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Define Plasticity

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Forum has been a little slow lately; so its a good time to ask this question: How do you define plasticity?  Practical, technical, or common answers are fine by me. Curious to hear what plasticity means in the pottery world in general.

Nerd

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Guest JBaymore

Ask 10 potters............. and you're gonna' get 30 answers.  ;)

 

best,

 

...............john

 

PS:  Interested in watching this one also.

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You know it when you see it.

 

I was taught that a basic test for plasticity was to roll a coil and bend it. If it bent without cracking then it was plastic, if it cracked it wasn't plastic. It didn't really measure degrees of plasticity, though. When talking about plasticity in regards to throwing, I think of how easily the clay will stretch. Porcelain, known to be low in plasticity, does not go where you push it- there's a rubberiness to it, it pushes back. You have to be more aggressive with it to get the form you want. Plastic bodies stretch very easily, and you have to be more careful or they'll go past where you want them. Make a pitcher spout with both and you'll get my point.

 

Everyone talks about adding plasticity to their clay bodies, but I think many commercial clay bodies are overly plastic, especially the very smooth white stoneware bodies and the cone 6 'student' grade bodies, like those typically labeled 'Buff'. They're much too high in ball clay for my taste, with very poor particle size distribution. But ball clay is cheap so they use it a lot.

 

When I worked for A.R.T. Clay we had a buff body that was mostly ball clay. It was our 2nd most popular body, and took twice as long to pug because it clogged up the vacuum chamber in the pug mill so badly because it was so sticky. The absorption rate was also too high. Whoever formulated it was an idiot. I added a bunch of feldspar and kaolin and silica and reduced the plasticity a bunch and got the absorption rate down where it should be. It then ran through the pug mill really nicely, was much nicer to work with and glazes were brighter on it. The materials cost to make it did go up a penny a pound, but the production output doubled, so that was okay. Not a single customer complained about the change. Ball clay is only good is moderate amounts. And should never be used in porcelain, IMO.

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Neil said it, roll a coil then bend it if it doesnt crack it's plastic. I actually prefer Little Loafers which some people think is too plastic but I like that I can push its boundaries and not have it crack. I've tried the Brownstone also made my Highwater and it's just not, you bend it wrap it to to form it like LL and it just cracks as you manipulate it... very annoying.

 

That is just this potter opinion.

 

T

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I was taught the same Neil, and over the years have adhered to that basic definition. However, I have found several adjectives to modify it, like firm, throwable, gritty, weak, and others. Most of these refer to the state of the clay when throwing, handbuilding, or pulling handles. Different clays for all of this, and for me the best balance is the clay that will do all three, as I often do a combination of techniques with the same clay.

 

 

best,

Pres

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Neil said- They're much too high in ball clay for my taste, with very poor particle size distribution. But ball clay is cheap so they use it a lot.

Cheap is the modern formulation criteria. Also depends upon the ball clay: no ball clay = no plasticity, unless very pricey additives are used. FHC (Foundry Hills Cream) is widely used, because it is very plastic and very cheap: but it is also very dirty. You know when FHC is used by the grey color it adds to the body. Plasticity is a loosely applied term in pottery, like many things clay. EPK (Edgars Plastic Kaolin) is not very plastic at all. OM4 is viewed as very plastic, but in fact has medium plasticity.

Is plasticity our term for elasticity? Is memory a direct reflection of plasticity?

Nerd

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Guest JBaymore

I don't totally agree with the argument that no ball clay = no plasticity. Fireclays and stoneware clays certainly have plasticity. Maybe not the same as ball clay, but they're way more plastic than kaolin.

 

I'm with that thought also. 

 

The best clay body I've ever used is from Japan... and there is no ball clay in that.   Mostly (90+%) it is a naturally occurring stoneware clay from 1/4 of a mile from where we are making pots, plus a little fireclay addition from Shigaraki.  The processing is the key to the amazing plasticity and strength.  Dug with heavy equipment, left outside in a pile to "age" for a year, run thru three separate blunging operations (first one screening out the bigger stuff), filter pressed, batch mixed in a blade type mixer, then pugged.

