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Happy Thanksgiving Neil- America, and those abroad.

 

Peter: the WOPL test measures the plasticity of clay. Take any ball clay you use: measure out 100 grams dry, and add 35 grams water: then knead into a pliable ball that you can form. If is tacky after that, then it is less plastic, 35 grams of water is medium plasticity, and if it requires more water: higher plasticity. You can use this test to gauge the plasticity of native ball clays, or any clay you are considering for a stoneware formula. The smaller the particle and platelet size; the more plastic it will be. It has been well established that high plasticity ball clays add mechanical strength to a clay body because they have much higher surface areas. If your stoneware body is stiff, then use this WOPL test to determine a ball clay that will add more plasticity.

 

Nerd

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It has been well established that high plasticity ball clays add mechanical strength to a clay body because they have much higher surface areas. 

 

Nerd

 

Ball clay bodies also have excellent dry strength. You can burnish high ball clay bodies when bone dry and not have to worry so much about breaking the pots.

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Nerd:

One of the cone 10 bodies we used to use a lot in grad school was the old equal parts formula that many folks have used for many decades:

 

20% OM-4 (ball clay)

20% EPK (koalin)

20% Hawthorn Bond (fireclay)

20% Custer (feldspar)

20% Flint

 

I'm sure every user modifies this for whatever type of ball clay or fireclay or feldspar they prefer, but I was wondering how this classic formula stacks up against the standards that you're establishing with PSD, mullite, molar limits, etc?

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Neil. I will run it shortly, just to further illustrate the new SAS standards I am proposing.

 

My Question: did I word it well enough for you to get it?

 

Nerd

 

I call your formula the 5/20 blend.... 5 parts of 20% each.

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Neil: your 5/20 blend equals:

alkali 2.84 molar    total fluxes 3.55 molar    Alumina 16.96 molar     Silica  78.54 molar.

 

SAS Evaluation:   Total SAS 2180 divided by 100 parts = 21.80 median SAS  (24.00 SAS is maximum for cone 10)

 

OM4 is classified as a kaolinitic ball clay. EPK is a finer grained kaolin. Although a stoneware body, it has high kaolinitic properties. Although usable as is, I would like to see alumina above 18.00 molar, and silica around 75.00 molar. So very slight adjustments.

 

Nerd

 

25% EPK, 24% OM4, 20% Hawthorne, 20% custer, and 11% silica =

alkali 3.01 molar,  total fluxes 3.83 molar,  alumina 19.98 molar,  silica 75.09 molar.

 

If you want more body: swap out 12% EPK for 12% Imco 400, or GoldArt.

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Think your WOPL deserves a new thread, as does your SAS.

 

Ballclays are just secondary kaolin which has been transported over some distance, along the way getting ground up in to much smaller bits and picking up a lot of junk. That is why ball clays have less alumina - because they have been dirtied up during transport with other non-clay materials, diluting the amount of pure alumino-silicate material they may have started with. They can also have lots of silica.

 

I think their chemistry has got to be a major driver for clay bodies - in both good and bad ways...

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Curt:

Sadly I think chemistry comes behind production costs and profits.

 

Now I need to find a used MOR testing bench: need some kind of measurable/variable standard. (MOR = modulus of rupture)

Doubt that I cause any change, or that my standards will be accepted: but I am trying to set some.

 

Nerd

 

If you're the only one formulating like you, and your clay is more expensive as a result, you'll go out of business before everyone catches on. Everyone will say 'the clay I've been using for the last 30 years has worked fine, so why would I switch to something more expensive?'. And they do have a point- humans have been making pots for thousands of years just fine. The trick will be to prove somehow that your formula limits will benefit the user in a very measurable way- fewer clay problems, etc; or benefit the producer in some financial way- lower materials cost, fewer unhappy customers, etc. That may not happen in our generation, but if the new generations of potters can be taught to use limits when formulating clay bodies, then eventually it will become the norm.

curt likes this

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And if my clay is only 3-8 ¢ a lb more: in some cases the same in general?

 

What if I could take 2lbs of clay and throw a cylinder than was 12": tall and 5" around?

What if I had stoneware that had 0 porosity? Could handle thermal shock?

What if I increased the COE, where glaze fit issues disappeared?

 

But yet I am a realist: most do not care. So perhaps I will go back to my cave and play some more :)

But then again I care; and that is the only motivation I need.

 

Nerd

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Sorry if that last post sounded negative. Just playing devil's advocate. I know how clay customers think.

 

Most don't care about the how or why, but they do care about the results. They'll pay attention to anything that increases success. Why was B-Mix so popular? It gave porcelain-like results without having to learn to work with porcelain. Why does the Giffin Grip sell so well? Because it makes trimming easier for folks who haven't learned to center well (I know there are other reasons to use one, but this is the main reason). People want things that make it easier to make pots. If clay formulas can give them that, then you're in business.

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Neil is right IMO and I would say his comments are the key to what your SAS write up needs in addition to what you have already written - namely, you need to spend some additional words making the tangible benefits of using your new SAS methodology very clear. Some testing or real world examples backing up your methodology and its benefits might also help.

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At the end of the day, I am a hobbyist deeply involved in his hobby.

