[Cob] more on plasters & cob time

[Buckaroo Bonzai]

Darel wrote:

Yes, I know about the Japanese master mason, going to
the U.S. for the session.

> We just returned from a workshop at the Steens'
> Canelo Project in AZ.  The 
> Japanese master plasterer used 90% straw in some of
> his finish plasters, and
> as little as 10% (by volume) in others, depending on
> the surface he wanted
> to create.  Additionally, in Japan it's typical to
> let the clay and straw  sit together for a year so
the elements can "marry."

This point is misleading however, unfortunetly.  
Ideally, this would be the case and in the case of
important buildings such as temples, shrines and rich
peoples homes this is done for even two years.  For
more common folk, in days of old, the mix would be
prepared in the autumn and aged though to the next
spring or summer when it would be applied.  It's also
a little bit more complicated as more new rice straw
is aged each month or   so and finally a few days
before application.   Also note that this is mainly
for the inner wall of a wattle and daub, not for the
next outer layers.

The current modern architectural standard in Japan
requires a minimum of immersing the straw into the
clay mud for two weeks prior to application.  The
intend was to make using soil a faster process so to
be a little more competitive.  The old methods
required two much time  and time is now money so the
old methods became uneconomical and unable to compete
with fast cement fixes where no skill is needed.  That
is why there are few of the master masons remaining
and their knowledge disappearing.  Though many of us
are trying to preserve as much of it as possible. 
Just a historical note for you.


This is different from the use of straw in a lime
plaster.

> The straw breaks down
> so thoroughly that it almost disappears, but
> apparently the fiber is still  there strengthening
the plaster.

More straw is added so you will have very decomposed
small with then graduated sizes....

> We saw "plaster"  corners that were 3"
> thick, both on straw bales and on cob bricks. 
Be sure to distinguish between the layers.  The finish
layer is the thinnest and normally has a different mix
or materials.

> Finish plaster was done atop
> those already thickened edges, sometimes another
> inch or so thick.
> 
> My own experience echoes Shannon's.  The mixes I've
> used for 1/2" plaster,
> for 2" plaster, for 2-12" thick cob, and for floors
> are all very similar.
> 
> Cob has always seemed painfully slow to me, but
> friends here are speeding up
> the process with tractor and tiller cob and also
> using form boards so they
> can do much bigger lifts at a time.  Mechanical
> mixing means a drier mix,
> which also can be stacked higher before spluging. 
> Two people can put up a
> cob wall of 20' in two days.  It's moving closer to
> rammed earth but does
> not contain concrete.
> 
> Barbara     
> 

Thanks for you input Barbara.  It was refreshing.

Sent by Darel to us.

Cheers


		
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