[Cob] plaster

[Shannon C. Dealy]

On Mon, 9 Feb 2004, Jilly wrote:

> Can you please post links for plasters suitable for cob? I am trying
> defend mixes, nothing is working the way I would like. I am looking for
> a smoother finish. So far, these are still "scratchy".

How smooth your finish is will be a function of how coarse your
ingredients are and how you apply it.  Sand comes in all shapes and
sizes and therefore could be a major determining factor, though
depending on your mix and application technique, even coarse sand can be
used and still result in a quite smooth surface.  Basically, what has to
happen is that the finer ingredients (clay / lime / gypsum / whatever)
have to fill in the spaces between the coarser ingredients: sand and
fiber (straw, manure, etc).  Of course the coarser your sand and fiber
are, the more finer ingredients you will need to fill in the gaps.

A simple plaster that will give a very smooth surface can be made using
a good high sand cob mix, but first run the sand through a fine mesh
window screen, and only use the small particles that pass through.  Run
your clay through a somewhat coarser steel screen (it would just clog
a window screen) to get out any major impurities.  For fiber you can use
broken up dried manure, or finely chopped straw -- I use a circular saw
on a straw bale (the side with the straw ends sticking out), with the
blade guard retracted, I sweep the blade of the running saw back and forth
across the face of the bale (sideways -- perpendicular to the direction
you would usually use for cutting with a circular saw).  Of course be very
careful when using a saw in this manner.  Now, using these refined
ingredients, make a cob mix in the usual manner.  To apply the plaster,
wet the wall with a spray bottle, then apply the plaster with a trowel,
your hands, or whatever else works for you.  Finally, smooth it out with
whatever tools work for you, and then float the surface, which is in some
regards kind of like polishing the surface, by working the surface with a
float you can effectively bring the edges of the sand particles and fibers
flush with the clay (and/or lime / gypsum / etc)  particles in the surface
of the wall.  While floating the surface, it may be desireable or
necessary to occasionally lightly mist the wall with water while you are
working it, but keep the water to a minimum.  If your surface is
irregularly shaped, you don't own a float, or you want to try something
different, try cutting the center out of the lid of a plastic container
(like cottage cheese comes in) and use that to float the surface (with cob
I find it's much easier because the walls I build are always curved).

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
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