[GSBN] The Reasoning For Hessien

Ian Redfern ian at adobesouth.co.nz
Sun Apr 20 18:16:39 CDT 2014

Good morning Sarah, Sven, Derek  & All,

I agree with Derek's summary  and have "another" piece of physics and
chemistry to add  :

Lime plasters (using fully hydrated quick lime) and of a high calcium
content with low or no clay contaminants  MUST HAVE  free water in the mix,
which acts like a catalyst,  to transfer the atmospheric carbon dioxide
(CO2) into the plaster matrix which reacts with the calcium hydroxide  to
create the calcium carbonate we seek as the durable white stuff we all love
!    This free water must be at the free (outer) surface ! ! ! ! ! !
The apparent surface temperature is critical for this reaction to occur and
prevent the surface not to dry and/or go slimy as the plaster freezes and
skins over ­ one good measure is to consider the lime skin as akin to ones
skin where the hypothermic effect of wind shearing over the surface causes
excessive evaporation and under cooling  =   not a good scenario in either
terms  a) for the surface drying and loss of catalyst free water  =  kills
the surface and inhibits further reaction within the plaster layer     b)
reduces the skin temperature to below the "freeze" temperature (around 8 ­
10 c) which again kills the surface  =  dead and crumbly plasters that do
not perform in your exposed environment !

Another consideration is the vertical chimney effect these high winds will
have on your proposal

Regarding key of plaster onto a strong substrate ­ I refer you to the
published works by Holmes & Wingate  or Schofield  who have historical
techniques to mechanically fix lime plasters ( 12 + thick) to solid walls

Following inspection of a Earthquake damaged straw bale  building where very
thin skin stucco with acrylic paint was used as a weather shield over
earthen plasters  I can vouch that this do not work, especially on exposed
faces ! ! ! ! !   Water will get in and cause loss of bond and worse

For  what it is worth a well mixed earthen plaster with sufficient clay to
bind and lots of grit and straw chaff (fine cob stuff) will weather and be
amenable to regular clay slip repairs and surface maintenance, as we imagine
to occur in historic Taos N M village life and other traditional cultures
where earthen buildings are more the norm and regular maintenance part of
the culture.
Here at adobe cottage we use a thin skin lime plaster render with a lime
wash (diluted lime putty brushed on) over cement stabilized adobes  as a
sacrificial weather skin and acknowledge this needs random minor repairs
where delamination has occurred on the exposed corner and walls.
(as an aside, lime plaster with the addition of a hand full of fresh cow pat
that has been well massaged onto a protected wall that has a good key
[roughened] and is kept humidified and protected from wind and cold has
proven effective and sticks like the proverbial . . . . )


Ian  E  Redfern

  www.adobesouth.co.nz   Ian Redfern
 Adobe South
 A:    5 Lancewood Rise, Onerahi, Whangarei
 P:     09 436 4040      M: 027 490 2324
 E:     ian at adobesouth.co.nz

From:  Derek Stearns Roff <derek at unm.edu>
Reply-To:  Global Straw Building Network <GSBN at sustainablesources.com>
Date:  Monday, April 21, 2014 8:02 AM
To:  Sarah Johnston <sol_design at yahoo.com>, Global Straw Building Network
<GSBN at sustainablesources.com>
Subject:  Re: [GSBN] The Reasoning For Hessien

Hi, Sven,

I¹m concerned about the phrase:

> The reason to install the hessien is to stop  the lime from cracking as the
> body coat dries and the reason to apply the lime while the body coat is still
> wet is to keep the lime damp in a very dry and windy climate.

Some historians would tell us that ³the reason that the Greeks sacrificed
goats before the building of a temple, was to protect the structure from
earthquakes, lightening, and other natural and godly risks.²

I fear that both of the asserted ³reasons" above have similarly large
appeals to human psychology, quite separate from the amount of support from
physics.  The ³reason² (motivation) for our making a choice reflects our
hopes, knowledge, experiences, and fantasies.  The ³reason² (cause and
effect) that a technique works or fails in a particular building reflects
the physics and conditions involved.  Our motivation may or may not overlap
the physics, but they are seldom identical.

