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amy.m

Bone China casting issue

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Hi all

Was hoping for a bit of guidance on this issue - I'm slip casting a large piece using bone china slip and seem to keep getting rippling on the inside of the cast.

I suspect it's a combination of factors but just wanted a second opinion, does any body think it's most likely caused by an issue with the slip mix itself?

Due to the size, it's being filled very slowly taking care not to move the flow of slip too much and also drains slowly.

 

Thanks!

Amy

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Followed the manufacturers recommendation (for every 10kg of granulate add 4.5 Litres of Water and 23gms of 75s Sodium Silicate)

Mixed a large batch of 120KG powdered clay + 52 ltrs water & 266g of Sodium Silicate (held back 2 ltrs water & 10g of SS)

Specific gravity at last test was 1.78, but requires testing again.

Seemed to cast ok testing in smaller moulds.

 

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13 hours ago, Min said:

They did't call for adding soda ash as well as sodium silicate? Just using the latter can make a "stringy" slip, I'm wondering if that's causing what you're experiencing.

Hamer&Hamer mirror Min's concerns on wreathing and deflocculation.  https://tinyurl.com/yyx5z39l

However they add that "swirling the remaining slip around the mould before draining may help to overcome the problem".

PS for what it's worth
Valentine's recommend an identical slip recipe for use with their Fine Bone China Granulate.
https://www.valentineclays.co.uk/product/fine-bone-china-granulate

When making casting slip from their plastic clays they recommend using both sodium silicate and soda ash for all except the bone chinas.
Irritatingly they also give suggested control parameters for all but the bone china.
https://www.valentineclays.co.uk/sites/default/files/downloads/Technical Analysis Chart - Industrial Products.pdf

 

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From Frank Hamer: "Wreathing or livering, which is a rippled or uneven surface on the inside of the cast, and roping, which is when this uneveness is in raised lines: The slip has insufficient thixotropy . The sodium silicate should be reduced and if necessary the sodium carbonate increased to maintain a working fluidity. Swilling the remaining slip around the mould before draining may help to overcome the trouble." Quote from this

From Walker Ceramics, description of wreathing: "small uneven ridges on the slip side of the article" and cause for wreathing is "slip thixotropy to low", fix "decrease Dispex addition

I emailed Valentine's and asked why there is no soda ash in their porcelain and china clay casting slip recipes and got this reply: "Generally speaking there is no Soda Ash used when making slips that don’t contain Ball Clays, Ball clays react better using a combination of Soda Ash and Sodium Silicate and these are both described as deflocculants"  

 edit: couldn't open your link to the Frank Hamer tiny url link @PeterH , found it elsewhere and quoted it in case there are issues with my link also

Edited by Min

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Thanks for all the replies on this -

Min - yes,  only sodium silicate was mentioned for the recipe - Interesting comment from Valentines, perhaps a reduction in the sodium silicate may be all that's required.

From the quotes posted by Min & PeterH, the symptoms sound certainly like wreathing - sounds like further experimentation is needed with the amount of deflocculent added to the mix. Unfortunatly the technique of swilling the slip around wont be possible as it's a big mould! but certainly worth trying if I get any similar problems with smaller moulds.

Neil - Thanks for the link for the slip mixing method, very useful guide to have to hand.

 

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