[Cob] RE: cob plaster

[Shannon C. Dealy]

On Mon, 9 Feb 2004, Mary Lou McFarland wrote:

> Exactly how fine does the shreaded straw for the plaster coat need to be?
> I've had some experience with "horsehair" plaster  (tearing out only), has
> anyone tried other fibers such as hair or  hemp rope fibers, cut and
> separated?  It seems to me that finer is better but just how fine...1/2
> inch...1/4 inch lengths?

This is kind of a matter of personal taste and technique.  After cutting
the straw with a saw, I generally run it through a 1/4 - 1/2 inch screen
to keep the large stuff out (you will end up with longer straw than the
size of the mesh as some with go through long ways -- the point is to
get the really big stuff out), though a number of people I know (all of
whom do alot more plaster work than I do), have started using much longer
straw in some of their plasters (even full length straw in some cases),
and doing alot more tool work in order to bring out the straw fibers in
the surface (some people perfer the straw to show, others prefer it to
blend in).  I haven't tried the longer straw, merely observed.  In other
words, the length isn't particularly critical, it just depends on
what you like.  Of course from a structural perspective, longer fibers are
better, so if you are using fine fibers (like manure, horse hair, etc.) I
would tend to leave them longer since they will readily blend into the
mix, where straw being coarser will tend to show particularly in longer
lengths.  As always, spending some time experimenting to find the best
tradeoff between your goals (looks, bond strength, structural strength,
etc.) is usually a good idea.


Shannon C. Dealy      |               DeaTech Research Inc.
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