Cob earthen plaster on wood

[Michael Saunby]

On 01 February 1999 05:52, Keith Breuker 
[SMTP:flyfisherkb at centuryinter.net] wrote:
> Does anyone have experience plastering wood with earthen plaster?
>
> I would like to plaster the poles and some other wooden areas in my SB
> house.  All wood is kiln dried and part of the interior of the house.  I
> would staple burlap to the wood and probably add some wood glue to the
> plaster mix to help with adhesion.
>
> Has anyone else done this?  What were the results?
>
> Keith

Plaster, earth, lime, and most other sorts I expect, adhere quite well to 
any porous surface.  The surface shouldn't be too dry though when you apply 
it, and of course if the subsequent expansion, shrinkage or any other 
movements are different then the plaster is likely to crack, but not 
usually to fall off. I'd be less concerned by adhesion than by the 
cracking.  Traditional earth plaster on my house (17th century) uses hay in 
the first coat and cow hair in the top coat.  The glue is more likely to 
alter properties of the plaster (for good or bad I couldn't say) than make 
it stick.

Another note, more to myself to remind me to investigate further, is that 
experience so far is that the inside of cob walls, even in VERY wet weather 
are very dry, they draw the moisture from the plaster far too quickly 
unless you get them very wet first.  I know that clay soils have strange 
properties where water is concerned, plants have difficulty drawing 
moisture from them, and large quantities of water can sit on the surface 
for long periods without much penetration to even short depths.  Also large 
pieces of cob wall are much lighter, than bricks of the same size.  A brick 
shrinks when made, but cob shrinks internally filling itself with small 
voids, like it's got bubbles in it, so the insulating properties of cob 
likely have nothing to do with straw, it's all to do with the properties of 
clay soils.  In a way the straw helps to hold the clay apart as much as it 
holds it together.


Michael Saunby