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Stoneware Clay Properties

stoneware plasticity pliable throwing

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#1 glazenerd

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Posted 18 February 2017 - 10:34 AM

I have been studying plasticity in stoneware bodies, as most know. I am finding some results that are making me question the accepted belief that plasticity equates to ease of throwing. Plasticity in general comes from the electrostatic charges on the clay particles; which changes as the body ages. I am looking for articles that specifically review the relation of sub micron ball clays, to the ease of throwing. I am trying to determine/figure out how mass plays a role in throwing.

 

Ron Roy and I had this discussion at NCECA; what is the cut-off point for large and intermediate mesh sizes, before those additions create a denser mass: which makes the clay harder to push around on a wheel. It is very common for stoneware bodies to have 80% total clay content, there are some even higher than that. So I still find myself questioning if mass is playing the larger role in determining if a clay is easier to push around? Not sure if I am articulating my thoughts correctly, but hopefully I have made the question clear enough.

 

As a comparison: everyone knows how easily porcelain moves around when thrown. The most common analogy is that it throws like cream cheese. That is because porcelain in general has 25% silica, and 25% feldspar; which has much less mass than fire clay. One of the major differences is mass: stoneware has more clay content; and much larger particle sizes. I have tested this theory by adding V-gum and macaloid to high percentage formulas of fire clay/intermediate clay. These additions are not the norm; solely done to test if plasticity is the determining factor in ease of throwing.

 

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#2 Pieter Mostert

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Posted 18 February 2017 - 02:20 PM

By 'mass', do you mean density?



#3 Tim T

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Posted 18 February 2017 - 03:05 PM

How are you defining/measuring plasticity in this instance?

Off the top of my head, there are probably at least 3 different factors:

- the force needed to move the clay, i.e. its resistance to your hands on the wheel

- the plasticity, taken as how much the clay can be deformed without rupturing - how tight a radius can a coil of clay be bent into before cracks appear on the outside

- the ability of the clay to support its weight in the thrown object, e.g. if you do a wide shallow bowl, will it collapse

The first and last are interrelated, except thixotropy means that a static clay object may support itself, but become more fluid when throwing.

Also, obviously, for a given clay changing the amount of water will have a significant effect.



#4 glazenerd

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Posted 18 February 2017 - 06:49 PM

Peter: density would be the better term.

 

Tim: the force required to move the clay.

 

Simple data: porcelain has 50% clay, and stoneware 80% (generalized). So the weight of the clay by volume obviously effects the amount of force required to move it. Plasticity by simple definition is the platelets sliding by each other: less drag, less friction-etc. There seems to me that a link is missing in here somewhere. Trying to figure out specifically if the amount of large particle fire clay (mass) is directly related to the throwing properties of stoneware: apart from plasticity. When I have blended high percentages of large particle fire clay with plasticizers: the body resists movement. Conversely, when large particle clay amounts is reduced, the body is easier to move. Which makes me question if plasticity is the only mechanical issue involved.

 

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#5 bciskepottery

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Posted 18 February 2017 - 07:56 PM

How are you measuring "force" -- by your own feelings while throwing or through some type of device that provides empirical data? In your comparisons, are all particle sizes constant with the only difference being the particle size of fireclay? And all other variables, e.g., wetness, are also constant? Just trying to limit the number of variables at play . . .

Wondering, if I gave a student a bag of Standard 181/182 stoneware and told him/her it was a Standard porcelain, would they know the difference? (You could likely pick two clays from any manufacturer, not singling out Standard). We seem to have so many preconceptions of how a clay should behave based on anecdotes or comparisons ("throws like crème cheese") that it may color our judgment or create a set of expectations.

#6 glazenerd

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Posted 18 February 2017 - 10:13 PM

Bruce: the only variable is force. I do however slab roll to 3/4" thick, and cut a six inch disc. Place it on the wheel, and use one finger to push against the sides until it begins to move. Empirical? it's clay. Even the testing currently used in studio does not meet the ASTM standards.

 

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#7 Pieter Mostert

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Posted 19 February 2017 - 12:37 AM

Have you measured the densities of the clay bodies you're using? Since this changes with moisture content, can you be sure you're comparing apples with apples?

 

Even if you can show there's a correlation between the density and the minimum force required to deform the claybody on the wheel, I'd be hesitant to conclude that there's a causal relationship between the two.



