Cob cob / ceramics

[Carol M.]

Lars Fields wrote:

>         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

Hi again, Lars -

I've been thinking about this a good bit since I read your original post and I think maybe
a good place to start with finding out what you'd be facing is with a brick maker.  From
what I understand, the materials used for making bricks are also carefully sieved to remove
any and all debris.  If small amounts of straw, small sticks, leaves and such didn't matter
in the firing or strength of the finished bricks, I really don't see why the companies
would go to all the trouble and expense of removing them.

The problem as I see it is that the straw is going to vaporize at a much lower temp than it
will take to harden the cob - if it can be hardened.  So the gases are going to have to
exit the cob long before it's hardened.  Ceramics are strictly clay and so can stand the
intense heat needed to fire it - IF the temp is raised slowly enough to allow any remaining
moisture in the piece to escape.  But remember, the clay is allowed to air dry completely
before it's fired so there is little remaining moisture.  I have no idea how long it would
take a cob structure to dry out that much.  And the cob is more than just clay and straw.
I think the straw would tend to hold the moisture inside the cob a long time after the
outside looks and feels dry.  And then controlling the heat enough to raise the temp slowly
is going to be one heck of a problem since you can't put the cob structure in a kiln.

Yet another line of thought is that since firing should make the home very low maintenance,
longer lasting and more weatherproof, if it was possible, then I think our early ancestors
would have done it.  They certainly knew about firing pottery about the same time they were
chipping arrowheads from flint.  I read a lot and one of my favorite things is
archaeology.  As far as I know, there has never been a fired earth home found - anywhere in
the world.

I also have strong doubts about being able to fire the walls so they are equally hardened
from the inside out.  I think it's more probable that you would end up with walls being
very hard on one side and not on the other.  Just firing clay pots outside, like our long
gone ancestors did, requires one heck of a lot of wood - lots more than you would think.
So you might well end up spending so much for your energy that you lose the low cost
benefits of a cob home.

Ever had a potato blow up in a microwave oven?  It _sure_ gets your attention in a big
way.  I'm not going to bet my last dime that a cob structure would blow up if is was
"baked" too fast.  But I'm not going to try doing it, or willingly be around a place where
it's being tried.  Testing this idea could be dangerous, so please go very slowly if you
decide to try it.  And stay a long way back! <g>

Carol