 

Feels nothing like any clay body I've used in the USA.  When you first touch it for something like wedging... you say "UGH!"  Grainy, soft, mushy, come to mind.  You'd swear that you'd never be able to work with it.  Then you put it on a wheel............

 

best,

 

........................john

 

PS>  Fired to Orton cone 14.

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It is more relevant to materials like metals, plastics and finished ceramics. More about the strength and breaking than working properties. It can still be applied for clay but... well... mumble mumble mumble.... so how about an example with cheese?

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Is plasticity our term for elasticity? Is memory a direct reflection of plasticity?

Nerd

To the first, yes. To the second, no. Porcelain is considered to be one of the worst offenders for having "memory," eg if you accidentally bump a piece while it's wet and push it back into place, you can expect the deformation to reappear in the kiln. It is also considered to be one of the least plastic clays to throw with.

These of course, are broad generalities.

 

I was taught that plasticity has to do with how well the clay particles are wettted, as well. "New" clay that has been mixed in something like a Soldner mixer but not pugged is quite short in the first six weeks. Conversely, the same recipe that has been slurry mixed and dried to a working consistency is much easier to work with right away.

It was also common practice during my education to speed up the clay aging process with some sort of lactobacillus (tub o' plain yogurt), vinegar, or urine (frowned upon in a group setting) in a fresh batch of clay. A few cups of reclaim in a fresh batch were also a recommended plasticizer. The idea was that acidification from bacteria (yogurt, urine), or just straight vinegar contributes to plasticity.

 

I recognize that some parts of this information are more accurate than others: I'm just relating common notions.

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Callie:

There is a specific scientific explanation that defines plasticity: but potters do not throw theorem. :)  The ability of the clay to absorb water (WOPL) is the correct application. EPK is 26 by the way, most fireclays run between 26 to 30: ball clays run up to 38, bentonites, hectorites, and gums even higher.

 

Do the college text books define plasticity?

 

Nerd

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Do the college text books define plasticity?

Does it matter? I thought you wanted to know how potters defined plasticity? ;)

 

It is what it is to each of us, mostly defined by some point of reference (using a clay body) in our potting experience. Hard to quantify or define what is so very subjective.

 

(Back to the cave.)

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Does it matter? I thought you wanted to know how potters defined plasticity? ;)

I do, and many have added their subjective opinions. Just curious if there is a text book definition? My thoughts are like clay, responses mold and manipulate them.

 

Terri.. nice pic of natural strata- the ability to move and conform to movement- good definition. By the way, Borax is used in metal refining- because it is a mineral with stronger properties than synthetics like carbonates. Not sure that is the reason, just a hunch.

 

Nerd

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So when I thought about plasticity and elasticity a bit more...

 

Materials first change in an elastic way. Like stretching a spring just a little. It returns to the original position.

Then the will deform in a plastic way. Meaning they take on a new shape. Like stretching the spring too far.

Finally, they break.

 

Now, most cheeses are actually elastic instead of plastic. The frozen cheddar still lacks a good plastic region.

 

---

 

With clay, the elastic part is "bad". This is property is part of what makes the troublesome memory.

The plastic aspect is good.

The breaking (short clay) is bad.

 

I guess an "ideal" clay would stretch forever and then not spring back at all. Unless that would be too much like chewing gum :-)

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Guest JBaymore

 

 

I guess an "ideal" clay would stretch forever and then not spring back at all. Unless that would be too much like chewing gum :-)

 

The ideal "clay" is........... hot glass!  Most plastic material I've ever used.  If I had discovered in in undergrad school...... could have derailed clay.  BUT.......... you can't TOUCH it.

 

best,

 

......................john

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"Plasticity" as discussed amongst potters is also highly subjective.

A number of the visiting artists that would come through the sessional teaching position at ACAD back in the day would complain without end about how short and unworkable the local clay options were. At the time, the local supplier was only getting stock from Plainsman, and B mix was an exotic import. Because Plainsman is more or less what I learned on and is most of what I've ever used, I have no problem with it.

 

Edited to add: the way plasticity is used amongst clay people is a misnomer. It would be much more accurate if we used the word "workability."

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Guest JBaymore

 It would be much more accurate if we used the word "workability."

 

And what you want in a throwing body is not what you want in a handbuilding body. 

 

best,

 

................john

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