Cob: Re: Cob

[Shannon C. Dealy]

On Wed, 21 Aug 2002, Darel Henman wrote:

[snip]
> First off, for example comes repairs.  Inevitably a nick, a scratch, a
> gouge, a really big gouge or hole in cob will happen.   Is the simple
> pluggin in of a new cob plug sufficient in strength for repars.

For the kinds of accidental damage that will typically happen to a
building (i.e. it wasn't due to someone ramming it with a car), you can
just mix a small amount of cob just like you did for the original
structure and plug it, though there are a few things to keep in mind:

  1 - Shrinkage: for larger holes, you need to keep in mind that unlike
      when the structure was built, there will be no uniform settling of
      the cob, the material around the hole is going to stay put, while
      the material you put into the hole is going to want to shrink.  In
      order to prevent/reduce this problem, you will want to minimize
      shrinkage of the cob by using a mix that is high in sand (which
      won't shrink) and and has a low clay/water content (which cause
      shrinkage during drying).  For larger holes, you will actually want
      to fill most of the hole with brick, rock, or a dried piece of cob
      in order to prevent shrinkage, and then use fresh cob to mortar
      everything together.

  2 - For a larger hole, you will want to put some smaller holes into the
      sides of the bigger hole in order to provide a means for the
      fresh cob to bond mechanically with the old cob.  This can be
      done with a small chisel, or a hand drill with a bit designed for
      cement, ceramic, or just one you don't care about anymore.  Some
      people like to just drive some old bent nails half way into the cob
      to provide a mechanical bonding area.

  3 - When applying fresh cob to dried cob, wet the dried cob around the
      patch area thoroughly and repeatedly, it won't really soften the
      area if it's good cob, but you need the surface clay to absorb
      some water for it to bond properly with the clay in the fresh cob (a
      spray bottle full of water works well for patching and plastering
      jobs)

  4 - Applying a little excess cob to larger patches so that they bulge
      out will help with shrinkage by providing extra material that
      can be pulled in as the patch shrinks.  Once the patch has had some
      time to dry, any excess can be trimmed.

> Coloring is a different issue.  You'd want the same color.

If you use the same materials in the same ratios, you will get a good
color match, but it will take some time since the patch must dry to the
same degree the rest of the wall has dried to, since the moisture content
affects the color.


> What about maintenance?  Depends on the surface finish I'd think.

Definitely.

>
> What about for a pure cob exterior surface... after say five years it
> gets worn down by the elements wind and rain.  How can this be
> maintaned.  How did the old timers do it for the last eight hundred or
> so years.   A new layer somehow keyed in and plastered over the one
> being eaten away by the elements?

Traditional english cob would get a fresh coat of limewash each year,
this protects the cob from errosion and wind damage, since it is the
limewash that actually got damaged.  Initially, these coats would
penetrate into the surface of the cob, and later coats would build up this
outer shell which over time turns into what is essentially a thin durable
limestone veneer over the outside of the wall.  Before everyone starts
thinking about how much work this will be, don't compare this with modern
house painting.  This isn't like modern paints and materials, there really
isn't any prep work, you simply apply the new coat.

For people who prefer bare cob, you can simply make up a cob plastering
mixture which is basically your standard cob made using finer materials:
finely chopped straw or dried manure for fiber, run your sand/clay/earth
through a sieve to eliminate rocks and break up any large clumps of clay.
Thoroughly wet the outside of the building and apply the mix in a thin
layer over the surface.  Done properly, the clay in the old cob and
new plaster should provide a decent bond, but surface cracks and
imperfections along with straw fibers sticking out of the cob wall will
improve the bond.


Shannon C. Dealy      |               DeaTech Research Inc.
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