Cob Clay, Fibers, & FC

[vtrac at alternatives.com]

>Greetings All,
>
>First off, I'd like to say how great it is to find you guys online.  I've been
>interested in cob for some time and finding a discussion group of fellow mud
>puppies was a thrill!
>
>So, for my first question/contribution to the list, I have a concern about
>clay.  In this area, we have two types of clay:  an orangey-reddish clay that
>is most often found in the upper levels of soil, and a sticky grey clay (it's
>called gumbo around here) that is often found a bit deeper.  While I suspect
>clay is clay, instinct tells me that the gumbo would be a better choice to use
>for cob.  From the broad range of experience out there, I'm wondering if you
>all feel there is any benefit to using one or the other?
>
>Second, I'm curious about what plant fibers work well in cob.  I've read the
>archives (yes, ALL of them!) and while I've read of folks using plant material
>other than straw, I'm curious if anyone has tried pine straw?  With all the
>pines in South Carolina, we have an abundance of the stuff.
>
>Finally, regarding fibrous cement: if I understand correctly, one of the main
>drawbacks to the material is water absorbency, making it unsuitable for
>exterior walls and such in wetter climates.  However, what about using it for
>lightweight, non-structural interior walls?
>
>Metta to all,
>
>Ron Cameron

Hi Ron,

Although I am only in the process of building and still learning, I have
found that clays can be very different and I am no longer sure that all
clay is suitable. For example, I have 1 load of a very blue/grey clay that
is extremely sticky. One might think that the stickier the better however
this stuff really absorbes the water and thus contracts and expands a great
amount. We started to get a lot of cracking until we started adding in a
lot more sand, almost twice as much as I am used to using with the grey or
brown clays. I have also noticed that when it rains ( my walls are still
exposed) the walls really soften considerable more than other walls I have
worked on in which heavy rain will merely soften the outer quarter inch or
so. Lesson being,  do lots of tests before trucking in your clay and a
small test brick will also act differently than a large wall.

As for pine straw I can't say other than the longer the fiber the better I
think. We have been using wheat and oat straw with 10 " stalks. You want to
avoid organic material that will decompose and you should be able to give
the fibre a good tug without it breaking. This gives the cob its tensile
strength.

I am not really familiar with fibrous cement, but I thought that the cement
would provide enough stabilization.

Anyways, good luck to you.

Ian Marcuse
Down to Earth Building Bee