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Shrinkage And Proportions


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#1 clay lover

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Posted 03 January 2014 - 10:22 AM

I know the shrinkage rate of the clays I use, but I am wondering if it is all round shrinkage, or more height than width?

When making tall mugs, I have been struggling to get the finished proportions what I want. It seems to me they shrink more in height than in width . I end up with pieces that resemble handled bowls, not what I want, and not what they look like freshly thrown.
Do you know how to figure wet measurements to get the proportions that you want?

#2 Chris Campbell

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Posted 03 January 2014 - 11:57 AM

The only solution I can think of is for you to measure the mugs three times. After throwing, when dry and after firing so you know exactly what is happening and when. Then you have true numbers to use to compensate for shrinkage.

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#3 JBaymore

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Posted 03 January 2014 - 12:05 PM

At a VERY "technicality" level, the forming process can slightly affect the shrinkage rate relative to how the forces are applied to the clay platelets. I have my students experiment with this in the ceramic materials ("clay and glaze chem") class. But it is a VERY slight difference.

 

I'd be surprised if it was significant enough to make the visual difference you are mentioning....... but maybe.

 

Chris has the right approach above.

 

best,

 

................john


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#4 Wyndham

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Posted 03 January 2014 - 12:07 PM

There one of several ways to  figure this out. Make a piece you want, mug , cup, etc, then cut a paper profile of the piece so that you can  snug it  against the piece and you're happy with the profile.

Check the paper profile against the piece after the bisk then also after the glaze firing.

Some distortion may occur due to thin/thick walls of the piece and this may help id those area's, if any.

You can then scale the paper profile up(copier works fine) to help get the finished size you have in mind

Hope this helps

Wyndham



#5 Mark C.

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Posted 03 January 2014 - 12:08 PM

I have some vague memories from clay and /glaze calc. class 40 years ago about pots shrinking unequally. I cannot recall height vs width- but do recall its not the same.I doubt you would even notice this unevenness but as Chris said measure then you will know.

m


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#6 Davidpotter

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Posted 03 January 2014 - 01:39 PM

in me class the level one students make a 12 inch ruler for their first project. good start for testing glazes on each inch and seeing the shrinkage rate them selves. but i have never seen any shrinkage more than a quarter inch


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#7 neilestrick

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Posted 03 January 2014 - 01:46 PM

The proportions should remain the same. It's a percentage game, so the longer the dimension, the larger the actual distance that it shrinks. So the height, being longer than the width, is more noticeable. No one ever mentions their platters getting shorter, just smaller in diameter. 2 inches in height only shrinks 1/4 inch at 12% shrinkage. But 18 inches in diameter shrinks more than 2 inches.


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