Cob Do I really need sand in my clay?

[M J Epko]

At 12:15 PM 7/28/97 -0600, FROG wrote:
>I have done some experimenting with cob/earth plaster 
>mixes and it seems to me that there is very little 
>correspondence in how many cracks develop and the ratio 
>of sand to clay.  Does anyone have any reason why I 
>can't therefore just dispense with the sand?

	Pete Fust did a sans-sand cob wall (about 7 feet tall, maybe a dozen feet
long, incorporating a few good-sized rocks & twisty wood; and he did this
in *one* day) at Black Range Lodge in New Mexico. I thought it was pretty
impressive, myself - but there's tons for me to learn. The wall's a year
old or so & hasn't been coated or 'touched-up' in any way. It doesn't show
any signs of significant initial or subsequent cracking.

	The thing I liked most about it was his "hose" demonstration: he turned
the water hose onto it, and it didn't erode at all except under significant
pressure. Then he took the hose to the sand-ified earth stucco on an
adjoining SB wall, and it immediately started eroding right off.

	Linda Smiley from Cob Cottage Company was there, and I asked for her
thoughts about it. She was very nice (she really is smiley), explaining
that the Cob Cottage opinion is that the sand is what comprises the bearing
capacity of the wall; that the clay is the binder. Her concern was that a
sans-sand bearing wall would be subject to load-induced failure over time.
She also speculated that even thought the local Kingston soil is almost
totally clay per the mayo-jar test, there may be enough non-expansive
little chunks suspended in it to get away sandless. She voiced concern over
the long-term viability of the wall, stressed the importance of
experimentation with and close examination of the local soil to understand
what it is and isn't capable of, but wouldn't under any circumstance
recommend omitting the sand.

	If we're talking earth-stucco only, bearing capacity's not a concern & I'd
tend to go with what performs best in the specific function.

	Once I get moved and figure out exactly what I've got to work with out
there, I'll be doing quite a bit of experimenting. I've been advised that
it likely is mostly clay, and I'll certainly be checking it out as-is.



   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                     M J Epko
              duckchow at ix.netcom.com
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     Nothing is in reality either pleasant or 
     unpleasant by nature; but all things 
     become so through habit.
                    - Epictetus, Encheiridion