#8 Joe_L

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Posted 19 February 2017 - 08:31 AM

As well as mass (density) and plasticity I expect there will be all kinds of effects from elasticity & rheology too. Much of this will be non-linear and pretty complex. Anyone into geotechnics modelling? (The person I've met doing a PhD sponsored by industry in this area is experimenting with statistical methods to avoid the limitations of quantifying properties for direct modelling).

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#9 glazenerd

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Posted 19 February 2017 - 09:55 AM

Peter:

 

I am doing everything I can in studio to maintain some semblance of standards. All experimental bodies start with 10% feldspar, 10 % silica, 10% ball clay and 28 oz. of water. I chose the 10% standard because it is the most commonly used ratio in stoneware bodies. I kept the ball clay lower as well, because it is the primary component of plasticity. I also let it sit in a sealed bag for a week before use to achieve some level of saturation. Then I use the disc as described above: so what I am applying force to remains constant. The remaining 70% changes proportionately between 35M fire clay and #6 tile kaolin. I chose #6 because it is a larger grain, and does not add any appreciable plasticity. By only using 10% ball clay, I am keeping a measurable amount of plasticity as a standard. The best standard I could come up with, without spending $100K for lab equipment. Although, I have given thought to buying a digital force gauge: they are relatively inexpensive. I could place a small piece of 3/4 plywood against the clay, and push with the gauge behind it to give a simulated effect of the human hand.

 

Joe: I am very much aware that there will be variables, but I am controlling those the best I can. Then again, I am trying to measure clay properties, which to my knowledge have never been tested?  The reason I opened this thread with the request for articles: I have not found any remotely related. All I know from experimenting with clay bodies over the years: there has to be other forces at play in stoneware beside plasticity that effect its throwing properties. Bruce is correct: plasticity and ease of throwing is highly subjective in the clay world. Then again, yet another issue in the clay biz that irritates me because no one has ever set standards. If your friend ever publishes his works, I would certainly like to see it. Clay makers typically only give absorption and shrinkage: because neither do not require testing equipment. The few that give COE data, either own a dilameter, or sub contract it out to others for testing. So even the clay makers do not have ASTM standards for the most part: they just mix it and ship it.

 

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#10 glazenerd

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Posted 19 February 2017 - 07:16 PM

Obviously I am going to have to devise a test that does not rely on human interpretation- as Bruce suggested: empirical.

 

So if I buy one of these and use it:

 

http://www.tequipmen...CFdgcgQod1uQKDw

My thought was to use my six inch round disc, and make a tool that would ride on a 4" radius, and be one inch higher than the disc. (1 3/4" total height). The idea is to replicate the human hand on the clay. I would put the probe on the back side of this tool and push until the clay rises to the top of the tool. ( 1 inch).. However, I think it needs to do so in lets say-10 seconds?.... at a constant force?.. not sure here.. Thoughts?  The result would be a measurable rate that would qualify the amount of force to compress the clay; which would equate the plasticity and the density.

 

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#11 Pieter Mostert

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Posted 20 February 2017 - 01:48 AM

I'm still not sure I understand your experiment correctly, so let me try to summarize my understanding and you can correct me where I'm wrong.

 

You have two bodies

 

Body F:

10% Feldspar

10% Silica

10% Ball Clay

70% 35M Fireclay

 

Body K:

10% Feldspar

10% Silica

10% Ball Clay

70% #6 Tile Kaolin

 

When you add a given percentage of water (whatever 28oz translates to) to both bodies, body F is denser and more resistant to deformation than body K.



#12 glazenerd

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Posted 20 February 2017 - 08:46 AM

Peter:  close enough.

 

Porcelain:  50% kaolin, 25% silica, 25% feldpsar, 2% V-Gum T --plastic, easy to move around

 

Body F is harder to push, or any combination of clay that totals 80%. It is harder to push, and yet it has more plastic material content than porcelain. So if the generally held belief that plasticity is the primary component of making a body easier to throw: then by that same principle stoneware should be easier.

 

Above that however, I think an industry standard test needs to be in place that rates plasticity by a numerical scale. I was reading the thread about Continental Clay, and the comment was made that no technical information was given for the products. Clay arts is the only trade that I know of that has such lax standards and quality control for products. Kudos to the makers who care enough to at least supply some working information.

