Cob cob recipe

[Will Firstbrook WCB of BC]

Hi Phyllis and Benton,

The ideal cob depends on the quantity and quality of sand and clay.
Ideally the sand sound be angular not smooth beach sand. The ideal clay
should be really sticky.  You can partly test the soil by the jar,
scrape off the organic top soil and screen some soil to use to remove
rocks and roots etc. Add the soil to the clear jar and add water to the
top. Really shake it up then let it sit. The contents will settle in the
following order from the bottom up: sand, silt, clay and water.  The cob
mixture needs to be 50 - 80% sand by volume for it's compressive
strength. The clay should be 10 - 30% of the mixture depending on the
stickiness. The clay is the glue that holds the sand together. There
will also likely be some silt in the mixture the less the better. 

Many times the soil needs to be fortified with either sand or clay (or
both if the soil is really silty). You can make a test batch of cob by
mixing up some soil and water really well. Try to form the mixture into
a ball in your hand if it falls apart it either has not enough clay or
too much water or too much silt. If it is really sticky add some sand.
Mix well then add a bunch of straw (not hay) and mix well. Form the cob
into small bricks for testing. You can vary the mixture for a number of
test batches. Ensure you remember the mix for the small batches and let
the cobs harden. Once dry you should be able to tell what combination
makes the best and strongest cob. You can usually find clay near ponds
and lakes. Sometimes what at first appears to be clay is high silt
content, the test is clay is very slippery and sticky in between your
fingers. Sand used for concrete works great for cob.

Good luck!

Regards, 
Will


>-----Original Message-----
>From:	Benton J. Miller and Phyllis Rameriz [SMTP:phillis at itlnet.net]
>Sent:	Monday, August 11, 1997 4:08 PM
>To:	coblist at deatech.com
>Subject:	Cob cob recipe
>
>
>Hello,
>Greetings cob people!     We are searching for information regarding
>the "recipe" for the mixture of our cob. What is the ratio of clay,sand,
>and straw?    We took a soil sample from the land we plan to build on
>and we are not sure how to test it properly. We put it in a jar and
>filled with water, but it just looks like a jar of red mud. We are
>in western Oklahoma and the soil is very red. Down at the creek on
>the land the mud looked very "clayish".
>Anyway, any help would be greetly appreciated.
>If we need to purchase clay for the mixture where will we find it that
>would be close enough to make it financially feasible? I've heard
>the Austin,Tx. area has good clay soil. Just wondering.
>Recipe anyone?
>
>
>Many thanks,
>
>Phyllis and Benton
>