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Callie Beller Diesel

Does calcined EPK rehydrate if it gets wet?

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My impulse is no, but I want to check with the chemistry minds. My understanding is that calcining drives off the chemical water, and that isn't always affected by the physical water. I know some things will reabsorb water over time, but that's things like soda ash that you roast, not calcine.

Whats the technical things that happen there? 

(Nerd, I want the essay version if you've got it.)

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Callie:

When chemical water is removed kaolin becomes meta-kaolin; meta-kaolin can be rehydrated to the kaolin structure over a verrrrry loooonnnnnggggg time at ambient pressure and temperature.  At high pressure and over a range of warm temperature the rehydration rate is increased to hours.  The data are available in the archaeology discipline literature within the last 30 years. 
Calcined kaolin probably has a measurable, but moderate level of water adsorption capacity - similar to zeolite materials or silica gel.    I would expect a drying temperature of about 300-400 F for removal of the adsorbed water.

I too would like to see the full version from Nerd.

LT
 

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With glaze making if part or all of the EPK in a recipe is calcined it reduces the shrinkage of the glaze slurry and prevents cracking / crawling issues with high clay glaze slurries. If the calcined EPK was able to re-hydrate in the glaze bucket then it would negate the properties of it and that just doesn't happen. Like has already been said, nope, for all intents and purposes it doesn't happen or if it does it's to a negligible amount.

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Gee, I have to write an essay on the day I decided to give up on Mrs. Folgers.

molecular moisture is the term most often used to describe hydrated minerals.

molecular moisture is driven off by 1050F. For this reason porcelain bodies are more susceptible to quartz inversion cracking. Quartz inversion occurs at 1064F (563C) at which time quartz (silica) expands. At 1050F, molecular moisture is driven off, which causes shrinkage of the body. So within a few degrees shrinkage from molecular moisture occurs, as silica expands; which makes inversion a problem.

Unlike other clays, kaolin is mostly a pure grade of alumina silicate. That said: the typical LOI for kaolin is 12.5%; with most of that being molecular moisture.  Secondly kaolin is a 1:1 clay particle: looks like a saltine cracker at 25,000X magnification. It does not have inner layers like ball clay (a 2:1 particle) that holds moisture. As LT pointed out: temperature plays a bigger role in the short term rehydration, rather than direct contact with water. 

In reading the work of A.F. Norton: who did the early research on slip casting ( Alfred 1948) he cited that a single water molecule is only fractionally smaller than a molecule of clay.  This in turn produces a "stretched membrane" ; in which water actually encapsulates a kaolin particle. The actual absorption rate is minimal pending temperature. At 60F absorption is minimal, at 90F it is accelerated. So absorption or rehydration is subject to a rate law: X amount occurs every thirty days. That rate is decreased at. 60F and increased at 90F. I have seen no literature that indicates that absorption is increased by direct water contact.

i know from personal experience that I have to calcine kaolin twice a year. At the begging of spring, and again in late fall. My studio can be 85-100 degrees during the summer, with humidity levels 35-50% on average. I dry mix my glazes and only add water before I begin to spray. My base crystalline glaze is usually mixed at 10,000 gram batches: suspender and colorants added before water.

would be easy enough  to test: calcine 500 grams, weigh after .. Let it sit out exposed and weigh it 90 days later: that will tell you how much rehydration occurs.

note: I have not read any abstracts that directly studies rehydration of kaolin. Need to qualify that. 

Tom

 

Edited by glazenerd
Note added

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After thinking about Callie's question, and being finished with a book on rock weathering, asked my computer a few questions: 

these searches will be good starting points if one is curious:
https://www.bing.com/search?q=rehydration+of+meta-kaolin 
https://www.bing.com/search?q=%2brehydration+of+pottery+artifacts
https://www.bing.com/search?q=%2b+Rehydroxylation+Kinetics+of+calcined+clay

The results should shed some light on the rate of rehydroxylation of fired clay minerals.

Rehydration is measurable with the modern analytical tools.  
https://www.osti.gov/biblio/6170623-rehydration-metakaolinite-kaolinite-evidence-from-solid-state-nmr-cognate-techniques 

Calcined Kaolin is currently being tested as an ingredient to lower the carbon foot for the concrete and the degree of rehydration of pottery shards is being used by archaeologists to estimate the time since the last firing of the pottery. 

I agree with Mark and Min's conclusion that rehydration of calcined kaolin is not a significant concern for most potters.  

However, if I were having problems with cementation in the slurry of a glaze using calcined kaolin as a glaze ingredient, i would look deeper into the use of metakaolin in cement pastes for insights on preventing the cementation.  Retardants are available that will prevent the hardening of cement.  I use the retardant technique when using Portland cement as an ingredient in a clear glaze. 

LT

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1 hour ago, glazenerd said:

 

would be easy enough  to test: calcine 500 grams, weigh after .. Let it sit out exposed and weigh it 90 days later: that will tell you how much rehydration occurs.

Tom

 

But couldn't it just be absorbing physical moisture, which could be dried out again by heating to a couple hundred degrees, vs absorbing at a molecular level that would require calcining? You'd have to weigh it, dry it out, and weigh it again to see which you're dealing with. I would think that all of our materials vary in weight to some small degree depending on the humidity in the studio at any given time, unless they are in well sealed containers.

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Thank you so much you guys! I am actually very exited about this level of reading this evening. 

2 hours ago, Mark C. said:

Tom

(I decided to give up on Mrs. Folgers.)

This should have zero effect since the amout of caffine is equel to the water content in calcined epk after on winter in Canadian ambient  air.

 

Are you saying we’re dry here, Mark?