Returning to the ³reason² given for using the hessien (hessian), the hope is
that it will ³stop the lime from cracking as the body coat dries².  So let¹s
think about how congruent this hope might be with what we know about the
materials.  The clay body-coat of plaster will still shrink as it dries, and
the hessian can't stop the shrinkage.  Hence, the hessian must either move
and shrink with the body coat, or move independently of it.  Each of these
possibilities increases the chance of delamination, between some layers of
the plaster system, and there will be some stresses between the layers in
any case.  Similarly, the slip coat on top of the hessian will want to
shrink as it dries, and will either move with the hessian, or not.  In the
version described below, it sounds like we have eliminated the clay slip
coat over the hessian described previously, and gone directly to the lime

Next we apply the lime over the clay-soaked hessian, or else over a
well-keyed, fairly wet clay slip layer, which shows our optimism.  Wet clay
plaster is very soft.  The maximum possible strength of a physical key in
wet clay is very small.  I have no idea about the bond likely between lime
and clay-soaked hessian.  We now have a thick body coat of clay plaster,
still drying, a layer of damp, clay-impregnated hessian, perhaps a layer of
damp clay slip, and a layer of wet lime plaster.  All drying and moving at
different rates.  But we plan for them all to cure strong and well-bonded to
each other.  

Further, we believe that the water in the clay layers will make a
substantial contribution to the moisture curing of the lime plaster exposed
to the "100km/hr winds" of "a dry and windy climate."  During this cure, the
moisture in the clay is entirely under the lime layer.  The wind and dry air
are entirely outside the lime layer.  So we would expect a significant
moisture gradient across the thickness of the lime layer.  The lime layer
will likely be pretty stiff within a couple of days, while the clay layers
will take a good bit longer to dry.

The suggestion is made to punch holes in the hessian after it is applied to
the clay body-coat, to ³provide a bomb proof key.²  I assume that this
reflects, felicitously, the very limited New Zealand experience with bombs.
I¹m unconvinced that we can get a strong, reliable bond this way.  I¹d like
to learn how you would go about punching holes in the hessian over wet clay,
without distressing the hessian/clay bond.

I started the next to last paragraph with the words, ³we believe², but in
fact, this system has too many unknowns and assumptions for me to believe in
it, without very specific testing.  I am well aware that Sven also has his
doubts about the proposals, and is trying to find a way to make things work
under difficult conditions.  I have great respect for Sven, and his
experience.  I fear that if he allows himself to get pushed into using this
plan built on someone else¹s fantasies, it could backfire very badly on him.
Putting your reputation on the line, using an untested system invented by
someone else (conjectured would be a better word), is very risky.  Be
careful, Sven.  

Best wishes,

On Apr 20, 2014, at 12:44 PM, Sarah Johnston <sol_design at yahoo.com> wrote:

>  Thank you to those who replied to my earlier post.  Here is some background
> on this project/concept.
> The reason to install the hessien is to stop  the lime from cracking as the
> body coat dries and the reason to apply the lime while the body coat is still
> wet is to keep the lime damp in a very dry and windy climate.  It is also very
> nice to be able to get all the plastering completed at one time rather than
> having to come back in 6 or 12 months.  While tarps are always used to protect
> the lime walls, it is very difficult to keep the tarps in place during
> 100km/hr winds.  Summer is too hot and winter too cold so the window of
> opportunity is very small for lime application.
> While I have not personally used this technique, a friend who does use it is a
> true believer.  My concern is delamination in the long run and his experience
> can not debunk that concern.
> Does anyone have thoughts on the following......
> The hessien is worked into the face of the body coat, wire brushed to lift the
> fibers then punch holes through the hessien to create a solid key into the
> body coat.  Lime would then be applied directly to the hessien and would key
> into the holes as well as the fine fiber of the hessien.  The hope is the
> hessien will prevent the cracking and the holes will provide a bomb proof key.
> This technique could  help in situations that Derek has pointed out, when you
> have a lime that does not like to bond to the clay, the hessien and the holes
> create that key.   Test walls are being prepared.  We will try 10mm dia, 20mm
> deep @ 75 centers and 10mm dia, 15mm deep @ 50mm centers.  sorry if you do not
> speak mm, I have mostly used US spelling :)
> This project is being covered by the TV show Grand Designs so the techniques
> that are used could be very well publicized, thus a double reason to make sure
> everything works very well!
> Would love to hear any thoughts!
> Thanks,
> Sven
> Sarah & Sven Johnston
> Sol Design, Ltd. 
> 50A Connolly Street
> Geraldine 7930  New Zealand
> 03 693 7369 
> sol_design at yahoo.com
> www.soldesign.co.nz <http://www.soldesign.co.nz>
> _______________________________________________
> GSBN mailing list
> GSBN at sustainablesources.com
> http://sustainablesources.com/mailman/listinfo.cgi/GSBN

Derek Roff
derek at unm.edu

_______________________________________________ GSBN mailing list
GSBN at sustainablesources.com

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sustainablesources.com/pipermail/gsbn/attachments/20140421/9922b7ca/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: CA533371-6F57-4A6A-A105-6E9517FA9FC0[14].png
Type: image/png
Size: 8206 bytes
Desc: not available
URL: <http://sustainablesources.com/pipermail/gsbn/attachments/20140421/9922b7ca/attachment.png>

More information about the Gsbn mailing list