 

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#13 bciskepottery

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Posted 20 February 2017 - 11:54 AM

Above that however, I think an industry standard test needs to be in place that rates plasticity by a numerical scale. I was reading the thread about Continental Clay, and the comment was made that no technical information was given for the products. Clay arts is the only trade that I know of that has such lax standards and quality control for products. Kudos to the makers who care enough to at least supply some working information.

 

Well, with the terrific job industry already does with standards -- e.g., this clay body works well between cones 5 and 10 -- I'm sure they'd do a bang up job with plasticity ratings (-1 if the clay has dried out while sitting on the shelf for over 12 months and +10 if made yesterday and still mushy in the bag).  Clay arts doesn't seem to do well with standards (just look at the various glaze books and see the variations in limits for safe and durable glazes or see if there common agreement on what is food-safe, etc.).  Mostly because clay arts represent the bottom of the user world and carry none of the impact that large volume consumers (e.g., commercial producers) can demand from manufacturers.  In short, we get the left overs, are the last to know when composition of materials change, etc. because we are a marginal percentage of their bottom lines.  And, on a customer-by-customer basis, they will make a batch to whatever specs we give them, so those who have specific requirements can get what they want -- for example, Mark C. has his distributor mix half his clay to be softer knowing it will be sitting/aging for six months before he uses that part of the supply.  Such a rating may be useful to some potters like yourself who want a specific type of clay body for a specific technique, e.g., crystalline growth, but most would not have any idea of how to use that information.  And, as the clay ages, the rating is likely to become outdated and/or no longer precise. 

 

Standards should be reserved for things that affect health, safety; plasticity does not rise to that level of concern where we ask for and submit to regulation. 


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#14 Pieter Mostert

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Posted 20 February 2017 - 01:54 PM

I wouldn't try reduce plasticity to a single numerical value, since there are number of aspects to it, as Tim mentioned. This survey article mentions various attempts to quantify plasticity.

 

Ideally, to determine whether density determines resistance to deformation, you'd compare two bodies where you substituted one ingredient with one that's the same with respect to all properties except density. As soon you have other properties changing, you open up the possibility that these are contributing to the deformation resistance.



#15 Min

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Posted 20 February 2017 - 02:29 PM

If all suppliers provided the info and quality clays that Plainsman Clays does I'ld be a happy camper.

 

If you haven't had a look at the info they provide have a read through of the Overview then click on any of the bodies listed on the left of that page. I use a fair bit of M370 and find what it is described as is pretty much what you get. If anything it is a little overboard on the care needed while drying and firing.


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#16 Diesel Clay

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Posted 20 February 2017 - 03:29 PM

Do industrial ceramic manufacturers apply, or even need, to quantify the plasticity of their clays, and if so, how do they do that? I'm talking the machine jiggered stuff, not the slip cast. It has been mentioned that clay artists aren't the primary market for almost any material that we use. Does someone already have some method of quantifying plasticity, and under what circumstances are those numbers applied?

I agree fully with Madeline about the kind of info Plainsman supplies as being probably the most useful sort for clay artists. I'm kind of surprised other suppliers don't do this more. I also agree with Bruce that having a plasticity number on the side of the box is going to have limited value over time, and even re-wetting some old, shelf-hardened clays will change that number. Some suppliers also sell their clays as dry mix, and then what?

#17 High Bridge Pottery

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Posted 20 February 2017 - 06:25 PM

 

Porcelain:  50% kaolin, 25% silica, 25% feldpsar, 2% V-Gum T --plastic, easy to move around

 

Wait, I have never found this with porcelain. Whenever I try and throw it it doesn't want to move.


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#18 glazenerd

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Posted 20 February 2017 - 07:19 PM

Bruce: excellent argument, and all valid points: sadly enough.

Peter: likewise valid points: yet I still find myself wanting to explore the concept of density effecting mechanics.

Min: Tony and I have talked about this via email. He "cares", makes all the difference in the world.

Callie: industrial/commercial clay formulations avoid plasticity at all costs.

Joel: it has already been established that UK clay makers are from Venus... :huh:

 

Once again I find myself exploring areas that really do not matter to anyone- but me.

 

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#19 glazenerd

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Posted 20 February 2017 - 10:10 PM

Peter:

While I do have an interest in the mechanical properties that density exhibits in clay: I also have other motives. Being able to gauge the pressure required to move clay would also be useful information in setting formula limits for stoneware.

Nerd



#20 oldlady

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Posted 22 February 2017 - 01:30 PM

:huh: nerd, while doing all this experimentation and research, do you find time to make pots?


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