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Balancing Of White Porcelain Slip


Helynn

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Hi Everyone - This is my first post on the forum, so I hope I'm doing it right!

 

I have made up a white porcelain using a recipe from a published book, so I had better not mention any names just now!  The recipe is made up of 2,200 grams China Clay; 1,250 grams Flint; 1,250 grams Feldspar potash; 300 grams Ball Clay; 13 grams Sodium Silicate; 13 grams Soda Ash and 2.2 litres water.  It has mixed together fine, and I put it through a 100 mesh seive, just in case.

 

When I pour the slip it sets up like greased lightening, about 5mm thickness in only 5 minutes.  It is slightly reluctant to release from the mould, but only a bit; but my main problem is that it is very dry and "chalky" and doesn't have any plasticity to it which is making it difficult to turn down on the wheel, which is what I want to do.  I have a feeling people might tell me that the sodium silicate balance is out, but I haven't a clue what to do to get it right, if indeed it is the sodium silicate and not something else.

 

Should I ditch this recipe and try a different one, if so has anyone any suggestions.  As I said earlier, I want to be able to turn some of my work down on a wheel, and fire to around 1230 degrees ish.

 

Hoping someone out there might come up with some ideas.

 

Helynn

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It’s my understanding that casting slips and bodies for wheel work are two different animals. The finer particled clays like ball clays and some kaolins are partly what give a body plasticity. Without this plasticity the clay is short and not not suitable for wheel or handbuilding work. Casting slips don’t need this plasticity and the finer particles in a throwing body, if used for casting, would lengthen the amount of time the slip takes to cast before the walls have built up sufficient thickness therefore plasticity is not wanted. You didn't mention which kaolin you are using, some are less plastic than others.

 

Hopefully someone who knows more about this than I do will chime in here.

 

Welcome to the forums. Yes, you did it just perfect  :)

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What you've got there is a basic cone 10 porcelain formula of 50% clay, 25% feldspar and 25% flint. They've substituted ball clay for a bit of the grolleg, but not enough to make it cast well. Grolleg porcelain bodies do not cast well. Porcelain has very low dry strength, and casting makes it even worse. I would decrease the china clay and increase the ball clay. The downside of doing that is that it will effect the whiteness if you're firing in reduction.

 

As for the viscosity of the slip and how fast it sets up, somewhere out there you can find the recommended specific gravity of casting slip, and adjust yours as necessary. Deflocculated slips do not fit recipes 100% because slight differences in the moisture level of the raw materials and how carefully you measure everything will have a noticeable effect. It all has to be adjusted once it's mixed.

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Thanks to people who have replied to me.  In England we call Kaolin china clay, and I’m using just what our local potter’s suppliers call china clay – they don’t go into any further details.

 

Maybe I’ll forget about the wheel bit, and just use the items as thrown, but they still set up very dry and chalky.

 

The trouble if that I don’t really know what I am doing when it comes to the chemical balances of slip.  Neil suggested decreasing the china clay and increasing the ball clay.  What percentages would you adjust to Neil?

 

I’m getting round to thinking that making up my own slip isn’t such a good idea after all and maybe I should resort to made up slips; with the amount I’m wasting at the moment it isn’t very economical, although if I “hang on in there†I might win in the end.  Maybe I’ll persevere a bit longer.

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Don't resort to the pre-made slips! You've come this far. You will figure it out, and be happier in the long run for knowing how to do it.

 

I agree with the above suggestions that slip casting and "turning" (which I think is what we call "trimming" in the US) should not be combined. Two completely different things. Instead, design your molds so that a finished form comes out of them.

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As Neil pointed out, a classic porcelain recipe with a twist: someone was wood firing this recipe. The only reason to add soda ash is for it to flash in the firing. Soda ash however is strongly hydrophobic ( rapid dehydration) and highly alkaline. ( caustic levels) soda ash is 12.95-13.25 PH, anything above 11 is caustic. Which also means you will suffer from rapid degradation of your molds. Your hands would turn a nice rosy pink if you handled it too long with bare skin....it will burn skin!!!!!

 

Secondly, absent the soda ash, the flux molarity is set for high fire: cone 10-12. The other reason I know this is a speciality mix.

 

A classic cone six recipe would be: 35% china clay, 15% ball clay, 20% flint! and 30% potash. The higher flux molarity is more in line with cone six. Then add your sodium silicate. You are in the U.K.: is your sodium silicate 75 or 150 degrees twaddle?

 

Might want to give your molds a quick wipe down, with straight vinegar to kill the high alkaline deposits the soda ash left behind, then followed by a quick rinse in water.

 

Nerd

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Min:

Did not know you were so well versed in clay chemistry.

The opening statement from Laguna states, that soda ash is beneficial in dissolving lignite in clay.

China clay has how much Ppm of carbon and what percentage of sulfates? Soda Ash is for specific use and purposes, much like talc. Not common at all, rather the opposite. Mr. Rhodes and I have disagreed before.

So again: " soda ash is not necessary from a purely clay chemistry stand point." The flux molarity required for a 1230Cish firing does not require soda ash, in fact that amount of soda ash alone would supply close to enough KNaO by itself.

 

Nerd

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Min:

Did not know you were so well versed in clay chemistry.

Hmmm, I think it's been clear for a long time that Min knows her chemistry.

 

I find soda ash to be useful for cone 6 for reasons other than flux. I have one glaze that contains a small amount. I didn't see why it was necessary from a flux standpoint, so I tried making a batch without it. The fired glaze looked exactly the same. The noticeable difference was how it behaved off a brush. This is a glaze I apply by brushing, and without the soda ash it dried way too fast for brushing. The soda ash makes the glaze "oily" enough to brush. I can see how this might be useful for slip making too, in terms of regulating the consistency.

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Soda ash (along with sodium silicate) is often used as a deflocculant in casting and other slips, and inhibiting the flocculation of any clay suspension (subject to being kept to a small amount - too much will make a mess).
 
As regarding "dissolving" lignite, it works more like a detergent works on oily substances and its primary effect will be to allow the lignite to be thoroughly wetted by the water in the slip. 
 
Soda ash may act as an combustion catalyst for carbonaceous impurities in the clay body, especially if the "carbon" species are in direct contact with the soda ash solution (water phase).
 
lt

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GEP:

I agree, Min is very sharp with glaze chemistry: but slip and clay is a different beast. Slip shares some attributes as glaze, and some with clay. Clay is entirely different set of values. Bottom line, the basic recipe is a cone ten porcelain slip: she only needs sodium silicate to get the job done. You can use soda ash, but it is not necessary in this case.

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