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glazenerd    816

Was reading the "Atterberg Limits" again today. Those wondering where our definitions for plasticity, short-long, slip viscosity, and water limits came from.

 

Nerd

 

Was happy to see SAS mentioned in relation to plasticity.

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Mark C.    1,798

 I used to make clay back when I was not using so much-it became just to hard on the body even at a younger age. It was not cost/and time efficient as well.

I choose to spend my time making pots instead of clay.

The upside about making is the total control of your clay body .

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glazenerd    816

Mark:

 

I am awaiting the publication of my work: the idea of selling five copies keeps me motivated. Although the knowledge of insomniacs reading my thread late at night and drifting off to sleep soon after, does bring some consolation. I am just a nerd; what can I say?

 

Nerd

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docweathers    79

Tom,
That is quite impressive, both your series of posts and your proposed article. I cut and pasted it into one big wordprocessor document. You should have been a lab chemist. They love doing that kind of stuff.

 

When I was an  administrator for a large mental health research project at the University of South Florida, I developed a way of dealing with conflict in the very conflict prone organization.

 

I was In a position  that many people envied and thus was prone to attack on whatever I said, just from jealousy. When this happened I would think for a moment whether anything substantively worth fighting for was at stake or was the battle just over who was right. It was only an ego battle over being right I would typically just look at them and not even answer. I figured that I had offered what insight I could and if they did not find it useful that was okay. If there was something substantive to be gained in the battle, I would argue as hard as anybody else. 

 

As Nietzsche said "that which does  not kill, you makes you stronger"

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glazenerd    816

Doc:

 

Ty for the vote of confidence. I would have enjoyed being a lab chemist; you are right about that. I see it as a jig saw puzzle that just needs the pieces put together in the right order. The chemistry side of clay is not of much interest for most; the results that comes from it is. If I posted a clay recipe that was plastic, could be thrown 30" tall with 1/8" walls, and cost 0.25¢ a lb: my inbox would be full. The bottom line is I do enjoy exploring the boundaries; and this summer I will be incorporating some elements that are far outside the norm of our industry. Many interesting chemicals and additives out there used by the technology industry; that are not used in ours. Additive A has many forms, and comes in very specific formulations. Imagine a tablespoon of additive causing more plasticity than a pound of V-Gum. Perhaps a 1/4 of a cup of a molecular inhibitor that gives clay the feel of being six months old, even though you just mixed it. I see many avenues of exploration that have not been traveled before..... we shall see...it depends.

 

Nerd

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docweathers    79

Hi Tom
Re: your email It is easy to support a good idea.

 

I'm interested in what you come up with on additive A. Some time ago I posted a message  on the board asking if anyone had tried it. Of course, nobody had but many posted irrelevant comments.

 

I too like to solve the problems, but more from the mechanical end than the chemical end. That  is a reason that I call myself the gizmo guy. Also being a welder helps me make strange gizmos to make pottery easier.

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glazenerd    816

Ty John

 

Perhaps I will send some research papers to Alfred to help fill in their blanks. They were questioning the role of electrostatic forces in the role of plasticity: it has been studied and verified by over a dozen Phd;s. The reason particle size is attributed to plasticity is because: as the particles get smaller, the presence of alumina drops as well. Perhaps an illustration: 

 

Bentonite, hectorite, and smectite clays typically run between 0.40 down to 0.25 particle size with alumina well under 10%

Ball clays: run  0.50 up to 0.95 particle size, but run 13-21% alumina.

Porcelain is above 1 micron and runs 25-37% alumina.

 

So the association between particle size and plasticity is correct; but it is not the particle size but rather the alumina content that actually produces the plasticity. Cation exchange is directly related to the alumina content ( as noted in all research papers); it is  the alumina that conducts and transfer molecular charges. Just like the aluminum high voltage power lines above ground that carry the current, alumina is the sub micron conduit of electrostatic charges. As the alumina decreases, so does the electrostatic charge: that just happens to coincide with particle sizes: with some exceptions. A negative charge equals plasticity.

 

Modern science has developed many man made polymers that will impart more plasticity to a clay body, than any natural plasticizers. They are used in tooth paste, facial creams, make up, and a host of other products to make them all creamy and dreamy. The article also stated there are no scientific ways to measure plasticity: actually there are several. Although, as will all things clay in pottery it would cost time and money to purchase that equipment and that would add money to a lb. of clay.

 

Nerd

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glazenerd    816

Every once in awhile the lights go on and everything clicks.  I have this figured out- all the pieces fit rather neatly I must say. 

