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glazenerd

Slip- Engobe Study

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I have concluded my studies on stoneware and porcelain: time to move on to a new topic. I have been reading some background studies done in the 40s, thru 70,s. on slip chemistry. I am interested in hearing experiences, thoughts, opinions, links to articles, abstracts, etc. 

This is an open topic, so as long as your post has the word "clay" in it: ramble on. Finite details welcomed. 

I have been reading studies from W.G. Lawrence and A.F. Norton; both Alfred PhD's on this topic. While viscosity was covered, there was much more emphasis placed on the water film, PH levels, temperature, and particle stacking. It was specifically noted that "particle stacking" is an entirely different principle than particle distribution. Was or is any of these principles taught, or still covered today? Any potter familiar with "terra sig" has delved into PH and particle stacking. Does anyone have links about the effects of temperature on ionic charges? 

Tom

i realize there are many links to collidal chemistry, PH dependent charges, etc: but interested in those specifically related to pottery use.

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Tom,

My interpretation of your post is that you want to understand the mechanical and chemical physics of the interaction of mineral particles in a aqueous slurry, (aka slips in pottery jargon).

I would look first outside of the pottery discipline to gain a reasonable understanding of the chemical interaction of minerals with an aqueous liquid phase.  The geochemical and civil engineering disciplines are more likely to have the best insights in what happens, and why, in these mixtures than any where else, except perhaps the mining and ore processing disciplines.   When my understandings  how  the individual properties of any solid particle and any liquid medium interact to produce the  measured properties of the combined system , then I would focus on using that knowledge to address specific questions about  pottery slips.  


The electrical charges on an ion are independent of temperature - an electron has a single negative charge at any temperature;  the effects of an electron in a medium -- water is the medium for slips -- wil1 be sensitive to the changes in the temperature of the medium through such properties as viscosity, dielectric permittivity, density, diffusion, ion solubility, etc.  These topics are addressed in modern textbooks for Physical Chemistry and/or Inorganic Chemistry.  
I recommend any of the last three editions of Peter Atkins Physical Chemistry textbook, or for a text with a geochemistry orientation, Greg Anderson's "Thermodynamics of Natural Systems" Cambridge University Press.   

 

Slips are a mixture of fine solid particles dispersed or suspended in an aqueous medium.  Many of the properties are related to solid surface phase interactions with the liquid phase.  
 
Adamson and Gast's book "Physical Chemistry of Surfaces" (ISBN 9780471148739) provide insights of interactions of solids in liquids. 

A starting point for internet insights are websites such as: 
Zachry Department of Civil Engineering
Texas A&M University
College Station, Texas 77843-3136, USA
URL: http://ceprofs.tamu.edu/briaud/
https://ceprofs.civil.tamu.edu/briaud/research_wip.html 


This lecture might also be an introduction to slips from the Geotechnical Engineering disciplines. 
The Twelfth Spencer J. Buchanan Lecture.  College Station, TX
 
SLURRIES  IN  GEOTECHNICAL ENGINEERING 
by 
Raymond J. Krizek Stanley F. Pepper Professor Department of Civil and Environmental Engineering Northwestern University Evanston, Illinois  
October 29, 2004
https://ceprofs.civil.tamu.edu/briaud/Buchanan Web/Slurries in Geotechnical Engg.pdf 


Looking forward to your synopses of the hows, whys, & wherefores  of aqueous slips.

LT

 

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LT

thanks for the links; certainly will look at all things relevant.

Lawrence gave a short section on the "water hull" theory in regards to slip chemistry. I actually learned that a water molecule and kaolinitic particle are the same shape and size: which plays a large role in slip chemistry. 

D.D. Button did some research on the effects of temperature and ionic charges in slip: he concluded there was a substantial drop in ionic charges. However, those changes occurred in the water film, not the particles themselves. Also found his abstracts on CEC exchanges between sodium, potassium, and calcium informative. Hard to believe that porcelain was mixed in one third ratios during the. 1800's. Kaolin, silica, and feldspar in equal parts: 1-2% calcium was used as the plasticizer. It had to age for months in order for ionic charges to develop, but it is telling about how plasticity really works.

Find any other related materials; pass them along please. Missing a couple pieces of the puzzle yet. Really interested in the PH factor, that intrigues me.

Tom

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Tom

When you use the term "PH" are you really referring to the pH of a solution?  If not I need a more information on regarding your statement: "Really interested in the PH factor".

pH is defined as the negative logarithm (to base 10) of the hydrogen ion concentration (moles per liter at 298.15 K )  in an aqueous solution.   pure water at 298.15 K has a pH of 7.0.   From a thermodynamic point of view pH is a means of measuring the equilibrium concentration of hydrogen ions in water at a given temperature.   pH does vary with temperature.  I don't remember exactly the relative values (that is why I have a CRC chemistry handbook) but temperature changes over 1 or 2 degrees are can have significant impact on pH.    Anderson's or Atkin's book has more details.  The importance to a pottery slip is that pH values below 7 are called acidic and above 7 the slip is called basic or sometime caustic.  pH 7 is neutral.   

