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Improving Clay Plasticity


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#1 banana

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Posted 16 March 2017 - 03:15 PM

Hi to all,

 

I am trying out some new, locally made clays, for throwing on the wheel. 

 while pulling the clay up, there seem to be tears in the wall, or sometimes these kind of cracks, but not from dryness - it's as if the clay is just separating. 

 

My thought was that there is a lack of plasticity in the clay body! But I got the recipe for one of the clays, and it has altogether almost 80% plastic ingredients. That seems like a lot to me... 

 

I would love for any advice on this matter:

 

First of all - is the problem really lack of plasticity?

What might be the cause of this problem?

What can I try in search of the solution?

 

Thank you for sharing your wisdom,

Dana



#2 yappystudent

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Posted 16 March 2017 - 04:58 PM

Also a relative newbie but I believe this may be an issue called "Short" clay, which is a condition of the clay when it has gotten a little dry, upon reworking it hasn't properly absorbed water into it's structure. Even though it may be almost entirely pure Kaolin it can still happen, in fact I'm trying to work with some white porcelain I bought recently and it tends to get "short" worse than any other clay I've used yet. It's fine right out of the bag but when trying to reuse the scraps, even though it seems fairly wet, it cracks and fusses. Folks with more experience may know further solutions, but the only one I know of is to let the clay sit bathed in a little (not much) water for about 24 hours. To do this, put the block of clay in a bucket. Poke holes about 1" apart right through the block with a long knitting needle while it's still in the bag, don't puncture the bag if you can avoid it, and pour in about a cup to a cup and a half of water for a whole block. Pour it over the holes. Hold the top of the bag closed, and fill it with water so the water covers the top of the block but doesn't get inside the bag. The water on the outside squeezes all the air out of the inside of the bag and clay. let It sit about 24 hours.

If you want to see videos of this there are a handful on Youtube, it's where I learned it and it works really well. Some poke holes, some don't. I like this one from Simon Leach:

 


If nothing breaks you're not really trying.


#3 Joseph F

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Posted 16 March 2017 - 05:17 PM

Thanks for posting that video yappy. For some reason I have never seen it, but I have been making plates and I am about tired of my clay being on the stiffer side. So I am going to do this with the next 50# I have tomorrow.



#4 Marcia Selsor

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Posted 16 March 2017 - 06:54 PM

I have poked holes into clay using a dowel. Maybes many as 8 or 9 holes. Fill them with water.
Wrap them up and leave in a big plastic container. I repeat the process until the clay is what I want.

But sometimes the clay is stiff just because it may be a little thixotropic. It has a stiff outside surface.I throw it onto the floor on each side. Sometimes twice. That softens it up.It helps to slice it up and throw it onto a wedging table and slice several times. Wedge. This eliminates throwing clay with a tough surface in one places.
Marcia
Marcia Selsor, Professor Emerita,Montana State University-Billings
[http://www.marciaselsorstudio.com

#5 neilestrick

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Posted 16 March 2017 - 08:40 PM

If the clay wasn't aged or pugged and de-aired it will be 'short'. The best to to make clay without pugging is to mix it up as a slip, then dry it out to a workable consistency. De-airing or mixing from slip ensures that all the particles are wetted, and therefore bond better.


Neil Estrick
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Owner, Neil Estrick Gallery, LLC
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#6 Magnolia Mud Research

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Posted 16 March 2017 - 10:16 PM

Neil,

 

Can you explain how 'De-airing' increases the wetting of the clay particles?

 

 

 

LT



#7 curt

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Posted 17 March 2017 - 12:42 AM

Or the combination of materials the manufacturer has used in the clay body may just not be a good blend - no matter the moisture content. You said it was new local clay bodies, has the manufacturer properly road tested them in throwing and/or hand building situations?

If the right combination of clays (ie, plastic) and non-plastic materials is not used in a clay body - or the right distribution of particle sizes of those raw materials is not used - clays that vitrify reasonably well in the kiln may nevertheless be almost unworkable in your hands.

Can you post a photo of the tearing in the wall of a pot you are throwing?

#8 alabama

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Posted 17 March 2017 - 03:27 AM

I was told to improve plasticity to pour the water of boiled potatoes or rice into the clay. That water by itself doesn't work as well. I never tried it though since if I didn't like the characteristics of one clay I'd find another clay vein at another river bank or add regular store brought clay to the bucket of local clay. I did find one dark blue clay that would only shallow bowls, The Gray clay below it would make anything so I mixed the two together then only dug the lower clay. And I've had to mix two different store bought clays together to improve one of them!

#9 glazenerd

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Posted 17 March 2017 - 07:02 AM

 

and it has altogether almost 80% plastic ingredients. That seems like a lot to me...

80% total clay content for stoneware is typical. This does not mean that all of the 80% are plastic clay: in fact plastic clay typically run less than 20%. Plastic is one of those clay terms that gets tossed around rather loosely. In order to be classified as plastic: then the WOPL (water of plasticity) has to be 30 or above. Unless you have some background in clay chemistry; the recommendations for hydrating is your only option. While moisture content does play a role in plasticity: plasticity itself actually comes from chemistry.

 

Porcelain will always be "short" if it is not mixed correctly with enough plasticizer to overcome the chemistry deficiencies associated with kaolin. In the case of porcelain: add too much water and it will only weaken the body.

 

 

Or the combination of materials the manufacturer has used in the clay body may just not be a good blend - no matter the moisture content      Indeed!!

Nerd






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