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RE: GSBN: Earth plasters
Sven chiming in....
During the workshop with Tom we made a last minute decision to try his
Due to the fact that we did not have the 'fermented water' which he likes to
add to the plaster and we were not able to allow the plaster to sit and
ferment, the proccess was given several 'boosts'. Silage was added to the
mix along with hay. I believe the hay was where the main issue arose as the
hay had been wet in the past and had some pretty good mould in it. The next
issue was that we were plastering the interior of an unheated space during a
cool and not entirely dry part of the year.
Very soon after the plaster was applied at 40mm thick we were seeing black
mould start to grow on the surface and not the farmiliar white fluffy mould.
I scraped the outer surface off to find the mould did not apear to be active
deeper into the plaster. Or perhaps I should say it was not visible. We
aplied lime water to the areas of mould and almost without fail the mould
was gone. Once the plaster dried all sign of the mould was gone.
In the future I would be happy to use fermented plaster but I will not use
mouldy hay in it and the silage may be reserved for animal food only.
By the way, this issue is on a wall which is in the living area and spans
into the dining area of the home. There will be a wood stove installed
within a couple of feet of this wall.
Does anyone have a suggestion for a finish coat? Would it be good insurance
to use Borax in the final earth plaster? Or something else??
Is the mould likely to be dormant and the wall MUCH more prone to mould
growth in the future? My thought is that it would be slightly more prone to
mould but given the location within the home and the wood stove we sould not
have to worry too much down the track. My thought is that the Borax will be
Thanks for any ideas!
From: GSBN [<a target="_blank" href="mailto:GSBN@...]On">mailto:GSBN@...]On</a> Behalf Of Mark
Sent: Thursday, October 11, 2007 3:49 AM
Subject: Re: GSBN: Earth plasters
Thanks for your submission and insight. The first I'd heard of 'fermenting'
straw (or paper) was from some friends from NZ, Sven and Sarah Johnston,
who hosted a workshop and a guest presenter was Tom Raven, the colorful
Frenchman, who uses a fermented mix, like bakers use a yeast starter for
baking breads. Tom swears by this fermenting and concurs with you on added
durability of the earth plasters. The Johnstons found they had some mold
issues, maybe as a result of this mix tho. Have you experience any issues
with molds? Maybe you'd care to chime in Sven?
Colorado Straw Bale Association
> [Original Message]
> From: Graeme North ecodesign@...
> To: GSBN GSBN@...
> Date: 10/10/2007 3:08:32 PM
> Subject: Re: GSBN: Earth plasters
> We have found the addition of pulped paper as a fine fibre very very good.
> Its made either from office paper or newspaper - it can be made by
> soaked paper into a concrete mixer with lots of water and just letting it
> roll until completely broken up , then draining off excess water before
> It gives a very fine fibre which is enormously beneficial in terms of
> increasing strength, and durability of earth plasters. If it ferments a
> even better (all ferment seems to help earth plasters - using water that
> had a bit of straw left in it for a week or two to start to fizz is good
> well) - there appears to be some enzymic reaction that takes place
> paper or ferment water and clay that helps workability and durability.
> Paper pulp eliminates dusting from earth plasters - helpful especially if
> the soil is a bit silty. It also sticks well - we use it over
> pre-compressed straw without meshing.
> We also use it as a finishing plaster over drywall - a very nice finish
> it sticks like ...glue.
> It is also very effective as an additive to earth bricks as well -
> robustness, durability, and also lowers density giving better insulation.
> The amount of paper pulp that can be added can be quite high - we often
> up to around 30% or so by volume to plaster , and have made mud bricks
> up to 50% paper pulp - cardboard houses anyone?
> I am currently looking at building with cob and mud brick mixes using
> roughly 2 parts clay-soil, 2 parts wood shavings (long fibered rippings)
> straw, and 1 part paper pulp - very tough, very light, easy to work, and
> looking promising. Very good insulation, and very nice to use as gravity
> increases - remember that there is plenty of evidence that gravity doubles
> in strength very thirty years.
> As to lime over earth - I9ve found it works well so far IF there is good
> keying and wetting - if the initial earth is a cob type mix with lots of
> (straw) fibre with plenty of fibres left hanging out of a rough surface,
> then that is good too for tying the layers together.
> I9ve just lost a patch off a bit of a small experimental earth plastered
> whitewashed straw wall where the earth plasters layers were not keyed well
> together. Keying is good.
> Graeme North Architects,
> 49 Matthew Road,
> RD1, Warkworth,
> New Zealand 0981
> Ph/fax +64 (0)9 4259305
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