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Calcium Bentonite Vs Sodium Bentonite In Clay Bodies

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As part of my ongoing clay body development project I have been looking again recently at bentonites.  I have access to a source of calcium bentonite and wanted to find out if others have used it any of their clay bodies, what their experiences were, etc.. 

 

Most of the discussion around bentonites seems to be around the hyper-swelling sodium bentonite (Wyoming) and the plasticity benefits it brings.  What little research I have been able to find suggests that calcium bentonite swells when hydrated, too, but only about a fifth as much.   If calcium bentonite even gets a mention in the context of ceramics, it is almost inevitably about how it can be chemically converted into sodium type via a cation exchange process (sodium bath).  

 

But what about using calcium bentonite as a more substantial addition to a clay body? Even though it doesn't swell like sodium bentonite, its does swell some, and its potential to add plasticity still seems much higher than most other alumina-silicate materials in a clay body (eg, china clay or stoneware clays).  While most people would only ever add a few % of sodium bentonite to a clay body (more than this it becomes sticky and harder to work with?), in principle you could add several (5?) times that amount of calcium bentonite.  Perhaps some wou 

 

Setting swelling capacity aside, bentonites (including calcium bentonite) in general have a small particle size which in theory could/should improve the green packing density of the clay body.   Apparently calcium bentonite is often used in foundrys for its combination of plasticity and favourable green strength compared to sodium bentonite.

 

Calcium bentonite might also have a helpful chemistry (or not!) for the clay body, which starts to become relevant when you are adding potentially 10% or 12% rather than 2% or 3% as a raw material.  I am one of those people who thinks that clay body chemistry DOES matter, particularly when you are want to ensure the body is functional.

 

I guess color and impurities of the bentonite would have to be taken into account the more you add, not unlike a ball clay in this respect.  Similarly, perhaps calcium bentonite could replace a portion of the ball clay addition under certain circumstances.

 

I also suppose that the cation exchange capacity of bentonites becomes relevant in the context of the other raw materials in the clay body (eg, sodium, Nerd?) the more that you add.

 

Interested in any thoughts or experiences anyone has.  Also interested in any bentonite specific research anyone has run across.

 

 

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Hmmmm.... OK, I guess I asked for that, Nerd. I get the gist of your points, but this thread is really about bentonite, not sodium (or magnesium sulfate). I will wait for your book to read about that.

 

Have you ever used calcium bentonite?

 

Flocculation is only part of the story for calcium bentonite I think. Particle size, plasticity, green strength, quantum are also important bits.

 

Why can't it be added as a major clay body constituent?

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Okay: I spent four paragraphs explaining why sodium ( including sodium bentonite) is not well suited for clay bodies. The last paragraph, heaping praise on the benefits on calcium as having better chemical compatibility.

 

So if I refuse to use sodium on the premises stated, and calcium is much better: which bentonite do you think I have been using the last two plus years?

 

Remember something about cation exchange: the numbers are about rate exchange ( time). Calcium at 80 meq will do the job of sodium at 150 meq: it just takes more time to do it. 4 days to be exact for porcelain, and 24-36 hours for stoneware.

 

Nerd

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Yes I thought you might be using it as I think you mentioned it in another thread sometime recently.

 

So could you say something about your experience with calcium bentonite? How much have you been adding to your clay bodies % wise? What differences have you noticed if any? Does it work in large percentages? Have any problems emerged?

 

Remember something about cation exchange: the numbers are about rate exchange ( time). Calcium at 80 meq will do the job of sodium at 150 meq: it just takes more time to do it. 4 days to be exact for porcelain, and 24-36 hours for stoneware.

Nerd

In practical terms, when you say "job" here I am not sure what you mean. What happens at the end of 4 days for porcelain and 24-36 hours for stoneware? I am guessing you are talking about some kind of chemical alteration in the clay body which affects its plasticity or other working properties. Can you clarify?

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No problem, Nerd, happy to continue the more technical part of this question over there.

 

And I apologise if it sounded like I was brushing aside your earlier post (the one you took down). I essentially invited you (by name!) to comment on sodium and then appeared to complain when you did (what the...!?) Sorry about that, didn't mean for it to come out that way and will choose my words more carefully next time. I value your many contributions in these forums (even if I do not always agree with them!) and am always up for an in depth technical exchange as I know you are. Long may that live.

