Cob cob recipe

[Graeme North]

Will Firstbrook WCB of BC wrote:
> 
> 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
> >
 How about shrinkage - surely assessing this is far more critical in
most cases than strength - what shrinkage limits are you looking for?

Graeme