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Clay Weight To Fired Object Ration?


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Is there a potters' rule of thumb for establishing how much clay to start with when making a vessel?

 

For example I need to copy a glazed (inside only) stoneware casserole-type object where the lid and base each weigh about 800grams and thinking to perhaps start with double those weights in wet clay - does that sound about right?

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Probably about right.  I've weighed some of the mugs I've made, just to compare pre to post firing.  My 1 1/2 to 2 lbs. mugs weight right around a pound after firing.

 

I guess that would make sense.  Water is heavy.  I guess that's one reason why ceramic artist order glaze as dry materials.  Why pay for shipping on a material that comes cheap out of the tap?

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If you want consistency, then most vessels are made from a certain weight of clay proportionate to their holding capacity when finished.

 

A 2 pint casserole takes about 2.25lb of clay and will measure 4" high X 6.5" wide

A 4 pint casserole takes about 4lb of clay and will measure 4.5" high X 8.5" wide.

 

I got these figures from another website and tested them recently and found them to be pretty accurate. The lids used about 1lb & 2lb respectively.

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Pugged clay is approx 20% water, plus there is a bit LOI (stuff that burns off). If you know how much your clay shrinks and how big you want your finished pot you could work the maths backwards then add a bit more if you are trimming the pot. 

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