 

That said, I am still very interested in your and everyone else's practical, hands on experience with calcium bentonite in comparison to sodium bentonite in the context of clay bodies. Anything you or anyone else can add in this regard will be appreciated.

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FWIW, my local supplier only stocks calcium bentonite. I haven't tried using it in more than 2-3% additions to clay bodies, but I'd be interested in the results of any experiments you might do with higher amounts.

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I've been using calcium bentonite for quite a few years. Seems to need 1-2% more, but I've used different ball clays over the years and they have an effect too.  Bentolite from Laguna seemed cheaper and easier to get than other calcium bentonite from Kraft Chemicals.  I'd like a NZ kaolin based body with 12.5% schrinkage. and so far the closest that I've come is about 1/3 each of calcium bentonite, POE, and v ee gum t.

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  •  

    Typical analysis: SiO2 58-64%   Al2O3 19-22%   CaO 0.65%   MgO 2.5-3.2%   Fe2O3 2.8-3.5%   FeO 0.2-0.4% Na2O 1.5-2.7%   K2O 0.2-0.4%   TiO2 0.1-0.2%   Moisture 6-12%   PH 9.0-10.5

     

     

    Bentolite

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Bryan, thanks for your input.  You mention Bentolite.  Are you actually using it?  And if so, which kind?  Also, have you tried substituting any bentonite for ball clay to see how it works?

 

I just had a quick look at Digitalfire and I can see 4 different types of Bentolite, and judging by their chemistries, I suspect that the usage rates, effects, etc.. would depend very much on which variety you were using. 

 

The issue is that bentonites ain't bentonites.  Most potters have been using sodium bentonite for so long that the word "bentonite" has become synonomous with the sodium bentonite kind.  But whether a bentonite is classed as a "calcium" bentonite or a "sodium" bentonite depends very much on the ratio of sodium oxide vs calcium oxide in its chemistry.  (acutally cations, but never mind)

 

Virtually every bentonite on the planet contains some quantity of calcium, magnesium and potassium ions in addition to the sodium ions, in varying amounts.  From a practical perspective, I think most people ignore this since they are only using a little bit, but from what I have seen there is enough variability in the chemistries that with larger amounts you would definitely want to know and incorporate the chemistry if you are designing glazes or clay bodies from scratch.  

 

Anyway, the point is that when sodium predominates over calcium (eg, Bentonite B or H) this material should probably act like the ubiquitous swelling kind most people know and use.  However, when the ratio is reversed,  and the calcium present is several times as large as the sodium component,(eg, Bentolite L or L2) it should take a lot more calcium bentonite to mimic the effect of a sodium bentonite.  About 5 times as much as far as I can tell so far from the studies I have seen.

 

The chemistry Nerd has posted just above appears to be a straightforward sodium version of Bentolite, based on sodium vs calcium (Mg seems about normal).  Is that the one you are using? Or something else? 

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I started with bentolite L, switched to Volclay, then to a calcium bentonite from American colloid through Kraft Chemical, sorry I can't be more specific on this. Back to Volclay, seemed to be contaminated with calcite and some black rock.  Now I am just starting to use the following, which seems to be European in origin, via Laguna:

 

BentoLite L-10 - A non-swelling Calcium bentonite and our lightest colored Bentonite. Excellent for use in clay bodies, especially in conjunction with Macaloid in porcelains.
 

Typical analysis: SiO2 58-64% Al2O3 19-22% CaO 0.65% MgO 2.5-3.2% Fe2O3 2.8-3.5% FeO 0.2-0.4% Na2O 1.5-2.7% K2O 0.2-0.4% TiO2 0.1-0.2%   Moisture 6-12% PH

9.0-10.5

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Bryan,  Something amiss here for me. 

 

The word description (edit: and chemistry) you provided seems to be from Axner's website (and maybe Laguna's but I could not find it?), and Axner do appear to be calling this product a calcium bentonite.  However, the chemistry you provided is pretty clearly (to me anyway) a sodium bentonite, as evident from the great predominance of sodium over calcium in the chemistry.

