Why aged clay is smoother?


Stoneware in particular changes characteristics over time, but all clays do to some degree. The common thought is because of bacterial growth (fungus/mold, etc. Bacterial growth is a reflection of how much organics is in the clay itself (ball clay primarily). If you are getting a lot of bacterial growth on your clay: it indicates high levels of organics: which means you need to bisq slightly higher, or with a hold to burn them off completely.


The "aged" effect is actually due to the clay particle itself. On a molecular level, clay particles look like Swiss cheese: porous. When you first mix clay it is all soft and gooey because the water is binding the clay particles together. However, when you bend or twist it: it has the tendency to snap because it is "short." As time passes: molecular H20 penetrates into the molecular pores of the clay: and then the full plasticity level of the clay is obtained. (WOPL= water of plasticity). You will also notice a change in consistency from very soft when first pugged, to various degrees of firmness as time passes. The clay has not lost moisture content, it has absorbed moisture content. Which is also the reason blunged clay is more plastic than pugged clay: because mechanical forces speed up the process of absorption.


Normally within 30 days there is a marked difference, which improves over the next 90-120 days. After about 6-8 months, the process begins to reverse because the clay is actually starting to loose water: dehydration. Absorbing water is hydration, losing water is de (loss of).


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Comment by Tom Anderson on April 9, 2017 at 1:33am

After years of studying, researching and testing: I made up three new porcelain bodies. Half of the ingredients come from standard pottery sources, half do not.

Body A is as plastic as any stoneware body.

Body B I threw 4" x 6" cylinder that weighed 3/4lbs. @ bone dry.

Body C I threw 4" wide by 8" tall with one pound of clay.

Body D still was wet enough to trim after three days.

** Now I need to combine all these properties into body E.


Comment by Norm Stuart on April 5, 2017 at 3:09am

Of course I'd like to read them. Post the links. You'll notice in the excerpt below that cation exchange with alumina depends on pH.

Comment by Tom Anderson on April 4, 2017 at 10:53pm

Norm: well said.. TY

Lots of potters get offended when I suggest new ideas or possibilities. We both have inquiring minds, we like to know in detail how something works. I will post some research links later if you care to read them. Most stated a direct connection between alumina levels and cation exchange. Which if our industry would really explore and apply those findings: then we could begin writing formula limits for clay bodies. Thanks for your input and insights- much appreciated Norm.


Comment by Norm Stuart on April 4, 2017 at 2:50am

I know cation exchange capacity is widely used in agricultural sciences to determine the ability of a soil to hold water, which is to say "being flocculated". It also addresses the pH change in flocculation.


"The main ions associated with CEC in soils are the exchangeable cations calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) (Rayment and Higginson 1992), and are generally referred to as the base cations. In most cases, summing the analysed base cations gives an adequate measure of CEC (‘CEC by bases’). However, as soils become more acidic these cations are replaced by H+, Al3+ and Mn2+, and common methods will produce CEC values much higher than what occurs in the field (McKenzie et al. 2004). This ‘exchange acidity’ needs to be included when summing the base cations and this measurement is referred to as effective CEC (ECEC)."

I did a lot of reading on soil science trying to discover the potential overlap with ceramic as farm scientists seemed more fact based in their analysis.

It's an interesting idea that bond polarity increases as alumina content diminishes. I'd love to know what the feedback is from the people at Alfred University. There's so much voodoo rather than science in ceramics that I'd love to be certain it's alumina level rather than the coincidental particle size because surface charge relative to particle weight make a lot of physics and chemistry sense to me.

I've always been skeptical of the idea I have heard from clay vendors and some on CAD, including Baymore, that clay plasticity can't be measured. Laguna Clay provides a penetrometer reading - how a spring loaded device penetrates that type of clay - as a proxy for plasticity, but it's really a measure of compaction.

Clearly the plasticity of a particular clay blend is going to vary significantly depending on the water content, but it's not intuitive to me why it should be "unquantifiable".

Most ceramicists use the polymers Darvan 811 and Darvan 7 as deflocculants, which the Southern California Metropolitan Water District calls a flocculant becuase the polymer strands bind to particles in the drinking water so they can be filtered out in sand beds.

I also ran across a very costly "clay texturizer" and looked up the MSDS. It appeared to be an ordinary polymer floor wax of the type schools would use once a year marked-up fifty fold in price. Typical.

As you say, I suspect the possibilities are limitless, especially when combined with CAD 3D extrusion machines.

Comment by Tom Anderson on April 3, 2017 at 9:55pm


Perhaps if I illustrate it, it will make more sense. Particle size is used in the clay trade because it is the most commonly known theorem.

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.

