Cob Earth plaster & expansive clay

[Will Firstbrook WCB of BC]

Hi Robert, 

Sounds like you have good quality clay thus little clay is needed. At
one point when we were building a cob structure we encountered what at
first looked like good clay was infact mostly silt which is not
desirable at all. The stickier the better. Your plaster will probably be
quite porous due to the high water content at the time of application.
Your assumption appears quite reasonable.

Another point you may consider is if the plaster dries in direct
sunlight I have heard it sometimes cracks from drying too fast. You may
want to try a number of mixes (record your mixes) varying the
ingredients and plaster on spare bales of straw exposed to the elements
and see what seems to hold up best. Create some real world tests of
blistering hot (mirrors on a sunny day) and driving rain (from hose)
etc.

Regards, 
Will

>-----Original Message-----
>From:	Robert Bolman [SMTP:robtb at efn.org]
>Sent:	Wednesday, August 06, 1997 10:30 PM
>To:	coblist at deatech.com
>Subject:	Cob Earth plaster & expansive clay
>
> 
>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???
>
> 
>
>
>