Cob Earth plaster & expansive clay

[Robert Bolman]

Martha Cather a NM geologist offered the following response to my earth
plaster test data plea:

> I would certainly avoid using swelling clay in a plaster, unless
> you have a climate where the humidity is going to be most regular.
> Swelling clays cause problems in everything from slabs to plugging
> up oil reservoirs.  One major factor is that there can be so much
> variability from clay to clay regarding the percentage of swelling
> clay in any given area.
> 
> That being said, most earthen plasters have a fairly small proportion
> of clay to sand, and you could probably add something like those
> polypropylene microfibres to help with the shrinking/swelling problems.
> But, given a choice, I'd try and pick the most stable clay I could.
> 
> Martha in NM, who is also a geologist (but not an engineering geologist)

My response:
Martha,
Thank you for your thoughtful response to my earth test data plea.

In an effort to devise the most environmentally innovative type of housing
possible, it means a lot to me to be able to use whatever clay soil is
available on site.  Immediately beneath the topsoil I have very sticky
clay.  Based on cracking that I've seen elsewhere, I would guess that it's
indeed a very expansive clay.  However, I've already installed an interior
plaster that seems to be working fine.  I mixed two parts clay slip to
seven parts sand to one part chopped straw.  It went on the bale wall just
fine with only very minimal cracking in the first coat.  The second coat
exhibited no cracking at all.  I feel that the clay was sticky enough that
I was able to get enough sand in the mix that cracking is not an issue. 

Given that the plaster went on the wall sopping wet and that the cracking
that occured was insignificant, can I not assume that subsequent changes in
moisture content based on relative humidity will be inconsequential?

Thank you,
Robert Bolman

Comments???