thought on cob chiminney.

[Mark A Hoberecht]

At 01:53 PM 11/21/96 -0800, you (Shannon C. Dealy) wrote:


>> Did the Cob cottage folks ever get around to doing a hybrd strawbale 
>> and cob house??
>
>They have done a few hybrids using strawbale for walls that don't receive 
>any direct sun, and cob for walls with solar exposure to store the heat.
>I had been thinking of trying to build a cob wall with straw embedded in
>the middle to improve the insulation value of the wall, but haven't had
>time to try it yet.


I built a hybrid cob/straw-bale structure this past summer in Ohio and
discovered some interesting characteristics.  I started with a sandstone
foundation that had a cob "cap" on it several inches thick to level out  the
wall for the straw bales.  I then built an interior cob wall about 6 inches
thick and an exterior cob wall a couple inches thick on the outside.  This
cob mix needs to be pretty dry or the straw in the bales can absorb a lot of
moisture.  Even cob that looks and feels dry to the touch can give up
moisture to the bales.  This will dry out as the cob dries.  Once the cob
becomes drier than the bales, the process reverses itself and the cob sucks
moisture back out of the bales, just as cob does with buried wooden framing
members (a kind of preservative).

I built the bale wall "on edge" to make the wall thinner, and it worked
great.  Because the strings were exposed on the inner and outer faces I was
able to tie adjacent bales together.  This really stiffened my curved walls.

>> 
>> I'm thinking of building some kind of pit to make cob in. I've heard 
>> that if the cob sits a day before building that it works better (I 
>> guess by works I think they mean more moldable. 

>Letting it sit for a day probably helps because it gives the water time to
>work its way into all the small clumps of clay.  I have found that it is
>easier to mix cob if you let the water soak into the earth mixture for a
>while before you start working it with your feet because the clay lumps
>break down more quickly.  Once the cob is mixed, it may also help to let
>it sit for a while to allow the straw time to absorb some of the water
>which should make it more flexible and easier to work.

All good points.  I used finely sifted clay in my mix and still noticed that
sitting overnight helped.  I think there's two different processes going on
that I found in one of my cob references.  The soaking that allows water to
split down the clay particles and increases plasticity, and a process called
"souring" that changes alkalinity to acidity through bacterial action and
promotes flocculation in the clay.

Mark Hoberecht
scarecrow at lerc.nasa.gov