 

 

And they do have a point- humans have been making pots for thousands of years just fine

 

The irony of this position: the pots made 2000 years ago, last longer.  Amazes me that the clay arts is in the 21st century technology, and yet clay performs worse than it did 40 years ago. I wonder if the Assyrians had problems with bloating, dunting, and crazing?

 

Nerd

 

Have already done some initial testing. Have a couple of bodies out in the studio that many would like to get their hands on. Waiting for a new shipment of test samples: none of which are sold in the usual outlets.

 

More words:  a 325 mesh with 0.38 platelet has an SAS of 24, but so does a 200 mesh with 0.75 platelet size. Potters choose the 200 mesh only because text books say to.

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At the end of the day, I am a hobbyist deeply involved in his hobby.

 

 

And they do have a point- humans have been making pots for thousands of years just fine

 

The irony of this position: the pots made 2000 years ago, last longer.  Amazes me that the clay arts is in the 21st century technology, and yet clay performs worse than it did 40 years ago.

 

How so? We won't know that for another 2000 years. And how do you figure that it performs worse than 40 years ago. Not saying you're wrong, just want to know your reasoning.

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How so? We won't know that for another 2000 years. And how do you figure that it performs worse than 40 years ago. Not saying you're wrong, just want to know your reasoning.

Because we are 40 years down the road with advanced milling, advanced sieving, advanced separating, and advanced knowledge of products. And yet we are still dealing with the same issues of 40 years ago. We have online glaze calculators, programs, and more books than a person can read: and yet our glazes still do not function as they should. We have industry making claims that a clay will perform across 10 cones: even though known chemistry says impossible!! We have failure rates that remind me of the 1890;s. We have come to accept variables; that should not be variables at all.

 

Nerd

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Hi Nerd,

Just picking up on this thread again. I've done a bit of work looking at clays for water content and plasticity, using the Atterberg test for moisture content for upper and lower plastic limits. Also, in the absence of a ny proper lab kit for this, a drop test to measure the actual plasticity - make up a 50mm ball, drop from a height of 2m into a bowl, and measure the diameter, plus take a piece and measure the MC - not perfect but indicative. What I initially assumed was that a wide moisture range would correspond to a high level of plasticity, but as I test more clays I find this isn't always the case. At the same time I put it through a set of sieves, which gives a breakdown on the medium to larger particle sizes. Just need to find time to see if there is any link between particle size distribution and plasticity. Also, with hindsight, I should have done the drop test at say 3 different MCs.

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Tim:

I get the premise of this test: the more water, the more plastic: therefore a wider radius when dropped. Particle size distribution (PSD) plays a secondary role in plasticity: although if the distribution is wide or narrow can effect plasticity. The determining factor in plasticity is platelet (depth) of a clay particle: it determines rather the particles slide easily against each other, or collide.

 

Particle size:

 

o o o o o

 

Platelet size:

 

- - - - -

 

It is the platelet size that creates mechanical force / friction: preventing particle size from moving around freely.

 

Nerd

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Interesting concept on the freshly pugged clay absorbing the water into pores. Would this notion apply to pugged reclaim clay? We save all slurry and trimmings, and then dry it somewhat on plaster to puggable consistency. Then pug it until thoroughly mixed and store the slugs back in plastic bags until using it. I've noticed some hardening/stiffening of the clay over time, but have attributed that to drying due to incomplete sealing of the bags during storage. I'm thinking this theory of water absorption would not apply as the clay has been wet ever since initial production, through usage, time in the slurry bucket, and the pugmill just remixes it to relative uniformity within the batch? Or am I missing something?

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oohhh, I like that Cream Test. I've noticed some differences in the "sloppiness" of the various clay bodies used by the students, and will watch that more closely to keep track of which ones do what. On the reclaim, we try to recapture everything, lots of throwing slop (which is mostly the fines) and lesser quantities of trimming scraps (which, as you note, are missing the fines), so the balance in the reclaim should be trending towards more fines than in the original = more plastic? Your cream test will be useful to gauge what actually came out of the mill.

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Working on informational tables for use with the SAS Formulation Method:

 

Example:

 

EPK:  Median Particle Size: 1.36     SAS: 28.52   WOPL: 26.0%   MOR: 300

 

Anyone want to take a shot at what this information is telling you?

 

Nerd

It's short as all heck by itself to try and throw with, but it'll cast just fine. The dry shrinkage isn't too bad but it'll dry quickly (and be possibly prone to cracking because of this). I'd have to go back to the beginning to figure out the MOR indications, but I think it has to do with green strength.

 

For the record, if you could formulate a clay and give me a firing cycle with it that would make it effectively non-porous for food use, (and preferably red in colour), I'd buy the heck out of it to save all the damn testing time! Yes, even if it's 3-8 cents more a pound. And to play devils advocate, my local clay supplier did import some Southern Ice, and it does sell a bit, even though it's $80 CAD for a 25 lb bag. The end result is all.

 

I'm only catching up on this thread now, but I think if you haven't emailed Tony Hansen yet, you should.

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Educational Post moved from another thread:

 

Posted Today, 09:27 AM

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).

 

Nerd

Nerds,

What are your thoughts about Mica? An old (term of respect!) and very experienced potter told me his clay had great properties because of mica. The platelets slip past each other but attract ionically. Clay improved by lengthy settling in tanks. (This is so interesting just to hear everybody's perspectives)

Joe

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