Clay Tips for Studio Potters

[Tom Anderson]

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).

Tom

[Norm Stuart]

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.

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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.

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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.

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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.

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

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

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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.

[Tom Anderson]

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.

Tom

[Tom Anderson]

Norm:

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.

Tom

[Norm Stuart]

Unless you have enough volume of clay scrap to justify purchasing a pug mill, my feeling is you're better off tossing clay scraps. For our studio a pug mill would cost more than we spend on clay over a 5 year period.

I noted how long the slaking and wedging process took a volunteer to accomplish - and they'd be far better compensated working part-time at McDonalds. I suspect the work at McDonalds might not only better paying, but also less strenuous and more interesting as well.

Our economics are influenced by our location, within 30 miles of Laguna Clay. So our clay cost after a 30% volume discount (with a $35 delivery charge for 1,000 pounds or more including all raw materials) makes it pretty inexpensive.

In fact, I can honestly say clay is cheaper than dirt. At least it was the last time I last bought a 50 pound bag of dirt from Home Depot.

[Tom Anderson]

Firing cone 5-6 stoneware bodies.

Firing to cone 10 achieves enough heat work, and requires more time: the combination usually resolves any issues related to lower cone firing schedules. In addition, iron becomes a much more active flux at cone 10: adding to the maturity/vitrification of the body.

Stoneware clay relies primarily on potassium as a flux: because many of the clays involved have naturally occurring levels of potassium to start with. Potassium is a late bloomer: meaning it begins to melt at a higher temperature than sodium: the common flux in porcelain. The later melt also means it is still off gassing, after sodium would normally have stopped.

The other issue is mullite formation in a stoneware body. Metakaolin changes to spinel around 2050F: and spinel is the basis of mullite. Spinel / mullite is formed from alumina and silicate. At 2180F, potassium has fully fluxed, and mullite production is at its peak. So if you are ramping through the 2050F to 2180F range too quickly: you are actually hindering the clays ability to mature/vitrify. In addition, you are also blowing through the period when off gassing is at its peak.

 

To Remedy (firing bisq)

Use your normal cycle to 2050F

130F from 2050F to 2190F- with extended hold.

OR

130F to 2230F- with much shorter hold.

 

** Kiln size will determine hold cycle.

 

Using this ramp cycle will mature the clay: which is much more important when firing functional ware. In addition, the incidence of pin-holing will be greatly reduced: if not completely resolved. If you are still getting minor pin hole issues: either slow the top ramp to 125F an hour, or add 10 minutes to your peak hold.

Tom

[Tom Anderson]

Fired Brightness

 

Whiteness and brightness only becomes an issue for stoneware when it is used for functional ware. Obviously the color of the clay can alter the glaze color: so whiteness in stoneware in meant to duplicate the whiteness of porcelain for glaze color and brightness. Most ball clay do not give the amount of carbon or sulfur content: so you have to gauge how bright the fired result will be using other values.

 

Clay                 Color                   Alumina             FE            LOI

 

M28                Lt. Brown             25.60                0.87           9.01

No.5               White                    29.01                0.88           9.79

M33                Dark Brown          25.01                1.71           17.0

No1 (glaze)     White                   29.37                0.87           9.97

Taylor              Lt. Tan                 26.40                1.06           9.98 

C& C               Off white              26.29                0.92           10.40           

 

(LOI) Loss on Ignition: indicates carbonaceous materials that burn off when the kiln hits red heat levels.

 

The color of the clay does give some indication: but it cannot be relied on entirely. The carbonaceous material in the clay burns off: which is reflected in the LOI values. The higher the value goes, the more carbons in the clay. M28 has an LOI of 9.01 which has a color of Lt. Brown: but C&C is white and has an LOI of 10.40. So color alone is not an exact indication of carbon content. 

 

When ball clay begins to cross the 25.00 alumina mark: it begins to be classified as kaolinitic pending carbon content. No5 and No.1 both have over 29.00 alumina and are classified as kaolinitic ball clay, coupled with lower LOI values. M28 is also designated as kaolinitic having only 25.60 alumina: but very low LOI values. The other indication of fired brightness is iron levels. M33 has a high LOI value, and also a high amount of Iron (FE): the high carbon/iron content is reflected in its color. Even if a ball clay has a lower LOI value, but over 1.00 iron: it can still add color to the final clay body. To achieve high whiteness/brightness in a clay body: the ball clay must have lower LOI values, and iron should not be over 1.00%. 

 

Note: EPK and #6 Tile kaolin are often found and used in white stoneware bodies for functional use. EPK will add high SAS value, in addition to some plasticity. #6 Tile provides a high alumina content, and a larger particle/platelet size for added body. They can be used separately or in conjunction with each other. It is not uncommon to find either or both in amounts up to 25% of the total recipe. The addition of kaolin/s with ball clay is the basis of a 50/50 porcelain body. When fire clay additions are made up to 15%, the property is often called "tooth."

 

In regards to using ball clay for glaze additions: No 1 (known as SPG1) is a much better choice than C&C ball clay. It has a lower amount of iron, and much higher amounts of alumina. Alumina is an important component for glaze because it adds strength to the glaze: which helps guard against cutlery marks. In addition, the lower LOI value will also help lower the overall COE of the glaze.

 

As you begin to learn the various clay values: it will make selecting ball clay easier. Of the ball clays listed: Taylor is the optium overall choice because is has higher alumina, and lower LOI values. More importantly, it has a WOPL value of 38: meaning it is a highly plastic, kaolinitic ball clay. In addition, it has an ultra fine sub micron particle size of 0.31: one of the smallest of all ball clays.

 

Carbon Coring: if you have ongoing problems with carbon coring after you have adjusted your firing schedule: using this section for whiteness will reduce the carbonaceous materials in your recipe.

Tom

[Tom Anderson]

The issue of drying, hardening, and water retention in reclaimed clay is a different issue. It comes back to the WOPL (water of plasticity) principle of ball clay. It is the ball clay in stoneware (primarily) that governs water absorption: a ball clay with 33WOPL retains less water, and a ball clay with 38WOPL retains much more. So the effect, what you are speaking of is determined by the amount of ball clay (ultra-fines) that is lost.

 

How do you determine the level of ball clay the recipe had to begin with?    The Cream Test

 

When you throw the original clay: how much cream comes up and on your hands?

 

Coats just the inside of your palms and oozes through your fingers over time.... lower levels.

Coats your palms, and oozes; have to clean a few time while throwing.... mid levels

Oozes quickly and constantly cleaning off hands....... high levels.

 

Most of the reclaimed scraps is from trimming; which has been stripped of the fines; which includes ball clay, silica, and feldspar. That would alter the properties of recycled clay: because it is the ball clay primarily that holds moisture in a clay body.

 

The fix:  blend 80% ball clay (Om4 or FHC), 10% silica, and 10% feldspar. Add 1 cup (dry) per gallon of slurry. The testing comes when you throw it after it has been reclaimed: how much cream comes up when you throw? Adjust to suit your taste.

For Porcelain: 80% EPK, 10% 325 silica, and 10% Nep SY. Add 1 cup per gallon of slurry.

Tom