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Monday, January 30, 2012

Mystery solved.... maybe.....

Thanks everyone for your comments on the last post. I spent a couple of hours with my clay books and my laptop and I think I have come up with an answer for the pieces that decided to pull themselves apart. As I said, they seem to be pulling apart at the seams, and it has been very random. All of the pieces were constructed the same way, I have ruled out the score and slip issue, I do that correctly, it was not moisture, everything is dried slowly and is bone dry when it goes in the kiln and I candle for a long time. 

All of these made it  :)

However, here is what I discovered:


Damp clay is made up of fine platelets which ride within a thin cushion of water. When a slab is rolled out, the pressure thins out the clay and also moves the platelets so that they are aligned with the direction of the force. In essence, the clay takes on a grain, much like the grain in a piece of wood. Unless modified, the platelets will remain aligned in that grain throughout drying and firing.


This is where problems can develop. Grain in a slab will affect the clay's shrinkage. The slab will shrink more along the width, across the grain, than it will along its length, or with the grain. If pieces are assembled so that the grains aren't aligned in the same direction, the pot can literally pull itself apart during drying and firing.  (I don't know what this gray shadow is, I cut and pasted this from the article, must have picked up a ghost!?)

these pulled apart... I loved these :(

I have been so puzzled by the randomness of this, but the article I found makes sense of this. The little houses that made it were all cut from the same slab in the same direction. The larger pieces that didn't make it got cut from different slabs and probably in different directions, against the grain.
This didn't happen to me before I got my slab roller because I threw all of my slabs and changed directions to get the thickness even. Since I got a slab roller I have been lazy, just rolling out the slabs and cutting out my parts. Somewhere in the back of my mind I am remembering a class I took years ago and the mention of rotating your slabs when rolling or throwing them came back to me. Ah ha! I am pretty sure this is the problem, so next round of slabs, I rotate them when rolling them out and we shall see if this solves the mystery.

16 comments:

Lori Buff said...

So glad you figured out the problem and sorry you lost 5 pieces before you realized you had a problem. I hope other people learn from this experience so your loss is everyone's gain.
The doll in the entrance does look fantastic. I keep thinking up stories about her. She just belongs there.

smartcat said...

Oh wow! This may be why some of my tiles warp and some don't. I cut them from the same slab; some warp, some don't......even when I handle them the same. I think I will try rolling several times starting thick and rotating with each pass. Thanks, I thought I was the nature of the beast....which it may be....we'll see, grasshopper, we'll see....

Barbara Rogers, Potter said...

Thanks for the info, Tracey. Love your work! I'm pretty new to slab building, and probably just did a whole slab wrong...went from thick to thin in just one roll. The bane of taking shortcuts! At least I haven't made anything out of it yet...

Tracey Broome said...

I'm not 100% sure this is what's causing my issues. I have ruled out everything but this, so we will see after my next firing with properly rolled slabs. It's always something isn't it? I think potters just like problems to solve!!

Linda Starr said...
This comment has been removed by the author.
Hollis Engley said...

Hope that's the solution, Tracey. Nothing should stand in the way of more barns and other such buildings ...

Kevin Carter said...

I hope that you have found the solution, Tracey, that's a lot of your hard work that came to nothing, more or less. Or did it? You learned something, which is going to help you in the future, so maybe, as they say, every cloud has a silver lining.
Maybe your ruined houses and doll parts could inspire a series - "Broken Homes" or some such.
Keep up the great work!

Michèle Hastings said...

I think you hit the nail on the head. Jeff is always reminding about the molecular properties in clay when I have pieces that crack etc...
I have to say... that sweet little doll has been waiting all it's life to be in that house!!

Julie Whitmore Pottery said...

The structures are amazing and what you've researched seems like the answer. I'm soon to be the owner of a rolling table too, after using a pizza rolling pin for ten years. Turning, smooshing and flattening in every direction and this info is going to be such a help when the table gets here.
thank you!
xx
julie

Melissa Rohrer said...

Hope that solves the problem. There's nothing like seeing hours and hours of work destroyed. Smart of you to research this.

Kings Creek Pottery said...

Fascinating...thanks for sharing the process, we all learn from it. Anxiously awaiting to see if the next batch holds it together more.

Love the "broken home" idea!!

:)

Cathy Kiffney Studio said...

Yep, definitely-maybe-might-be it! Haha...clay always keeps you guessing. Has anything changed since your last successful firing? Is this group of pieces from a new batch of clay?
And with the rolling of the slabs, roll in all directions then flip and roll in all directions on the second side, kind of an even amount. Hope it works!

Tracey Broome said...

Hi all: Sorry I haven't been commenting, I am trying to get some photos of all this work before it heads out the door! Did I mention, I hate photographing all this stuff!

Anna M. Branner said...

Great detective work Tracey. Makes complete sense. Fingers crossed!

angela walford said...

poor dolly... somebody pinched her appendix and didn't cover the hole :(( you could change clay bodies or try paper clay, get something like a 30# nice and open for hand building ...

cookingwithgas said...

your hard work will pay off- sweet pigeon toed baby girl...