As particle sizes  decrease, so does the alumina levels. As the alumina level decreases, so does the bond polarity. There has to be a negative charge for plasticity to occur. So yes in the clay arts it is equated to plasticity; but in chemistry negative bond polarity is the actual mechanism. (Cation exchanges).Every abstract I have read written by Phd's in soil sciences all contribute plasticity to cation exchange that is directly attributed to alumina levels.

John Baymore, the professor at New Hampshire Arts sent me the email for a couple of professors at Alfred, and recommended I email them to discuss this further. Have also emailed back and forth with Tony Hansen, Ron Roy and a few other clay junkies out there. Clay Arts have sorta fallen behind on modern technology and discovery: most of the books are just rehashed materials from the 50's,

There are all kinds of polymers and other ionic agents out there, that are not used in our trade. I have been playing with a couple of them. Yesterday I threw a small bowl without using any water-- lots of things are possible.


Comment by Norm Stuart on April 3, 2017 at 12:45am

The diagrams are from the Alfred University Graduate course on ceramic materials, a 10-part sylabus.  I've uploaded them on this website someplace.

Comment by Tom Anderson on April 2, 2017 at 10:57pm

Hi Norm:

Looking at your material, looks like it came out of a studio potter handbook. Particle size is relevant in clay body formulation: more so applicable to stoneware due to the large particle fire clays involved. On the Ceramic Arts Daily Forum I have written extensively about WOPL, particle sizes and distribution. The pottery industry is the only one that uses particle size as a measurement for clay size: all other industries that I am aware of use SAS (specific area surface) or SSA (specific surface area. (same thing). Particle size is only the face of the particle, but does not include platelet depth. A clay particle may be sub micron in size, but its platelet may be larger than one micron: the reason only the pottery industry uses it.

Although there is a measure of truth about particle size and plasticity: it is not entirely accurate.

In research articles studying plasticity of soils, clays, and monmorillonite (bentonite) minerals: the cation exchange capacity is solely dependent on the alumina level in the particle itself. The pottery industry classifies monmonillonite as a clay, it is actually a mineral. The again the pottery industry uses a lot of definitions for the ease of application that are not entirely accurate. There are numerous organic negative ionic polymers that will impart more plasticity to clay; than any clay itself. V-gum T is an example: synethized from hectorite; but a polymer non the less.

While bentonite is the most commonly known and used in the pottery world, the smectite clay classification which includes hectorite imparts much more plasticity: from which macaloid is processed and refined from. Ball clay/s are typically sub-micron in size: their plasticity comes from the lack of alumina. De-flocculation is the most dramatic effect of cation exchange a potter will see visually, but it gives insight to how CEC works.


Comment by Norm Stuart on April 2, 2017 at 10:02pm

Tom - The monograph you posted about plasticity is correct in noting the differential structure between clay and the double layered structure of montmorillonite which allows in additional water in the Montmorillonite.

Particle size plays a much larger role in plasticity - the smaller a charged particle, the more plastic it is.

The level of flocculation always plays an important role in plasticity.

Clay absorbs more water over time, with Brownian motion, becoming more plastic -- but older clay stored in plastic also loses water due to evaporation through the plastic bag and becomes less plastic.


Why is the size of the charged particles so important?

The surface is the result of the size squared while the volume/weight is the result of the size cubed.

The ratio of the surface charge to the particle weight increases as the charged particle is smaller in size.  Thus more plasticity.


This to scale illustration makes the reason for this more obvious. 

A plastic clay or suspended glaze need tremendously of the tiny little clay particles to fully coat the surface of larger particles like Grog, Feldspar or Silica. 

Ball Clay is 1/10 the size of Kaolin and Bentonites are much smaller than Clay.


Clay is not generally porous. But the larger particles of material like feldspar do exhibit porosity due to chemical weathering. The interior surfaces in the pores of these larger particles can become coated with clay and water over time.



This is how much smaller Ball Clay is than Kaolin. Bentonites are even smaller still.


Here's a list of additional materials in particle size in numbers.


Finally, increased flocculation increases clay plasticity by rearranging the structure so more water is held within the clay agglomerate.

Increased flocculation is typically due to an increased number of free Calcium and Magnesium ions.

Comment by Tom Anderson on April 1, 2017 at 8:57am

Understanding plasticity,  memory, and general clay properties.

For those interested in the chemistry of clay:

CEC (cation exchange capacity) is the exact mechanism behind clay plasticity. It is common knowledge among clay junkies that the particle charge determines plasticity. A negative charge means the particles are repelling each other: which is commonly expressed in the clay arts as; "sliding by each other." A neutral or positive charge then means clay particles are attracted to each other: which is the basis of memory. Mechanical forces (throwing/rolling) distorts the grain boundaries formed by a positive charge: but the positive charge attracts those distorted particles: drawing them back into a positively charged particle alignment. 