 

Are you sure the product you are using is not Bentolite L-10 C??  BYK's website (which is where the Southern Clays' (manufacturer of Bentolite?) website defaults to for me) shows chemistries for both L-10 and L-10 C.  They do not describe L-10 specifically as a sodium bentonite, but nor do they call it a calcium bentonite.

 

However, L-10 C they do specifically call a "calcium bentonite" and the chemistry seems clear that it is. 

 

Is there a mix-up somewhere here, or am I missing something?  Just trying to get to the bottom of which one you are really using to understand your experience and comments better.   

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So, Bryan, it was hard to avoid the impression that the "calcium bentonite" on Axner's website was just mis-labelled sodium bentonite.  Was that what you determined as well?  Just curious. 

 

Will have a look at Pol Ox WSR although at this point I am really focused on the plasticity-reducing benefits of calcium bentonite, since I am interested in using it as a major clay body ingredient (in which case sodium bentonite would give me way too much plasticity).

 

 

Bentonite fun facts:

 

 

One gram of montmorillonite has a surface area of 750 m2/gram, nearly as much as the best activated carbon grains. Twelve grams have a surface area that covers an entire football field

 

Lets say you are throwing a one pound cup, comprised of fine particle ball clay and bentonite: you are holding enough particles in your hand to cover 37 football fields.     You will never look at clay the same again :)

 

Nerd

 

I think this is more than just a fun fact!  :lol:   In fact I have been trying to wrap my head around the particle size differences between bentonite and other clay body ingredients for a couple of weeks now.  The problem is that it is hard to visualize what the particle size of bentonite actually means in the context of the whole clay body. 

 

It is really the green-packing structure that I am thinking about.  How to understand bentonite's role in the green-packing of a clay body?

 

We have all seen close-up (SEM) pictures of "books" of kaolin and other clay body ingredients in studies, online, etc..  We can process those pictures visually, at least to some degree, and I think in our mind's eye we tend to see most clay body ingredients as more or less the same general size, kind of like looking at gravel in your driveway.  Yes, some pieces are 4 or 5 or maybe even 10 times the size of others, but they are close enough in size to compare one to the other, and this makes the whole visualization of what's in a clay body somehow comprehensible to the human brain.

 

But of course this is not even close to accurate.  The relative sizes of different clay body constituents are wildly different.  Kaolin and other clay particles are something like 1 to 5 microns in size, and you cannot see them (the lower limit of visibility for the naked eye is about 40 microns).  Feldspar, silica and many other standard clay body ingredients are hundreds of times larger than clay (eg, 120 mesh materials = 125 microns).  And sand and grog are larger still, say 1000 microns (1 millimetre).      

 

However, bentonite particles are more than a thousand times smaller than clay particles. (!)  In a glaze bucket, or a clay body, or anytime they hit water, their tendency is to split into almost the smallest platelets they are made up of, which we often see as a gel.

 

From a green-packing perspective, it seems pretty clear that bentonite is not like some small particle that nicely fills up the nooks and crannies around other larger particles in the clay body.  If a clay particle was the size of a suitcase, and grains of feldspar the size of your house, then bentonite would be like oily leaves coating every surface in sight.

 

This brings a different notion to plasticity than where I started out.  It also changes my impression of how the body might be maturing at temperature (what is melting on what, etc.).  And other things I am sure.    

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Curt said: "It is really the green-packing structure that I am thinking about. How to understand bentonite's role in the green-packing of a clay body?"

 

You are getting very close to figuring out the formulation blunder of the last 50 years. There is such a thing as too much plasticity, although that is commonly ignored in clay formulation. Ball clay/ s absorb and hold water: as do both bentonite varieties: the difference being one swells, while the other does not. High plasticity also means high absorption rates while throwing: they are inter- related properties.

The common understanding among potters is that sodium bentonite swells up to 4-5 it's size: that is not the problem. The problem is, it absorbs up to 18 times it's weight in water. So when bentonite is used in stoneware especially, that already has up to 50 % of clay that absorbs water: what is going to happen when water hits it while throwing. The more water absorbed while throwing, the weaker it becomes. S- cracks are primarily caused by absorption, compounded by sodium properties: and compounded even further because formulators keep dumping ball clays and bentonites into the mix.