 

Tom

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anchorman    2

This is very interesting information.  Thanks, glazenerd/tom for sharing it.

I'm still a little shaky about how one would look at the SAS, so perhaps you could walk me through it?

Clay body has:

32% hawthorne bond fire clay 50 mesh
32% foundry hill cream

12% om-4 ball clay

4% redart
12% custer feldspar

8% mulcoa m47-48 grog

Most of this clay body was chosen probably for all the wrong reasons, but price and availability are pretty important reasons.  Equal amounts of the hawthorne bond fire clay and the foundry hill cream were used initially to make mixing easier, since it meant dumping in a bag of this and a bag of that, with less time spend scooping and weighing.  We used to use a body that was very high in goldart, but the sulfur in the goldart was eating up our electric kilns, so it had to go.  I had the goldart tested for sulfur content, and it was something like 100x what was in the fireclay.

The foundry hill cream seemed like the best substitute for a variety of reasons, but it involved changing the clay body overall.  I like the way this throws, and pots dry without too much fussing or babying.  They dry much nicer with fewer cracks than the body with the goldart did.  All the same, anything I can change in proportions of ingredients, especially regarding particle size, in order to improve it strength-wise, etc, would be nice to know.  I'm also curious how things would change/need to be changed if switching to a coarser version of the Hawthorne bond fire clay.

Thanks in advance,
jon

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glazenerd    816

 

32% hawthorne bond fire clay 50 mesh   SAS 20   x 32 =  640 x 1.27 = 813

32% foundry hill cream                                SAS 27   x 32 =  864

12% om-4 ball clay                                       SAS 24 x 12 =   288

4% redart                                                        SAS 23 x 4    =    92

12% custer feldspar                                           Total SAS for clay only : 2139 (divided by total clay percentage of recipe 80) = 26.73 median SAS value

Most clay mines will give you the SAS value for their clays; some do not have them.  I have them for Hawthorne, FHC, and Om4.  The Redart I estimated knowing somewhat about it. All the clay ingredients are simply multipled by their SAS value. EX  32% FHC x 27 SAS = 864.  All fire clay has a wide variance in particle size distribution. Fireclay can run from 20 mesh to 325 mesh in a single bag, so the 1.27 multiplier used on the Hawthorne compensates for this variance in particle sizes. The rest of the intermediate and finer meshes are simple math of % of clay times SAS value.

 

18 SAS is coarse particle and 28 is ultra fine particle. So when you look at the medain SAS value of 26.73, it gives you a sense of particle distribution and plasticity. If 18 is coarse, and 28 is ultra fine; then 23 (middle of the two ends) would be considered medium plasticity. So your body reflects a highly plastic recipe at 26.73. The 26.73 also indicates you have an abundance of sub micron particle sizes in this recipe: typical stoneware recipes range from 23.00 up to 24.50 on average. In this case however, because you have 32% large particle fireclay in the recipe, you need a much higher SAS value to ensure you have enough sub-micron particles to fill the voids. The 12% feldspar also indicates to me you are firing up to the cone 10 range? This level of feldspar addition would not provide enough alkali molarity in a cone 6 firing.

 

Both the fire clay and the FHC, both have high levels of carbon/s (sulfur) You will have to be careful about flux additions and silica/alumina ratios due to bloating issues. I would also recommend a higher bisq fire; up to the 1860-80F range to burn off extra carbon. Almost all carbons in clay come from lignite(coal) which produce high levels of carbon ( sulfates/sulfides). The sulfates will burn off easily, the sulfides will not. You could clean up this recipe by increasing the OM4 by 25% and reducing the FHC by an equal amount. I personally like Old Hickory #5 in lieu of OM4 and FHC. OH5 is lower in carbons, higher is SAS value, and produces better green strength due to the higher alumina levels. To clean it up even further: reduce the FHC to 20% and replace the 12% with EPK. (SAS 28.56). It will clean up your recipe, provide more alumina, and is very clean burning clay.

 

Christy Minerals (Hawthorne) recently updated their website with new chemical analysis and particle size distributions. You will find very little difference in particle size distribution between 40 and 50 mesh: not worth the trouble in my opinion.

 

Nerd

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anchorman    2

Thanks!  that all makes sense looking at it now.

