Cob: Which straw?

[Shannon C. Dealy]

What you are looking is a source of fibers to provide tensile strength to
the cob mix.  A typical cob mixture will contain around 10% straw or other
fibrous material.  The source of the fiber is not critical so long as it
can meet some basic criteria:

   1 - Typically has an average length of 6 to 15 inches.  Shorter fibers
       can be used, but at some point the tensile strength of the cob will
       start to suffer.  Longer fibers can also be used, but at some point
       you will find it almost impossible to break a batch up into smaller
       "cobs" for you to carry to the wall (I made this mistake once and
       had to hack the batch apart with a machette before I could use it
       - oops :-) of course longer fibers can simply be cut into shorter
       lengths before using.
   2 - The sand and clay in the mix must be able to "grab" onto the 
       surface of the fibers, if it cannot get a good hold on the fibers,
       it will significantly reduce the strength of the cob.  The only way
       I know of to test for the strength of the bond between the
       sand/clay and the fibers is to make some small test bricks, break
       them into two pieces (by striking the center of the brick on a
       sharp corner - edge of a steel barrel, rock, or piece of concrete)
       and see how readily the fibers pull out of the two halves.  This is
       also a good way to check the tensile strength of your mix, in a
       reasonably good one, when the brick is initially broken, the two
       halves will remain held together by the fibers which still connect
       them, and will be extremely difficult if not impossibly for you to
       pull apart by hand.
   3 - It must have adequate tensile strength - this is the strength a
       material has when you pull on each end of it.  The dried stalks
       from a wide variety of grasses, including those used for grain
       production (wheat, rye, etc.) generally have sufficient strength,
       even though the individual stalks seem rather weak.  A simple
       (though highly subjective) test which will give you an idea of how
       strong these dried stalks can be is to take a one foot long (or
       longer) bundle about 1 to 1-1/2 inches in diameter in your hands,
       twist them together (one complete rotation should be sufficient),
       and then try to break it by pulling on each end.
       
All sorts of plants can be used in cob, though obviously some are much
better than others.  Some possibilities include: very thin vines, native
grasses, weeds, etc.  One possibility you might check is to see if you can
find out what plants were used historically for rope making in your
area.  One of the more common traditional materials - the insides of tree
bark is probably a very bad idea (deforestation), but people have
traditionally made rope from many other plants, and if it works for rope,
it probably will work for cob.  The ultimate test as always is to make
some test bricks with the materials you wish to use and see how it holds
up to attempts to break it.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 451-5177 |                  www.deatech.com