Cob Re: Suitability for art nouveau architectural material?

[Shannon C. Dealy]

On Sat, 7 Nov 1998, Bill Martin wrote:

[snip]
> "diamonds in the rough". I wonder how refined a medium this is for this 
> purpose... can the finished surface be hewn somehow to a glass-like 
> surface in the finished product?

While I haven't seen any "glass-like" cob finishes, it is possible to get
a very fine smooth finish from a cob plaster by doing the following:

   1 - Use only very fine sand for your mix (run it through a fine mesh)
   2 - Use very fine fibers in the mix (dried cow dung is a good source)
   3 - Run your clay through a fine mesh to break up clumps and get rid of
       any large impurities.
   4 - Once you have applied the cob plaster to the wall or other
       structure, polish the still wet surface with a smooth flat object,
       a lid from a small plastic tub (with the outer edge removed) works
       very well because it is flexible and can follow irregular surfaces.

> Could it be used as a cast for pouring 
> molten iron... or is this precluded by the limitations of the medium?    

Maybe - I've never tried it, but I used to do alot of metal castings.
I think you would need to fire the cob like a ceramic pot before pouring
the iron, since if there is any trace of water in the cob, it would
turn immediately into a super-heated steam when the molten iron makes
contact with the cob.  This would at the very least crack the cob, and
if there is alot of moisture, it could literally explode splattering you
with molten iron.

> What are the strength and temperature tolerances?

Strength depends on your cob mix, but cob is generally very good in
compression (though it's compressive strength will drop as the sand
content of your mix decreases).  Cob doesn't do as well from a tensile
strength perspective, but this will generally increase as the fiber
content of the mix increases (within limits).  There are no hard numbers
available for strength, since it will vary greatly depending on the type
of clay and fiber used as well as the particular mix ratios.

With regard to temperature tolerances, once it has dried, it shouldn't
have any problem with temperatures from well below zero, all the way up to
a few hundred degrees.  Prolonged continuous exposure beyond a few hundred
degrees, would probably eventually oxidize the straw fibers inside the 
wall and significantly reduce the tensile strength.  Short term exposure
to a couple thousand degrees (such as a fire in your house) probably won't
raise the interior temperature of the wall high enough to cause any real
damage, though it would be interesting to run some tests along these lines
to determine at what point damage occurs to the straw fibers.

> If the straw is ground to the consistency of a powder, would that make
> a significant difference in the finish?

Yes, but the straw fibers are what ties the cob together, if you grind it
to a powder, then it can't perform it's job.  For use in a fine finish
plaster, I think that 1/4" - 1/2" fibers would be close to the practicle
minimum, though you could switch to a thinner fiber (such as from cow
dung) to improve the quality of the finish.

> Can it be sanded...

You could apply a sander, (I've never tried it) but I wouldn't expect
it to improve the finish.  A better bet (assuming the cob has already 
dried) would be to thoroughly wet the surface and rub it with a smooth
hard object like a river rock, or to use a fine cob plaster as described
above.

> and/or blow-torched...

Applying a torch won't hurt it, but it won't do much either since unlike
wood, cob doesn't burn so you won't get that charred effect, all you will
get is a deposit of soot from incomplete combustion and a few bits of
charred straw where it sticks through the surface of the cob.

> and/or guilded? 

Haven't a clue.


Shannon C. Dealy
dealy at deatech.com