I forgot to mention that we primarily fire this to cone 10 as you gathered.  We've used it at cone 6, and it makes usable ware.  With no feldspar added it dunts at cone 10.  It did less so when fired very quickly and less cristobalite was able to form, but eventually would crack nonetheless.  With even just 5-6% feldspar added, that problem goes away.  I haven't pushed the upper limits for how much one can add before it starts bloating and doing other bad stuff, but additions of upwards of 20% or more didn't cause obvious problems with glaze fit, so I split the difference until I could do proper absorption testing.

I'm not sure what constitutes high levels of sulfur to you.  I haven't had the FHC tested, but I did have the goldart and fireclay tested a while back.  The goldart had a mean sulfur content of 7.5 ppm , and the hawthorne bond fireclay had mean sulfur content of .67ppm.  Getting goldart out of the mix changed it from being intolerable to be in the kiln room when firing during the sulfur burnout stage of things to being able to be in there even if the vent were off.  We of course always run the vent when the kilns are firing, but it was amazing to not the difference.  I can now just barely not a hint of sulfur when the kiln is late in the bisque firing.  7.5 ppm doesn't seem like much, but when you have a kiln full of 100 lbs of clay getting fired, that's still close to 37 grams of sulfur getting burned out in a clay body made up of 50% goldart... and that ends up making a rather good volume of sulfur gases eating away at things.

I noticed the same thing Regarding the fireclay with the 40 and 50 mesh clay, I was looking at the 20 mesh, but perhaps that is a little too big for a throwing body?

 

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anchorman    2

So if I'm understanding correctly, we should likely increase the amount of feldspar in this recipe to raise the KNaO molarity from ~2.3 to closer to 2.85/2.9 in order to get a better overall melt?  I'm going to do absorption tests before fooling with things much more.

Also I want to be sure that I'm getting the molar percents right.  I've got a unity formula, so I'm adding the total RO, R2O3, and RO2 numbers together, and then dividing each individual chemical by the total x 100 to get it's molar percent?  this is an interesting way to look at it.

 

Many thanks again for looking into this and explaining it all so well. 

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glazenerd    816

Anchorman

I finally had time to look at your recipe on page 3.

Alkali 2.73. Al 20.85. Si. 73.43. Si/al. 3.52

The silica level is slightly elevated for stoneware. Usually not critical unless higher carbon clay is used in percentages above fifteen percent. Alumina levels in stoneware are of equal importance as flux because spinel forms from silica/alumina.

 

I made this simple adjustments:

Hawthorne 30. Fhc 20. Epk 12. Om4 12 Red art 4. Custer 14. Grog 8

Alkali 2.92. Al 22.08. Si. 72.14. Si/al 3.27

 

You will find little to no difference in plasticity. Green strength will improve somewhat, and absorption will be lowered. The balance between sodium and potassium is good: sodium is reactive when you first pug the recipe, and then later causes other problems. The term is electrophoretic: meaning there is an instant reaction to sodium anions (positive charges) . This is the same reaction that occurs when sodium bentonite hits water: gelatinous . Higher sodium in clay recipes causes a thinning (think sodium silicate) at first, giving the impression/ feeling of softer moist clay. As the reaction reverses ( over weeks) the clay then stiffens because the moisture content is low. Sodium overall is a rather caustic flux, after all hydrochloride acid is made from sodium chloride. Same reason salt firings eat bricks for lunch.

 

Nerd

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curt    117

(At Nerd's request this technical question has been cut and pasted from a broader discussion elsewhere in the forums on the use of calcium bentonite vs sodium bentonite in clay bodies. If you want to look at that topic search these forums for calcium bentonite)

 

glazenerd, on 15 Jun 2017 - 7:40 PM, said:

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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glazenerd    816

Finally decided on a book title: "The Clay Codes"

 

There are codes: they are called formula limits, chemistry, and packing densities. (SAS).

 

And there is a third classification of clay minerals; that are never mentioned in your books, or sold at your local pottery house. They are rather complex, and have a unique chemistry of their own. When you stop trying to reformulate the commonly sold clays, and start looking beyond the formulations standards that started 60 years ago: you will realize the possibilities.

 

Nerd

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curt    117

Hi Nerd,

 

Sounds like a great title.  Looking forward to the book coming out. 

 

There was originally a post up here just after my question above, from you, responding to my question?  Do you know what happened to that post?   Did you take it down or has it just disappeared somehow?

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glazenerd    816

Well, my field studies on clay formulated according to the outlines given are complete. Now I just need to compile all of my notes, and finish collecting information from the various mines across the country. Thanks to those who chimed in. Perhaps now I can actually go out and enjoy making something, instead of analyzing every little detail.

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