Clay fatigue?

[Sheryl Leigh]

Is clay fatigue really something that happens?  I admit that I don't understand  - or is this another one of those long-and-dearly-held myths?

[glazenerd]

Not a myth.. But would like to hear your description of "fatigue" before I weigh in.

Tom

[PeterH]

2013 discussion on Something Exploded in the Kiln - Was It Clay fatigue?
http://cone6pots.ning.com/forum/topics/clay-fatigue

[Min]

I think of clay fatigue as fatigue from overworking clay on the wheel. Clay gets flabby with excess water and collapses. I don't think of the cone6pots post example as clay fatigue.

Potter Fatigue, overworking with clay, very common this time of year. 

[liambesaw]

Yeah that phrase could mean many things.  I also have heard it used on recycled clay that has lost it's fines.  

[glazenerd]

I would define fatigue as: loss of or change in particle charge. 

Plasticity is the direct effect of negative charges on clay particles. As those charges are lost; the clay becomes short. Fine particle ball clay carries the highest negative charge: when they are lost during throwing ( fines), there is a change in the net charge of the body. The result being the formation of flocs ( flocculation) which results in slumping or folding of clay.

T

[liambesaw]

27 minutes ago, glazenerd said:

I would define fatigue as: loss of or change in particle charge. 

Plasticity is the direct effect of negative charges on clay particles. As those charges are lost; the clay becomes short. Fine particle ball clay carries the highest negative charge: when they are lost during throwing ( fines), there is a change in the net charge of the body. The result being the formation of flocs ( flocculation) which results in slumping or folding of clay.

T

Any reason it seems to happen all at once?  Clay will feel fine until suddenly it's not at all.  Has happened a few times when trying to throw smaller stuff with reclaim.  For some reason larger stuff with reclaim was much more forgiving.  I'm guessing it's user error because I am much more careful and slow with large work and quite fast and sloppy with small stuff.

[Dick White]

Interesting. Tom, if the plasticity is degraded by a loss of negative charge as fines are washed out during throwing, do you think it can be restored by throwing with slightly alkaline water?

[glazenerd]

Well, if I answer this fully: Holly @ Ceramics Monthly might not like my giving away an article.

The simple premise in clay chemistry is:

acidity = flocculation / alkalinity = deflocculation.  With the underlying  application: alkali (fluxes) have negative charges, while vinegar ( acid) has a positive charge.  Ball clay is a 2:1 particle: skip the chemistry and think Oreo cookie. The two outer layers of the cookie are silica, and the center is alumina. And just like an Oreo when you dunk it in milk, it gets soft from absorption. After you dunk a few, a nice cookie film floats across the milk. Back to clay: calcium,,potassium, and sodium are held on the surface of the clay: and when wetted, are released into the water film creating the negative charge:- plasticity. When you throw, you wash away some of the cookies ( clay) supplying the negative charge; but you also weaken the negative charge in the clay water film. Once it hits a certain loss ratio .(unknown) flocs form and the material begins to fold. Sorry, alkaline water will not replace the ionic changes.

sorry for the Oreo cookie analogy. You either get cookies or four paragraphs on isomorphic substitution.

Tom

[glazenerd]

Just now, liambesaw said:

Any reason it seems to happen all at once?  Clay will feel fine until suddenly it's not at all.  Has happened a few times when trying to throw smaller stuff with reclaim.  For some reason larger stuff with reclaim was much more forgiving.  I'm guessing it's user error because I am much more careful and slow with large work and quite fast and sloppy with small stuff.

The Oreo cookie analogy above: you dunk them in milk, and they get soft from absorption.... Hold that thought.

ball clay has a plastic limit (PL) and a liquid limit (LL). The plastic limit is measured as water of plasticity. (WOPL)  OM4 ball clay has a WOPL of around 35: which means 100 grams of OM4 will hold 35 grams of water to form a nice pliable ball of clay. Once you cross the plastic limit of 35 WOPL of OM4 by adding more water by throwing you begin to enter the liquid limits. That cream that comes up on your hands when you throw is ball clay that has surpassed its plastic limits and have broken down by liquid limits: we call them fines. 

