to keep it as minimal as possible.   I agree with Jeff Longtin that this sounds like a case where you need to adjust your molds to the material you are using.

When I mold and cast my equine sculpture art pieces they are highly complex and super fine detailed.   If I were casting them in earthenware I could make a many piece, complex mold that allows me to cast them whole and entire, or as close to it as the form makes that possible.  But if I'm casting in porcelain or bone china, I need to cut up my original and make many mold pieces for the individual pieces I'm going to cast.  Like each leg, the head, the neck (if bent enough to warrant it's own molding), the tails, the base,  etc and then I will assemble these complex forms after they are cast.  Once you practice this assembly you can get pretty fast at it.  I can demold a complex piece like my Porthos sculpture and assemble the entire horse (with a tightly bent head and neck position and a back cocked leg that crosses somewhat over the other back leg) in less than an hour.     Then it's just a matter of final clean up and redetailing anything I need to once it's assembled.  I use a wet box to store pieces in so that I don't have to rush while working with the damp clay.

Now, as you say you are working on very thin forms so that light can pass through them.  Only you will know if it is at all feasible to cut up the form and mold/cast it in pieces which you can then assemble by slip sticking in greenware before you bisque fire.  But that is the traditional way to do it with bone china and some porcelains for the very reasons you state. 

I'm sure there are others here who can tell you more than I can about the wonderful material of bone china.  I make my own bone china slip from scratch in studio rather an use Valentine's but I hear good things about their slip and I have friends in the UK who use it.  Good luck!

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Here's a couple of examples of the kind of work I do.  Here's two of my sculptures shown drying in the leather hard stage.  I mostly cast the foal entire but add the tail and other details after demolding.  But the various Porthos draft horses, especially if the head and neck are turned sharply enough (it isn't on this example) can't be easily cast whole.  So I cast them in pieces and assemble them and attach them to their bases last (if they use a base).    If I am casting in earthenware I can cast them entire or nearly so, but foe bone china I cast them in pieces and assemble them.

Before & After Bisque Firing Custom Porthos and Wee Bairn SM1.jpg

Edited by Hyn Patty
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