 

the aging you mentioned is due to the slow leaching of some species from the solids.   the slowness is a kinetic issue.  After the surface layer of the dissolving specie is lowered the surface concentration is lowered and the rate of dissolving is slowed.  The properties of the slip (clay body) will be changing with time until a thermodynamic equilibrium is obtained.  Anderson and Atkin both have paragraphs of this part of the chemistry that is taking place in a slurry system -- their vocabulary is likely to be significantly different than the common pottery slip jargon.  

LT

 

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Yes, the PH of an aqueous solution.

water molecule being a dipole. From that basis the addition of NA until the PH hits a point of equilibrium. After that; there is a rapid rise in PH that directly corresponds to viscosity.. Which indicates  that the concentration of OH ions are increasing rapidly. Once the +/- sites are filled in the crystal,lattice: then maximum particle stacking has been achieved. From the graphs, that appears to be somewhere in the 5.5  M.E/ per 100grams. (Electrolyte)  

what I am trying to figure out: modern slip is based in specific gravity: 1.5 to .1.8 seem to be the numbers most often cited. Yet maximum particle stacking occurs at 0.90 to 1.1 S.G. Trying to figure out if that change came about because of long chain ionic binders (Darvan) or because some chemistry has been lost along the way. More research required on my part. The whole "water hull" or "stretched membrane" theories in slip casting are based on disrupting the water molecule. These theories are given as "base" exchange. Still need to get more clarity on these theories; they are pottery specific.

the other issue: sodium silicate is measured in degrees twaddle. ( concentration)  Did some research on SS currently sold in the pottery trade: sodium silicate levels are all over the place. So at this point, need to collect all kinds of background info.

interesting links.. TY.  Tom

 

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12 minutes ago, glazenerd said:

Yes, the PH of an aqueous solution.

water molecule being a dipole. From that basis the addition of NA until the PH hits a point of equilibrium. After that; there is a rapid rise in PH that directly corresponds to viscosity.. Which indicates  that the concentration of OH ions are increasing rapidly. Once the +/- sites are filled in the crystal,lattice: then maximum particle stacking has been achieved. From the graphs, that appears to be somewhere in the 5.5  M.E/ per 100grams. (Electrolyte)  

what I am trying to figure out: modern slip is based in specific gravity: 1.5 to .1.8 seem to be the numbers most often cited. Yet maximum particle stacking occurs at 0.90 to 1.1 S.G. Trying to figure out if that change came about because of long chain ionic binders (Darvan) or because some chemistry has been lost along the way. More research required on my part. The whole "water hull" or "stretched membrane" theories in slip casting are based on disrupting the water molecule. These theories are given as "base" exchange. Still need to get more clarity on these theories; they are pottery specific.

the other issue: sodium silicate is measured in degrees twaddle. ( concentration)  Did some research on SS currently sold in the pottery trade: sodium silicate levels are all over the place. So at this point, need to collect all kinds of background info.

interesting links.. TY.  Tom

 

I've seen you mention degrees twaddle but I've still never seen it in the wild.  Everything I've seen for sale, whether for concrete or radiator fixing is in percentage.  Is this unit only used in text books?

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Unfortunately my CRC chemistry handbook doesn't get a lot of use these days other than as a weight to press things! Interesting topic so I'll lurk around in the background ( plus I work with terrasig for my smoke fired pottery and I'm just getting into slip casting. The particle stacking makes sense in thin, low SG  terrasig as it buffs up nice. )

Are you looking at variation in temperature during the making of the terrasig ?

You need to talk to Tony Hansen, and the person he's quoting in this article about terrasig:

 https://digitalfire.com/4sight/education/super-refined_terra_sigillata_274.html

he talks about using heat to concentrate the particles in the solution.

 

 

Edited by terrim8
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Liam:

degrees twaddle came from The British Ceramic Society. Pending where you live; S.G. Is measured in Twaddle or Baume. Or the standard S.G. Scale commonly used in pottery. Twaddle is S.G times 0.005.  The interesting hiccup: at 60F/15.6C there is a notable drop in viscosity due to  changes in the water film, just as there is a change in SG above 90F. We had a thread not long ago about slip cast pieces warping. The OP placed a space heater and raised temp to 65F, and warping dramatically decreased. So the effects of temperature is part of slip chemistry that seems to be lost along the way.

Terri: terra sig is a lesson in collidial chemistry, and a prime example of particle stacking. Particle stacking is different than distribution, although it arranges particles in perfect stacking order for maximum distribution. I use grains of rice to describe it: the sides of the grain being negative and the ends positive. The term used is card stacking: meaning each grain of rice is perfectly arranged with each end pointing north and south, and each side arranged east and west. In this perfect particle arrangement: the particles settle in perfect stacking order as moisture is removed. S.G. Changes as PH changes, and SG changes by temperature. Mr. Hansen and I have exchanged lengthy emails over the last few years discussing clay chemistry: interesting exchanges.

tom

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Terri:

part of the problem with terra sig is the clays sold in pottery are not exactly ideal. I have a ball clay that is 0.31 microns, with a CEC of 18.8 that I special order from a mine. Most ball clays sold in pottery houses run 0.60 to 0.75 microns with 7-9 CEC. Which means much more work for small rewards.

