Cob: light clay ratios

[Shannon C. Dealy]

On Sat, 28 Apr 2001, Robert B Krueger wrote:

> 
> Why would you chop the straw when making cob.  The whole purpose of cob
> is to have long strands of straw to assist in reinforcing the clay and
> sand mixture.  Then when sewing this together, the straw is forced into
> the underlying cob wall making a nice tight bond.  

Two good reasons that come immediately to mind:

   1 - If you are making a cob plaster or other thin layer finishing mix,
       short straw will give a much nicer look, and since this type of
       layer is not structural, the longer lengths are not needed.

   2 - There is such a thing as straw that is to long for cob.  In the
       first batch on my zero dollar building, I was using hand harvested
       grass which was on average two to three feet long.  When I
       completed mixing the batch and was ready to put the cob on the
       wall, the long grass straw held the mixture so tightly together
       that I could not pull off a "cob" to place on the wall, and while I
       am fairly strong, there is no way I could pick up an entire batch
       as one piece to place it on the wall.  I was ultimately forced to
       hack this batch apart with a machette in order to place it on the
       wall.  After this I used the machette to cut all the grass down
       to size before adding it to the mix.  For most people using
       conventional small rectangular bales as their source of straw, this
       is not a problem since the straw in the bales is usually not overly
       long.

Shannon C. Dealy      |               DeaTech Research Inc.
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