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 22, 2018 at 6:00am

Cone 6-10 Firing Schedule for dark or red bodied clay bodies.

"Originally published in May 2018 issue of Ceramics Monthly, pages (68-69). http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission."

These bodies are typically high in iron; but all include higher levels of carbons such as sulfur and lignite coal particles.  White stoneware bodies are excluded. Firing defects include bloating, carbon coring, and blistering.

There is a division among researchers dating back to the mid 1920’s on the exact mechanism that causing these firing defects. Roughly half concluded that CO gases escaping from carbonaceous materials reduced the iron. The other half concluded that the sulfur (sulfides) from lignite coal and/or from iron sulfide reduced the iron. Due to modern milling techniques and my own experiments in this area: I tend to lean to the sulfide part of this equation.

Either way, bloating, coring and blistering is caused by the early reduction of iron: causing it to act as a vigorous flux which in turn begins to form glass as early as 1850F. This is nearly 200F degrees earlier than normally found in a firing cycle. The early formation of glass from reduction of iron creates an impermeable barrier which traps off gassing feldspars, and traps any remaining carbons.

Pyrite (iron disulfide) is the most common source of iron in raw clays. Iron is what gives stoneware that warm toasty color potters love, and the red/brown colors. However, the sulfur that is chemically bonded to the iron is problem child.  Heat too fast, and starve the oxygen in the kiln: and bloating, coring, and blistering will be the result. Reduction can occur in an electric kiln that is heavily packed, sealed, and heated quickly.

*Edward Orton “The Role Played by Iron in the Burning of Clays”. Amer. Cer. Soc. 7:112

A.G. Bole/F.G. Jackson “The Oxidation of Ceramic Wares during Firing. Amer. Cer. Soc. 7:183

Kramer/Fritz  “The Role of Oxidation in Porcelain and Ball Clay”  Amer. Cer. Soc. 12:13

While these researchers reported their findings mostly in the 1920’s: Brownell, West, and Lawrence found similar results in the late 1950’s and into the mid 1970’s.


From this research the following temperatures were reported  as carbon burnout phases:

Jackson reported that carbons began burning off at 800F, and accelerated to 1760F.  which others confirmed. Orton believed sulfurs were completely removed between 750-1110F. Others concluded from 750 to 1700F, all carbons were burnt off. The consensus among all the researchers was that high levels of oxidation were required to completely remove all carbons. In the research done nearly thirty years later: the consensus was carbon removal occurred mostly between 1200-1750F. It was Orton who concluded that ferrous sulfide began vitrification as early as 1800F; which resulted in carbon trapping, and trapping of off gassing spars. F.G Jackson performed that most extensive laboratory research of the effects of sulfur, including decomposition and the evolution of reactions on other materials.

Two primary results came from all of these studies.

  1. The kiln must be heavily oxidized, especially from 1200-1750F. Prop the lid open slightly, remove bunges if need be.
  2. The ramping temperature must be slowed down. The higher the iron level and carbon content: the slower the ramp needs to climb. (discussion to follow).

Orton originally recommended a rate climb of 75F an hour for high iron/carbon clay bodies: which was later adjusted to 108F an hour. Others suggested a rate climb of 125F an hour in the 1200-1750F burn out zone. Only testing will conclude which rate is correct for the clay body you use. However, as a rule of thumb: the higher the iron content- the lower the rate of climb.

Observations and conclusions I have made from testing:

  1. If carbon coring is occurring: oxygen needs to be increased and ramping rates lowered by up to 60 degrees an hour. Carbon coring indicates that heavy reduction of iron is occurring around 1800F and high levels of inorganic carbons are present.
  2. Bloating occurs when iron is only partially reduced: creating patches of glass instead of the hard shell associated with coring. This means some off gassing spars are being trapped, while some is escaping. This indicates a partial starvation of oxygen and iron being reduced in the 1900-2000F range: just prior to off-gassing spars. Ramp speeds need to be lowered by 20-30 degrees, and pulling a bunge or propping the lid to add slightly more oxygen.
  3. Blistering (large craters) indicates that minor reduction in iron is occurring. A complete barrier has not formed, but the clay surface has become extremely dense causing escaping spars to push through under more pressure. Slowing down the ramp cycle by 10-20 degrees an hour during the burn out temperatures should resolve this issue. Pulling a bunge if the kiln is heavily loaded is also recommended.

I cannot give you an exact firing schedule because red bodied and dark bodied stoneware has such a wide variance of iron and carbon levels. I can tell you that ramping between 108-125F an hour from 1200 to 1750F, while supplying lots of oxygen inside the chamber will resolve most all of these issues. If they continue after these recommendations: you have a heavily contaminated clay body that is probably best just to discontinue its use. It is also advisable that you do not start any type of reduction until 2050F, when a normal clay body begins to seal up.

“Carbons” are most often used in an inclusive/broad sense when addressed in most pottery books. Organic carbons such as peat, twigs, bark, and other decayed plant matter burn off with little effort and no effect. Inorganic carbons such as lignite coal, iron pyrite, or other forms of sulfides are the primary contributors to coring, bloating, and craters.


Comment by Tom Anderson on March 8, 2018 at 3:42pm

"Originally published in April 2018 issue of Ceramics Monthly, pages 60-61s).http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission."

Article discusses clay formulation, with numerical values that determine minimum limits for functional use.

Comment by Tom Anderson on December 27, 2017 at 2:50pm

Cation Exchange is the chemistry behind clay plasticity. To learn more about it, including how to use it in clay body formulation: read Cation Exchange on pages 60-61 in the January 2018 edition of Ceramics Monthly.

Originally published in January 2018 issue of Ceramics Monthly, pages (insert page #s). http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission."

Comment by Tom Anderson on April 8, 2017 at 10:33pm

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 12:09am

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 7:53pm

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 3, 2017 at 11:50pm

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 6:55pm


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 2, 2017 at 9:45pm

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 7:57pm

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.



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