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21 minutes ago, glazenerd said:

Terri:

part of the problem with terra sig is the clays sold in pottery are not exactly ideal. I have a ball clay that is 0.31 microns, with a CEC of 18.8 that I special order from a mine. Most ball clays sold in pottery houses run 0.60 to 0.75 microns with 7-9 CEC. Which means much more work for small rewards.

Would ball milling xx sagger or om4 get that particle size down?  Is there anything but raising surface area to make cation exchange higher?

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A new (to me) word!

"The Twaddell scale is a hydrometer scale for reporting the measured specific gravity of a liquid relative to water. On this scale, a specific gravity of 1.000 is reported as 0, and 2.000 reports as 200. … named after the scientific instrument manufacturer W. Twaddell of Glasgow, who first developed hydrometers on this scale at the start of the 19th century."

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Liam:

doubt ball milling would have any effect on sub micron particles. CEC has specific chemistry, little can be done to alter that.

like all things pottery: SG evolved into a studio friendly measurement. Not trying to change it per se; just wondering if we have lost some important info along the way. Temperature variations/ effects appears to have been lost; seems PH as well.

T

hulk: the available sodium silicate I have converted, run 75 to 225 degrees twaddle. Which would play havoc on a favorite recipe as potency changes.

Edited by glazenerd
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Anecdotally, I don't think the temperature affecting casting slip rheology is information that's entirely lost: it's just not showing up here, or maybe isn't well documented if the documenters in question don't live somewhere cold. I have a friend who owns a paint your own pottery place who began casting her own figures to save money. Because of some space concerns, at one point she was casting in her unheated garage before the weather really warmed up in the spring, and was having a lot of trouble. She consulted with a local expert who told her about the temperature thing.

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I don’t think it’s knowledge that’s (nearly) been lost, but rather knowledge that hasn’t been (consistently?) applied to a studio context.  I’ll bet every ceramics factory in existence currently has the temp of their production floors optimized for casting production—it would cost too much in failures not to.  A porcelain bathroom fixture is bigger, more complex, and requires more steps to production than all but the most intricate of large scale work.

 

It’s easy to forget we are a small minority in the ceramic world and for that matter, what a large percentage of what we live with is slip-cast ceramics.  

 

There’s a whole field of study devoted to this, it’s called ceramic engineering.  I feel like acknowledging that this discipline exists and its sources of information may help.

 

In the context of this discussion, Science of Whitewares I has “Mechanism of plasticity in clay water systems” by Onoda 1996 may be applicable.

 

The Science of Whitewares II specifically discusses issues regarding the pre-firing side of things from a strictly ceramics perspective (from minerology to production).

 

These may be old reads for those in this thread, but these publications tend to have come to very different conclusions than what’s been discussed here.  Perhaps the information doesn’t meet with the methodological rigors of our forum participants, however, in which case I apologize.

 

-Tyler

 

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Well LT: the answer to temperature was right under my nose. Temperature increase= increased ionic movement. Temperature decrease =...basic thermal principles. Apparently 60F is the point that motion decreases to the point where it effects the  negativity created by cation exchange. Less negativity moves the solution from defloc to floc.- which in turn particle stacking: which in turn produces anywhere from warping to compromised green strength.

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I made a comment in a different thread about large covalent sodium ions disrupting the water membrane. That relates back to the age old question of the difference between calcium and sodium bentonite. Why does sodium bentonite congeal, while calcium does not?

calcium is a smaller ion, with less energy than sodium. Calcium does not disrupt water molecules as noted below:

W= water. Ca= calcium

image.jpg.5e5a02b5994269a890fa9a2091fdd3b1.jpgsodium is a larger ion, with stronger ionic energy that looks like this. The sodium ion disrupts the water membrane, which in part plays a major role in creating the gelatinous nature created by sodium bentonite.

image.jpg.0ae9654f4ad9cf891e00796f8e217185.jpg

 

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I have been reading some journals from D.D. Buttons and W.G. Lawrence on suspension, ionic charges, and other thing slip chemistry related. For those who have ever wondered about the particle sizes of Terra-Sig: 0.21 up to 0.67 microns or 25-30,000 mesh. 

To answer a second question: at 60F, the charges that hold particles in suspension drop by nearly a third at this temperature, and decreases further as temperature drops. The term is called particle stacking: which means the negative charges that hold particles in near perfect alignment diminishes: with larger particles dropping out of suspension. This ionic disorder results in warping of fired pieces, with increase in absorption. As the temperature increases above 60F: the ionic charges increase: resulting in a high density green ware and absorption below 2%. ( if formulated correctly.) 

Tom

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