 

My clay keeps slumping and cracking because it absorbs too much water! We'll let me fix that by adding more materials that absorb even more water........perfect!!!!!!

 

Plasticity has a number, a very defined number....

 

Nerd

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I have tried as much as 5% calcium bentonite (Bentolite L 10 and one from LMV Nevada), and found it does not help workability/plasticity at all, unless activated with soda ash or Li2CO3.  The green packing property , because of the tiny particle size, might make for a tighter clay body, and you maybe could use far more calcium bentonite than sodium, but I haven’t tried that, above 5%.  I recently ordered a sample of Bentolite H from (KYT?, formerly Southern),  and Millwhite, Brownsville, TX.  8 bucks a sack.  But with the idea of activation, cation exchange.   Sorry for the delay here, but I was just looking for information on bentonite and stumbled on this.  Hope someone finds it.

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Sequoia:

i assume from your reference to " Cation Exchange", that you read my article on that topic in the January 2018 edition of a Ceramics Monthly. CEC ( cation exchange capability) is rated on a scale from 0-150. Sodium based bentonites, hectorites, and smecites are usually rated at 100 CEC and up. So the assumption being they have greater plasticity values; which is incorrect. Calcium based bentonites, hectorites, etc. are actually more potent even though they are rated from 50-75 CEC. Calcium is 200x more likely to exchange ions than sodium in the presence of a liquid binder. 

The other big issue: sodium based bentonites swell, and hold up to 18% their weight in water. For the record: and clay material with a CEC greater than 11, crosses into the "swelling" clay class. Calcium bentonites are far less prone to swelling, and do not produce that gelatinous mess sodium does. Personal opinion: sodium bentonite is a rather useless product. 

 

Nerd

 

Edited by glazenerd
Off topic content removed
Sequoia Pottery likes this

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I wrote an article in ceramics monthly about cation exchange for the two calcium bentonites I mentioned, https://ceramicartsnetwork.org/shop/ceramics-monthly-pdf-september-2014/  .  I read several patents going back to the 1930's in Germany about that, and some other sources about cation exchange of calcium bentonite, and that is what the article was about.   Regarding which of the bentonites, calcium or sodium, produces a more workable clay body, here's a test.  Make two clay bodies, identical except use 3% calcium bentonite in one and 3% sodium bentonite in the other, with the same weight of finished clay, and the same stiffness.  Say, 30 feldspar, 30 silica, and 40 kaolin, not that this is an ideal clay body, but it will show up the effect of the bentonite.  Make a pot with each.   Dollars to doughnuts the clay body with the sodium bentonite will be far easier to throw and will hold up better to the process. That has certainly been my experience, repeatedly.  When most potters say "plasticity" we mean a clay that is responsive and does what we want it to do.  This may differ from the strictly technical meaning of the word, maintaining shape upon deformation.   They BOTH maintain shape on deformation.

Wow, I am sure glad I came across that post.  Very much looking forward to your response.  I'll go look up your article right now.

Edited by Sequoia Pottery

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Ray:

the differences between sodium and calcium bentonites in the USA, are a bit misleading. If you go through the chemistry of each one sold: most have 2-3% sodium! and around 1% calcium. ( highly generalized comment) L10 however has 3% calcium! and 1% sodium: so it gets labeled as a calcium bentonite.  From the studies I have read on calcium verses sodium: values have to be well above these to be effective.

Tom

 

Edited by glazenerd
Post revised per request to keep within topic limits.

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Hi Tom

Thank you so much for res[ponding.  Right now I am taking an opportunity to go up to my studio and home for the first time in 3 weeks.. My wife and I are taking care of her mother, and normally we have people coming in to help, but, things happen.  So I've got to go.  And no internet service where I "live".

But I WILL be back.  As you might be able to tell, I am VERY interested in this subject, not only bentonite, but clay "plasticity", and all the tricks involved with that, and yes what is plasticity anyway. And all that, how it is related to glaze, etc.   So please be patient.

Ray

 

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