The negative charge any given ball clay supplies is measured by CEC ( cation exchange capacity). oM4 has a CEC of 7.2: which on the CEC index is medium plasticity. High plasticity ball clays run 9-11 CEC, the higher number indicates higher plasticity. But higher plasticity comes at the cost of higher absorption rates: a ball clay with a WOPL of 38 and a CEC of 9.5 will absorb lots of water. The faster it absorbs water, and the more water it can hold: the faster it moves from plastic to liquid limits. When the  clay you are throwing reaches its maximum plastic limit  by the water you are adding: it " suddenly" enters it's liquid limits: and folding ( fatigue) occurs.

the cream on your hands will tell you what kind and how much plastic ball clay is in your clay body. If you are constantly cleaning cream off your hands, and it oozes through your fingers as you throw: you have a high % of high plasticity ball clay in the formula. Which means it will absorb water faster, and reach its liquid limits faster: and reach " fatigue" faster.

Tom. 

[Dick White]

Ok, thanks Tom.  Another brilliant scheme from my overactive imagination shot down in flames by someone who knows the facts better than I do. I was saving the Oreo cookies for Santa next week, but I think I'll dunk one right now...

[Sheryl Leigh]

Excellent information, thank you!  I suspected it had something to do with too much water (the context was me learning about porcelain) but didn't know if there was something else missing.  People speak of it like you've just beaten the clay to death. 

 

Thank you again!

 

[Roberta12]

On ‎12‎/‎17‎/‎2018 at 5:27 PM, Min said:

I think of clay fatigue as fatigue from overworking clay on the wheel. Clay gets flabby with excess water and collapses. I don't think of the cone6pots post example as clay fatigue.

Potter Fatigue, overworking with clay, very common this time of year. 

I'm with Min.  And I would like an oreo now.

r.

[glazenerd]

 

Plate of "fines" from throwing with too much water.

[Benzine]

Nerd,

Quit trying to science everything.  Everyone knows, when something bad happens with clay/ ceramics, it is because you in some way slighted the Kiln Gods!

[Selchie]

I want more "fines", but only the kind that glazenerd has!

Is there a reference article or book about clay chemistry, glazenerd? Curious.

I am  also wondering about how much water is enough. The clay comes moist, but some addition of water when throwing decreases friction on the surface, so (I'm assuming) is a good thing. I am well aware of the actions of too much water - have watched clay, that in my imagination is going to be 10" high, decide it wants to become a  poor imitation of a "George Ohr" pot on the wheel.  

I would be interested in seeing some of the physical properties of clay in electron microscope detail. 

Thanks for the extensive explanation - definitely helps and peaks both interest and cookie cravings.

[glazenerd]

Selchie:

most of my research comes from abstracts written from the 1920's to the mid 1970's. The PhD's of the time submitted most of their research docs to the American Ceramic Society. They included, West, Lawrence, Brown, Orton, and Norton. Ougland % Brindley from the British Ceramic Society researched the reactions of feldspar, silica, and alumina to high temperatures ( cone 10)  I am fortunate because I have access to most of the books and research written by the above researchers. A potter friends father attended Webster University in the late 60's early 70's and purchased and kept most of these books. He also kept some of the research papers.

There has never been a book written on clay chemistry, with formula limits: although I have been working on one for the last five years. Clay does have definable chemistry, and it does have limits. I subject my fellow forum members to my rantings; they are a patient lot. The plastic and liquid limits for clay have been defined by: The Atterberg Limits. Some clay makers in Europe actually use his scale to define plasticity of their clays. Plasticity comes from soil science: cation exchange and isomorphic substitution. If you do a search; the initial results demonstrate  how both supply nutrients to plants. You have to get to the engineering side of those principles to see how they develop plasticity in clay. Actually there are 25,000X microscopic images of clay particles on the net. Kaolin (1:1) looks like saltine crackers and ball clay (2:1) looks like microscopic sponges- which they are.

i learned awhile back to use food to illustrate chemistry: it holds the attention longer. There is a list of clay chemistry articles I wrote last year for Ceramics Monthly on my profile page. There will be a few more out in 2019- January being the first one. 

Tom

 

[Selchie]

Thank you!!! I will go on a more focussed search now!  

I have a bit of an understanding of the physics of plasticity and elasticity of solids (and ultimate failure point - which is the one I have reached frequently as I have been learning to throw).  Adding the chemistry of the clay, liquid limits, rotation of the wheel and the clay, upward force of the potter's hands - and I think the end result is just basically magic!