[Cob] purpose of straw

[Marlin Nissen]

Historically Pharoah punished the Israeli slaves by "forcing them to make bricks (adobe) without straw" in the Old Testament.
   
  I think it's still up in the air how/what is an adequate replacment....there were some great discussions (archive) about pine needles, shredded paper, grasses/reeds etc. BET that the key is fibrous tensile strength kindof anchoring one area of CLAY to it's neighboring chunks of clay.
   
  An anectdotal observation is the Cob I've made with the most long, strong straw has been excellent and areas where the clay content/straw ratio was high was crumbly.
   
  My take on Cob:
   
  1) Sand are the rocks that you build the compressive wall upone
  2) Clay is the glue/cement
  3) Straw is the rebar/fibrous material that makes the wall HAVE to move as a whole rather than individual particles moving (also helps to dry the mix)
  4) Water as solvent for the clay enough to get it to smear AROUND the sand particles.

   
  Marlin
   
  
Stacey <painterjane23 at yahoo.com> wrote:
  Outside of tensile strength, straw is also very useful
to add insulation to cob walls. 

If you are trying for natural conditioning (passive
solar) design, this is especially important for north
facing walls. In fact, to build the entire north wall
out of straw bales is not a bad idea. But then again,
in Georgia, you might not need that so much:!


> What is the purpose of straw in a cob mix? Nobody
> seems to "really 
> know" what the role of straw is anyway. Is it there
> to hold the cob 
> together while the wall is still wet (like a free
> form), or to keep 
> the wall from crumbling incase it cracks later (like
> reenforcement), 
> or to allow air/water to move through the wall
> (because straw is 
> hollow)? The problem is that nobody knows the reason
> they used straw 
> because they didn't leave behind notes on how and
> why they built that 
> way, and it's been a while since they lived here.
> What do they do in 
> Africa? Do they use straw "in" the cob? Can any
> other plants be used 
> as tensile such as long grasses? I'm almost ready to
> start cobbing 
> but straw is just unavailable in GA right now, and
> what straw there 
> is has a very high price on it. I'm not willing to
> pay three times 
> the price for it if there's a substitution. I would
> love to just go 
> out in the field and get some tall grass if it would
> suffice. It's a 
> heck of a lot cheaper!
> 
> Chow,
> Damon Howell
> North Georgia, US
> 
> 
> 


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