Cob Clay, Fibers, & FC

[Bob Bolles]

Ron, Will, Avalon, and all

I believe that it would be an error to think that there is "a recipe" for
making cob, adobe, light clay or earth plasters.  With cob, light clay and
plasters each layer must bond together, whereas with adobe and light clay
blocks, the mortar bonds the blocks together.

In my limited experience, the ONLY way to determine the "right" mix for the
clay in your area (as mentioned) is through experimentation with the
clay(s) in your area.

The first step is to determine the clay content of your source.  The
simplest way is to perform a glass jar test:  using a wide-mouth jar, fill
it approx. ¾ full of soil - add water to near the top - mix thoroughly,
screw on the top and shake.  After the mix settles, the heavier aggregate
will settle to the bottom, and the finer materials will be at the top.  One
note of caution:  silt is nearly indistinguishable from clay, and there is
no bonding characteristics to silt. 

Your final mix will need 15-25% clay, but the real test is how it performs.

I have found that the easiest way (for me) to test a mix is to make test
blocks (bricks) with a wooden form (just right, Will) - smaller blocks dry
more quickly and are easier to handle.  I "pack" the mix into the form.  As
Will mentioned, keep a good record of the mix for each test block.

Not enough clay, the blocks will crumble, and the surface will be sandy.
Too much clay, and the blocks will crack.

It is my understanding that much of the original adobe blocks were made
with little or no fiber, but I always use some sort fiber because I believe
that it makes a stronger end-product.

It would be interesting to know if anyone is using, or has used  light clay
mixes for their cob-style construction.  

As a point of clarification regarding how I use the term, adobe-style
blocks with no fibers would be heavy-clay, and the addition of any sort of
fiber (or lighter material) produces light-clay.  We have used wood chips,
sawdust, perlite, vermiculite, straw, grass, etc.  I would think that pine
needles would work well, but I have not personally used them - not an
indigenous material in Mexico/So. Cal.  

I also look forward to using paper-pulp with the clay mix.  I have yet to
figure out a low-tech way to break down the paper and mix it with the clay.

At this point, I still have deep reservations about the papercrete and the
actual bonding characteristics of the Portland Cement.  I do, however,
think that a lime plaster would solve the water-absorbency characteristics
of the material.    

Regards
Bob