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Alternative Deflocculants for Slip Casting


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I started this discussion on whether or not paper pulp is a good temper for slip-casting. After further reading and YouTube videos, I have learned the importance of using deflocculants in slip-casting. The most common deflocculants I find in modern slip-casting recipes are sodium carbonate, sodium silicate, and Darvan®. With the possible exception of sodium carbonate, all of these deflocculants seem to require an knowledge of modern chemistry or at least medieval alchemy to synthesize. I have read that slip casting may have developed independently in China in the construction of early Jingdezhen pottery and in the construction of clay pan flutes in the first century of Peru (here) so alternative deflocculants must have been in use in these two independent locations in order to form a thick enough casting shell. Two other potters I've consulted suggested using wood ash or tannic acid as deflocculants sincetjese materials are easy to obtain and still have deflocculating properties. I will have to test these two previously mentioned substances first to confirm these recommendations.

Before I do any tests with wood ash or tannic acid, I want to comfirm if any potters in this forum have ever used alternative deflocculants in slip-casting.

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Interesting topic. Trying to find some old documents that detailed deflocculants used in the 18th and early 19th centuries. I would point you to libraries and galleries at Kent on Stoke in the UK. I do recall reading several documents detailing this issue: but again too long so my memory is fuzzy. I also recall Taxtile Doat (Serves in France and The University Pottery in St. Louis) detailing this topic as well. I could go on a deep dive about the chemistry, but that will not help you with a definitive answer. I do recall calcium phosphate being used as a plasticizer.

Tom

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Wood Ash is out my wheel house: but I do recall reading some on its chemistry. It will be high in sodium, potassium, and or calcium; pending which is most abundant in the soil where it grew. I also recall most ash has phosphates as well.  I also recall reading an article on Meissen poreclain (Germany 1790's), and they used calcium phosphate as a plasticizer, and to create a paste.

Tom

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On 11/12/2022 at 7:24 PM, Ryan M Miller said:

Before I do any tests with wood ash or tannic acid, I want to comfirm if any potters in this forum have ever used alternative deflocculants in slip-casting.

The non-standard deflocculants are likely to give you a poorer quality casting slip, so why would you want try them before trying the industry/craft standard deflocculants?

Genuine question, as I don't appreciate what's driving this decision (like the absence of supply chains or significant per-pot costs). Could you clarify what concerns you have with using  the standard deflocculants?

PS You may find this of interest
Slip Casting - Alfred's Clay Store
https://www.yumpu.com/en/document/read/34584429/slip-casting-alfreds-clay-store
image.png.33aaabfdc0cdd230a3749c6039495167.png

image.png.e0e7a6862a23bc71c6b5d52f5c813cdf.png
 

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I second what @PeterH said. You may have a good reason for exploring alternative deflocculants, but it’s unclear what that is. I haven’t used Darvan, or tried to procure it, but it seems like a winning choice and not hard to get. I know sodium carbonate and sodium silicate are easily available and relatively inexpensive  in many parts of the world, certainly so in the US. You shouldn’t have to synthesize anything. Yes, it is possible to convert sodium bicarbonate (baking soda) into sodium carbonate (soda ash) in a household oven, but why would you when you can buy it? 

And congratulations on learning how important deflocculants are to slip casting! There is lots of good documentation on the process, people have been doing it a long time. As mentioned above, soda ash will eat away plaster molds, especially intricate details. 

 

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On 11/12/2022 at 7:24 PM, Ryan M Miller said:

I have read that slip casting may have developed independently in China in the construction of early Jingdezhen pottery and in the construction of clay pan flutes in the first century of Peru (here) so alternative deflocculants must have been in use in these two independent locations in order to form a thick enough casting shell.

I would love to read more about these processes.

However it's worth mentioning that some beds of clays contain:
natural deflocculants (eg humic acids & lignates)
natural precursors to deflocculants (eg lignin) which are activated in alkaline environments (eg sodium carbonate).

So it's quite possible that you could get a usable slip by careful selection of the clay used. (And a poor slip by modern standards would still be usable and open up a craft/industry).

