Cob cob / ceramics

[Lars Fields]

Carol M. wrote:
> 
> Lars Fields wrote:
> 
> >  > lars wrote:
> >  >        this is kind of an aside - maybe pat newberry might know best:  does
> > > anybody know why it is inappropriate to mix straw with clay if one is making
> >  > bricks for a fired structure 'a la nader khalili'?
> 
> Hi Lars,
> 
> I don't know all that much about cob but I do know something about firing ceramics -
> have had my own kiln for over 10 years and do a lot of things hand built with clay
> that I later fire.
> 
> Any air bubble at all left in clay that's being fired will cause it to explode - in a
> big way.  So if you got the cob hot enough to bake hard, most likely at least some of
> the straw would vaporize and the expanding gases would cause explosions also.  I sure
> wouldn't want to be anywhere around when you were firing a cob structure.<g>  One of
> the most important steps in working with clay is to be sure all contaminates (leaves,
> twigs, sticks, etc.) are out of it before you start building a pot.  So I'd think
> that firing cob would be a rather hair-raising experience.  And one where a lot of
> people could be badly hurt.
> 
> Mind you, I'm not at all experienced with cob, but you sure do need to check with
> some other people before trying to fire it.  If you know a potter in your area, who
> does hand building with clay, that might be a good place to start for basic info.
> 
> Carol

hey carol,

	thanks for the warning. i thought that the cob might crumble or crack under 
these conditions, but i never expected _explosions_! 

	but, if i remember correctly, it is trapped gasses that cause the explosions, 
right? so, if the density/proportion of straw was high enough, then maybe the trapped 
gases would have avenues of escape and therefore wouldn't build up to pressures high 
enough to explode. what do you think? 
	or maybe if most/all of the straw was of long lengths (as another person, sorry 
forgot your name, suggested), then one could be relatively sure that each and every air 
'pocket' would have an escape to the outside of the brick or wall, as each peice of 
straw would stick out of the brick cause they would be longer than the bricks. 
	i suppose that might lessen the insulation properties, as air might be able to 
flow through the fired cob easier, but that might be taken care of with an earth 
plaster applied to the wall after it's been fired.


lars fields