Water clay weight

by Randall
(Iowa)

Pottery Clay Query - Winged Lion by Randall:- I was wondering if you might know about how much water clay weighs when dry in comparison with it's WET weight.

The clay I'm using shrinks about 5% during the drying process, I guess a bit more during firing beyond that.

I know the model I am hand-building took 825# of moist clay, I was hoping to get an idea what it will wind up when it's bone dry.

Randall

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Reply by Peter (admin)

Hi Randall

Very nice sculpt, may I say. Looks very much a pro job to me. I know how hard lion faces are.

Yes, I think I can help on this one.

It never ceases to amaze me how much simple maths and chemistry there is in ceramic sculpture and pottery clay.

Artists are supposed to be creatives, not scientists, right. Wrong!

If I were you here's what I would do.

Take a handful of clay from the bag. Weigh it. Let it dry out totally. Weigh it again.

Calculate the factor of decrease.

For example: if the wet weight was 15kg and the dry weight was 10kg, the factor is:

15 X ? = 10

? = 10/15

? = 0.6666r (or 2/3's)

The factor of decrease is 0.66666r or two thirds.

So, all you then have to do is take your 825 and times by 0.6666 (or times by 2 and multiply by 3) and your dry weight is 550.

Note: It makes no difference to the water content of the clay how big the bit of clay is. The big variable to watch out for here is how wet the bit of clay is when it is weighed. You can have clay fresh out the bag which is very wet, and clay which has been hanging around for a while which has lost a significant amount of wetness. The trick is to weigh a piece of clay which is about the average you were working on when you calculated the 825 weight.

Hope that helps.

Peter