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shrinkage dilema

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Maths never my strong point, so a bit confused about shrinkage (I really shouldn't be) but when I threw some mugs recently I thought I'd sized them correctly wet but they ended up too big, they did not shrink enough...

So, if my stoneware clay has a shrinkage rate of say 12%, if I made my pot 114mm high x 114mm wide, I'd end up with a pot approx 100mm x 100mm – is that right? 

Also, how much shrinkage should one expect at bisque (say 1050 degrees C) and how much between bisque and fully fired?

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Your maths is correct!

It's worth doing a shrinkage test. Cut a bar of clay exactly 100mm long (by 10mm, by 7mm) from a slab of reasonably soft clay.

Let it dry (flat!), and measure it.

Biscuit fire it, and measure it again.

Take it to glaze temp, and measure it for a third time.

You'll then have a set of figures from which you can work - they may differ from the clay suppliers. You'll be able to work in both directions - from just thrown to finished, and from finished to just thrown. Saves a lot of trouble.

So, for my own clay, shrinkage is:

Soft to dry: 100mm down to 96mm

Dry to Biscuit: 96mm to 96mm (no change for me, but this may depend on how high you biscuit fire)

Biscuit to Glaze: 96mm down to 92mm

So, I can happily say that 'end size' will be 'thrown size' x 0.92

And, 'thrown size' will be 'end size' x 1.087

Hope that makes sense!

Edited by Sputty
Orthography
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