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2 hours ago, neilestrick said:

But couldn't it just be absorbing physical moisture, which could be dried out again by heating to a couple hundred degrees, vs absorbing at a molecular level that would require calcining? You'd have to weigh it, dry it out, and weigh it again to see which you're dealing with. I would think that all of our materials vary in weight to some small degree depending on the humidity in the studio at any given time, unless they are in well sealed containers.

Correct Neil:

but the idea is to get some sense of rehydration that occurs in a 90 day period. That would give Callie some sense of how long her mixed glazes would remain stable. Because temperature plays a big role: then pending where you live is going to extend or shorten that time. 

4 hours ago, Mark C. said:

Tom

(I decided to give up on Mrs. Folgers.)

This should have zero effect since the amout of caffine is equel to the water content in calcined epk after on winter in Canadian ambient  air.

 

Mark:

i have had a 30 year affair with Mrs. Folgers: we met at Maxwells, house. 

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Just now, glazenerd said:

Correct Neil:

but the idea is to get some sense of rehydration that occurs in a 90 day period. That would give Callie some sense of how long her mixed glazes would remain stable. Because temperature plays a big role: then pending where you live is going to extend or shorten that time. 

But wouldn't that stability depend on which type of hydration we're dealing with? If it's physical moisture, it's not an issue because it's a bucket of liquid glaze. But if it's chemical rehydration, then the glaze will be affected.

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11 minutes ago, Callie Beller Diesel said:

Thank you so much you guys! I am actually very exited about this level of reading this evening. 

Are you saying we’re dry here, Mark?

yes humidity wise in winter I assume is dry there

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hmmm, trying to get my head around this. My previous understanding (which, like so many other things I thought I understood, may be a colossal misunderstanding) is that the (theoretical) kaolin molecule  is Al2Si2O5(OH)4 . As it is fired towards and past 1000F +/-, the molecule reorganizes into the conventional ceramic oxide molecules of one Al2O3 and  two SiO2, plus two H2O molecules that immediately evaporate. Once the extraneous water molecules are gone from the calcined kaolin, you can't put them back in conventional time and ambient conditions. (Of course, in geologic time and conditions, all sorts of chemical comings and goings can occur.)

Kaolins and other clays are mixed with physical water to create plastic clay with various working properties dependent on how much water is involved (the whole WOP lesson Nerd has been teaching us), but then the physical water is allowed to dry out before firing it. If you want to, you can mix it back with more water and it becomes plastic again. But once it is fired past the magic 1000F, the molecule is changed, the extra hydroxyls are gone, and it no longer can be remixed with physical water to become plastic again.

This is the main reason we calcine part of the kaolin in glaze recipes that have a lot of clay in them. With a high proportion of clay in the glaze recipe, the glaze slurry is just a really soupy plastic clay body with some extra fluxes to force melting at maturation. And such a slurry then shrinks a lot as it dries, causing all sorts of problems. So calcine some of the clay so that it is no longer plastic-capable, and the glaze slurry is now more manageable.

If the calcined kaolin is left out in an open container in humid warm weather, it might pick up some physical moisture and become clumpy, just as zinc oxide and wollastonite are well known to do. But that humidity is still physical water and does not recombine with the alumina and silica oxides to become a plastic clay molecule again. One might need to dry it in a warm kiln for optimal usage, but chemically it's still calcined kaolin.

At least that was my previous understanding. What seems to be revealed here is that after a long warm humid summer, the calcined kaolin will revert to a plastic state as the water recombines with the alumina silicate oxides. Or that if a glaze recipe with a lot of clay is modified with calcined clay to improve its working properties, it will revert to its lousy pre-calcined properties if it remains in the bucket for too long.

confused as ever, dw

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Citing the work of W.G. Lawrence, A.F. Norton, and D.D. Buttons ( all Alfred engineers)

the question was rehydration: the application is molecular moisture: meaning H2O has chemically bonded to the crystal lattice of kaolin. As DW pointed out above- nice job!  As DW noted: once calcined, it takes time (lots), temperature will accelerate, as will pressure.  In reading the works of those three listed: the only reference made to rehydration from direct water contact was "minimal" over an extended period. No definition of "minimal" or "extended" was given. Although in reading other abstracts: 5% rehydration occurred. Note however that is 5% of the 12-14% of original molecular moisture lost in calcination.

so why is heat and pressure required to accelerate rehydration?   Below is a quick diagram I made of a kaolin particle encapsulated by a molecule of water. Kaolin is a 1:1 clay particle: one side is alumina and the other side is silica. It has no inner platelets that will absorb water like ball clay does. Ball clay is a 2:1 particle. Kaolin will only hold water on its surface, and will not absorb water unless heat and pressure is applied. The resistance to absorption is part of the "stretched membrane" theory introduced by Norton in 1948. A molecule of kaolin is fractionally larger than a molecule of water: so the water molecule is slightly stretched as it encapsulates the kaolin. Applied, this means the water molecule is under increased surface tension which effects how the clay/ water film reacts.

image.jpg.80de0b251b459fd390c376e2e3a04632.jpgthe two inner circles are a kaolin particle ( alumina and silica) The outer ring to the inner jagged ring is a water molecule. The space labeled "ionic disorder" is the area between the kaolin and water. This area is where negative or neutral charges from the kaolin react with water: which is a dipole. (Long winded discussion on dipole here) This disordered ionic area creates a chemical barrier which retards moisture absorption. Again, kaolin being a 1:1 particle naturally resists absorption.

edit addition: sorry got called away.

temperature accelerates rehydration because temperature increase is a direct measurement of atomic motion. The "disordered area" becomes more organized  in relation to positive and negative arrangement of atoms. 

Pressure becomes relative because increased pressure closes the disordered space between kaolin and water.

T

Edited by glazenerd
Additional info added

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