Terra sigillata deflocculation from local clay

[wens]

Hi all,

I'm making some terra sig from my backyard drainage creek red clay in NC.  I need to deflocculate it, but didn't start with dry clay, so am not sure how to calculate the amount. I've got about a gallon of watery stuff that's almost there; a nice orange color. It's been strained to 100 mesh, then left to settle and removed the bottom sludge. It shines up when applied with enough coats, but it's too watery, and it could be better. I haven't measured the s.g. Do I actually need to dry this stuff out completely to weigh it, or can I approximate the deflocculant somehow? I could assume a half pound of clay and use something like 2g of combined sodium silicate/soda ash, but that might be too much? Should I try less and see if it separates?

If I start over and dry out the initial clay stuff , rocks and all, would that be easier? I don't want to lose the finest particles coating the small rocks, but not sure how to figure out how much is clay and not-clay.

Robin

[PeterH]

I think the usual form of Brongniart's formula will give you answer. 

This often uses a value of  2.6 for  the density of "typical glaze solids". Which seems close to the density of clay particles.
Particle Density
https://link.springer.com/referenceworkentry/10.1007/978-1-4020-3995-9_406
Densities of clay minerals range from 2 to 3, but many are near 2.65 g cm

PS An old but good ref for t-sig is
Super-Refined Terra Sigillata
https://digitalfire.com/article/super-refined+terra+sigillata
... but note that it doesn't mention the more modern deflocculants.
... the use of hydrometers has been replaced by weighing an empty and full calibrated syringe in many glaze applications
... it mentions several ways of concentrating thin t-sig, eg  As another option, use a crock pot set on medium heat with the lid off. In either case, the rate of drying will depend on the atmospheric humidity and the mount of heat applied. It will take some experimentation to learn the ideal conditions. You may find a cheap crock pot at your local thrift shop.

PPS Two threads on the use of  Brongniart's formula for glazes "in the bucket". Including references to the formula and a calculator.

Edited April 13 by PeterH

[wens]

Thanks for all that, Peter! I've made terra sig numerous times from dry clay (Redart, etc.) and have a hydrometer. It's so watery that I couldn't get a good reading.

Reading the last quoted thread, I got an identical bucket and filled it with water to the level my terra sig is, and weighed both. The difference is 102 grams, which should be the weight of the solids, since I may have not measured the volume accurately.  But it's a good enough start for the deflocculant.

I may have to start over with a larger amount of clay, to get a accurate reading, but I'll see how it goes.

[Magnolia Mud Research]

I've been there.  and learned to ignore the "official methods" and simply crushed the dry clay as small as possible using a hammer and kitchen screen utensils to get rid of the "big" rocks.  
Mix the clay with water to make a slurry that is somewhere between the buttermilk and the milk level (or a little less).  Let the mixture set for a while to let the heavy particles to sink, and then pour the liquid into a separate container which will contain most of the sig.  Stir up the original container again and let it set a while, and repeat pouring off the water to get remaining sig. Yes, this is not the official method, but it has worked just fine for the clay I take from my ponds and roads to them.  I have also used that same method for making sig from scrap commerical clay bodies. 

If you are able to get wet clay from the backyard just start with that; add water and mix well; if you have sand, it will sink along with the heavy particles.  I have a pond that collects fine clay every time there is a decent rain. After the rain there will be a thin layer of fine clay above the big paricles.  My first round on terra-sigillata I followed the "official" method; it worked ok.  After noting what was happening in the ponds and just getting a bucket of pond mud and water I begin to make my own method which has worked just as good as the textbook. 

The best "official" recipe I have found (beyond my own) is Vince Pitelka's version:
http://www.vincepitelka.com/wp-content/uploads/2022/10/Super-Refined-Terra-Sig.pdf   

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[wens]

I think you may be right. Where I live, it's all red clay, rocks, and tree roots, but I did try to dig some that was less organic. I've followed Vince's method for all my other sigs, but there are so many methods out there that are more "shake it up and let it settle and pour off." Maybe I should give up some control.

I don't know what color it will fire to, but here's a bone dry mini-bowl with Redart sig on the left (didn't stir it up very well), and ditch sig on the right. Hard to see the shine in the photo, but it does buff up nicely. I had some of it evaporating in a shallow pan with the dehumidifier on.

 

[PeterH]

11 hours ago, wens said:

Reading the last quoted thread, I got an identical bucket and filled it with water to the level my terra sig is, and weighed both. The difference is 102 grams, which should be the weight of the solids,

Just to point out that your approximation does not allow for the volume occupied by the clay particles. That's what Brongniart's formula addresses.

[wens]

Agreed, but with it so thinned out I couldn't get a true specific gravity to plug into the formula - it was too close to 1. Next batch, I'll try not to thin it out too much. Appreciate all the info!

[Kelly in AK]

I make terra sig regularly with my local clay and follow something similar to @Magnolia Mud Research’s post. I don’t go to great lengths to extract any remaining “finest particles” from the dregs. I played with that a bit but found it was counterproductive.  Letting it slake a few hours then mixing it really well gets the all good stuff in suspension. 

I’m sure this is derived from Vince Pitelka or maybe Pete Pinnell, but it’s what I go with:

1 quart water to 1 pound of clay. 2.5 grams deflocculant per pound of clay.

My deflocculant is half soda ash/half sodium silicate. 

[Callie Beller Diesel]

On 4/13/2024 at 4:16 PM, wens said:

Maybe I should give up some control.

Probably. 

Part of the fun of working with found materials is the variability. Sometimes close enough is all that’s needed.