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[Clansgian AT wmconnect.com]

> That makes sense at first thought. But thinking further, I wonder why  
> each new firing would not dry the moisture out of the clay.

We have the disadvantage of not having done this.  So in lieu of experience, 
I'm relying on the literature.  Can't take it as Gospel, of course, and in 
time I might think contrary-wise, but as the ball lies now, here's what Kiko 
Denzer says in his book "Earth Oven"

"An earthen oven, like a living thing, breathes.  When baking, it "exhales" 
steam.  Putting a non-breathable, waterproof finish (like concrete or paint) 
on 
an oven is like putting on a rubber suit to exercise - it traps moisture.  
Just as you'd get soaked with sweat running in a rubber suit, steam from 
baking 
will condense against a non-breathable finish and soak back into the oven.  
Enough of it can soften your oven, and even collapse it.  At the very least, 
you'll have to dry it out every time you bake, which takes exta time a fuel.

"Cement also cracks, allowing water in faster than it can get out.  When the 
mud gets soft enough, it will leak out with the water.  Or water freezes, 
cracks grow,and your oven erodes.  So ovens are best finished with water 
*resistant* plasters such as lime sand or clay-manure mixtures.  Such 
plasters are 
breathable, and are traditionally used in earthen buildings ..."   </HTML>