Google "cation exchange capabilities for more information." See table at bottom of page.

As you can see: kaolin and halloysite (new zealand kaolin) have very low CEC values, while montmorillonite (bentonite) and vermiculite have very high cation exchange capacities. (CEC) Vermiculite is classified as a 2:1 clay structure: which is the same structure classification given to ball clay. In simple terms: a 2:1 clay structure has two inner platelet structures, and one exterior platelet structure. Kaolin is a 1;1 structure: meaning is only has a surface platelet, with no interior structures. This is the important distinction between porcelain and stoneware: because the clay variety dictates how the formulated body reacts/acts. A 1:1 clay structure (kaolin) can only absorb water onto its exterior platelet, while a 2:1 clay structure can absorb water onto the face of the platelet, but also into its interior structure. This particle structure then effects the clay type ability to A. carry its own electrostatic charge, or B. conduct/transmit a electrostatic charge.


Kaolin has a 1:1 particle structure: so it can only absorb water onto the face of the platelet. Secondly, it has a neutral electrostatic charge (polarity), so that neutral charge means it has no ability to influence or transmit that charge to adjacent particles or other elements mixed with it. This property means kaolin will therefore be readily influenced by whatever electrostatic charges of the materials mixed with it: and that charge will only be carried onto the face of the platelet. Sodium and potassium both have strong positive electrostatic positive charges: sodium moreso than potassium. So when flux is added; the positive charge of those fluxes are carried on the platelets of the kaolin. A positive charge then A. greatly reduces plasticity because a negative charge is required for the particles to slide past each other. B. increases memory properties because the positive charge attracts the adjoining particles when mechanical forces separate or distort them.


Ball clay has a 2:1 particle structure; which can absorb water onto its exterior face, but also into it secondary platelet structures. Ball clay has a naturally occurring negative charge: which is why ball clay is so plastic. The negative electrostatic polarity resides both on its platelet face, but also inside its interior structures. In this case, when positively charged fluxes are added: while the positive charge effects the platelet face: the interior structure retains its negative charge. The negative charge is the over-riding polarity in the clay body (stoneware) and its plasticity is preserved. In addition, the negative charge is the over-riding influence in the body: effectively eliminating the memory caused by a positively charged clay body.


The 1:1 structure of kaolin limits absorption of water; being bond to its platelet face only. This also determines it's drying time, because the platelet surface shed water at an accelerated rate: causing a much more rapid drying time. Ball clay being a 2:1 clay structure absorbs water onto its platelet face and into the secondary inner structures. This results in a much longer drying time because the water absorbed into the interior secondary structure takes much longer to dry. The absorption of water into its secondary structure is what also determines how soft or firm a stoneware body feels to throw pending on the ball clay type, the percentages used, and the amount of fire clay added. What feels like :mass", is actually the additional water absorbed into the interior structures.


The other key in differentiating kaolin and ball clay is alumina levels. Kaolin have much higher percentages of alumina compared to ball clay. While it is widely accepted that kaolin and ball clay are both aluminosilicates separated only by the amount of carbons: that is not a true assessment. Alumina is a metal oxide that is solely responsible for the lack of a negative charge in kaolin. Alumina typically has a much higher positive charge (up to 3x) that of silica: the other major component in clay. The lack of alumina in ball clay is what effects its valance polarity: being negatively charged giving its plasticity. The  higher levels of alumina in kaolin is what causes it to have a neutral electrostatic charge, causing it to have much less plasticity; and much more memory. While kaolin and ball are both clays: it is the alumina levels that makes the distinguishable differences between them.


Stoneware being mainly comprised of low alumina ball and fire clay does not require additions of negatively charged ball clay or polymers to produce its plasticity. Porcelain being primarily kaolin, requires negatively charged ball clay or polymers to create its plasticity. In addition, the structure of the clay also determines how much water the clay particles will hold: which also determines how fast it will dry.


Comment by Tom Anderson on December 26, 2016 at 1:08am


Given your unique proximity to a major supplier, I could understand the position. Economics usually drive most decisions around the studio. I know more than a few that have drive over 2 hours one way to find clay, and many more that drive 4 hours one way. So everyone has their own motivations to reclaim; or not.

I keep five gallon buckets around: over a span of time when they get filled: then I reclaim. Do not need a pugmill, most potters do not own one: they just slurry scraps down and wait for it to dry down. I can reclaim 50lbs in under an hour: not that complicated to do. So the helpful tips are for those whose economics require it. I use to buy clay by the ton, I just make my own these days.



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