PS Off topic but well worth reading.
A Secret of Chinese Porcelain Manufacture
https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.196306971
The secret of old Chinese porcelain is the technique by which a material, capable of forming extremely thin-walled articles, was obtained from a slightly plastic kaolin. Intercalation compounds, which form by treating kaolinite with decaying urine, give the key to the preparation of this material.

Unfortunately I cannot find a freely available copy at the moment, so from memory ...

It seems that the Chinese eggshell porcelain industry was born when the amazingly plastic  properties of a specific clay bed were discovered.  It was found under the ruins of what had been a long-lived stable, and the properties were due to the action of stale urine on the kaolin particles. They then found out how to duplicate the effect under controlled circumstances.

Basically kaolin occurs as platelets, which aggregate into stacks. Under suitable conditions the large stacks can be delaminated into smaller stacks or even individual platelets -- giving the body the "fines" necessary for plasticity.
 

So in theory:

image.png.d8412f1ca25fd158cc51a1940130be88.png

... and in practice:
image.png.dc7b12db15ffbc9881ac5b3170efaae0.png

Edited by PeterH
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On 11/13/2022 at 5:52 PM, PeterH said:

The non-standard deflocculants are likely to give you a poorer quality casting slip, so why would you want try them before trying the industry/craft standard deflocculants?

I want to test claims that slip-casting developed prior to the 18th century in Europe by confirming what deflocculants could have been used.

I recently did some further reading on sodium silicate and its possible that there might actually be naturally occurring minerals that contain sodium silicate. Unfortunately, the only article I could find was a paywalled article from science direct. A Google search for naturally occurring sodium silicate yielded the following auto-generated quote from the paywalled article as the top result:

"Sodium silicate minerals can form by evaporation of highly alkaline interstitial brines in near-surface horizons. The most common sodium silicates are magadiite, kenyaite and kanemite."

If any of these three minerals exist in Peru or the Yingdezhen region of China, they might have been used in slip casting after multiple trial and error discoveries.

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Just maybe relevant:
Slipcasting https://npceramics.wordpress.com/2017/09/06/slipcasting/
Historically, there’s seems to be some debate as to when slipcasting was actually ‘invented’. It is fairly widely documented as being a process that became established as a viable manufacturing method around 1745. This is when both deflocculated slips and plaster of Paris moulds were readily available. Prior to this ‘water slip’ had been used for casting, but it had a variety of disadvantages; the main on being the saturation of the moulds. This not only drastically shortened their working life, but it also took ages for the plaster to dry out between each casting. And any casts that were then successfully removed were prone to high rates of shrinkage and distortion, due to their high water content. Overall, this meant the failure rate from start to finish was uneconomically high.
... it then goes on to mention Peruvian panpipes

My understanding is that  the Dawson's paper you referenced is suggesting that the pipes were cast individually* and then joined by adding additional clay by hand. This sounds like a relatively undemanding requirement, so perhaps a relatively low-tech casting technology would be adequate.
* Although the set of pipe moulds for a complete panpipe were held in a common structure.

Chinese slipcasting is probably a different story, but so far I've only found statements that is was done and not found any description of the pots produced (let alone the process).

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A low-tech way of achieving a deflocculated slip is find a bed of clay which deflocculates well when alkalies -- such as soda ash or wood ash -- are added. When this happens it can be because the clay contains tannins or lignite/lignates.

Taking a more chemically informed and proactive route both tannins and alkali can be added directly, as in:
Mechanisms for efficient clay dispersing effect with tannins and sodium hydroxide
https://tinyurl.com/m5w2cyz8

Tannins appear to be fairly easily extracted from natural sources, and historically may already have been in use in the tanning industry.
How to Extract Oak Tannins https://www.ehow.com/how_12001502_extract-oak-tannins.html

A paper recommending the use of tannins for deflocculating drilling mud:
Evaluation of a Naturally-derived Deflocculant (Terminalia Chebula) in Bentonite Dispersions
https://tinyurl.com/5n6crm5r

PS Also found this reference, but have not managed to access the text. The book is available for ~£35
image.png.efd0873f47506b2577c9f1a452396c36.png
...  I expect this relates to current practice, and is not inconsistent with the use of something more like water-slip than deflocculated slip.

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