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Lonna

How To Complete A Mistake? Please Help

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Ok, if your sculpture is a whole body with a hollow space filled with air in the middle of it, it will blow up like popcorn. Just make a hole in the underside of it's base; 1cm in diameter is more than enough.

Please do this, it's just a hole in an invisible place.

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Ok, if your sculpture is a whole body with a hollow space filled with air in the middle of it, it will blow up like popcorn. Just make a hole in the underside of it's base; 1cm in diameter is more than enough.

Please do this, it's just a hole in an invisible place.

 

This is a really great analogy, and explains why everyone is right in this post, as a practical matter, because even if you know don't know why you're putting a hole in your pot, it works!  

 

Popcorn feels 'dry' when you hold it.  But it isn't!  Popcorn blows up because the moisture in the popcorn flashes to steam.  (See Wikipedia,   https://en.wikipedia.org/wiki/Popcorn)  If it was just air in the popcorn, popcorn wouldn't pop. Its the water in the popcorn that makes it pop. The water is trapped inside because the popcorn kernel has a hard, impervious skin that won't let the inside of the kernel dry completely. If the popcorn's skin is damaged, the kernels won't pop, because the moisture content falls too low. 

 

So, when we dry our clay art, we're trying to keep it from acting like popcorn.  The moisture in the clay, and the moisture in the air inside hollow clay, needs to escape before we fire it above the boiling point of water.  If we don't, the steam will blow up our pots.  The clay needs to be "dry" for this to happen.  Unlike the shell of a popcorn kernel, unfired clay is generally porous enough that the moisture escapes through the surface.  Candling your kiln (at near boiling point of water) can help get the moisture out of the clay and any air inside voids in the clay.  Holes help dry hollow parts, because they allow the dry air outside the hollow parts to exchange with the wet air inside the hollow parts.  The clay dries more efficiently, and there isn't any moisture trapped (like in popcorn) to blow up your pot.  If your clay is very thick and heavy, the moisture doesn't escape well through the surface, and there can still be enough water in it to pop. 

 

So, if you poke holes in your popcorn kernels, you'd better pop it fast, because the inside will dry out and then you'll just have dry corn.  And that's lame. But if you poke holes in the voids of your pots, the inside will dry, and your pots won't pop.  And that's awesome!

bciskepottery likes this

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great analogy.   i just wish the original poster would come back to us and show how the marvelous sculpture came out.  she disappeared in january.

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You can fire totally enclosed forms with no holes in them.  The change in volume of the contained air from ambient to say 2400 F produces wall pressures (in sq. in. terms) that are well below the structural strength of typical clay walls.  If you made a clay wall as thin as a plastic balloon...... MAYBE.

 

Run the gas pressure / volume laws.  Look at MORs for clay bodies.  Been there, done that, bought the t-shirt.

 

However put a TINY bit of moisture in that same space..... and the change of state of a tiny amount of water produces a HUGE amount of steam pressure.  Bang.

 

A hole helps facilitate DRYING.  So a hole is good for THAT reason for sure.  But I think it IS important for ceramists to know a bit about why they do what they do.

 

best,

 

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

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