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"Lime over clay" is a topic just about as heavily-debated as
"drainage under bale walls", it seems...<br>
<br>
We've seen mixed results of lime over pure clay, but too many
variables at play to draw any conclusions. Our regular practice,
however (when not building in a rainscreen), is to apply a pure lime
finish plaster over a lime-stabilized clay base coat, very similarly
to your procedure with similar experiences (makes for an incredibly
durable base coat). We started doing this, based off the precedence
set by Paul Lacinski and Andy Mueller of Greenspace Collaborative,
because it would allow the base coat to set up more quickly, with
less likelihood for molding (we frequently use manure in our
formulations) particularly in late-season jobs, and would ensure the
base coat could make it through a winter without wear as we were
rarely able to get a finish lime coat on with enough time to cure in
a single season (we're in the cold and wet northeastern US). We
strongly favor the use of clay-based renders as base coats to help
control moisture (a big issue for us in our climate), and lime is
favored as a finish for its durability (relative to clay - high
exposure/low maintenance projects generally receive rainscreen
cladding).<br>
<br>
We've since embraced the benefit of a lime-stabilized clay base to
support a durable and long-lived transition to a lime finish coat.
First and foremost, a really good mechanical key needs to be in
place - we are very disciplined about keeping a scratch tool close
to hand while plastering, and getting a good accurate scratch (not
too shallow, not too deep, covering all surfaces) when the timing on
the base plaster is right - not too soon (overscratch) nor to late
(underscratch). Having a chemical similarity always made sense, and
empirically the stabilized base plaster seems more compatible in its
physical properties. But in chemical theory, this was further
supported by a great thread on the old CREST (or maybe SB-R-US?)
listserv asking the same question. My recollection (and I'm taking
big paraphrasing liberties here) is that Harry Francis, the lime
guru, explained that if too small an amount of lime is added to a
clay soil, it essentially turns the clay to silt, and a significant
amount of lime needs to be added to induce enough of a chemical
change for the lime to enhance the viability of the mix (this is
easily felt empirically while mixing, as suddenly the mix becomes
much stiffer and stickier as the lime is added, depending on your
mix methodology). This balance varies mostly on the type of clay
used (assuming a standard Type S hydrated lime), and is driven, at
least in large part, by pH (needs to be high enough), if I remember
correctly.<br>
<br>
On the wall, one hypothesis about why lime over clay can delaminate
(and I believe this was more theory than proven fact) is that the
application of lime turns that outer surface of clay plaster to silt
- enough to flocculate the clay molecules, but not enough to fully
stabilize it - and sufficiently weakens that outer surface as to
contribute to premature delamination, especially where solid
mechanical key isn't provided. I've heard tell of colleagues, and
had success myself, applying a coat or two of limewash between clay
and lime, and having good success there. This would serve to work
quite a lot of pure lime deeper into the pores of the clay plaster
and stabilize that outer layer (again, working off theory here), as
opposed to just smearing a lime plaster over the surface.
Stabilizing the base coat goes a big step further, ensuring the
entire base coat is compatible, and saving a step between coats.<br>
<br>
Does anyone else have recollection of that thread? I don't have time
right now to search my archives but will try to do so later.<br>
<br>
Cheers,<br>
Jacob<br>
<br>
<div class="moz-cite-prefix">On 4/22/14, 7:16 AM, Rikki Nitzkin
wrote:<br>
</div>
<blockquote
cite="mid:95EF8D90-270B-48EB-9F17-51CBD03D3F10@gmail.com"
type="cite">Hi all,
<div><br>
</div>
<div>I have only skimmed over these emails because I am quite
busy, but I don`t see anyone making comments about applying Lime
Plaster over Clay plaster. If I repeat in my comments, forgive
me.</div>
<div><br>
</div>
<div>I am concerned, because I have found that applying Lime
plaster over clay plaster has given very "mixed" results here in
spain. Half the time the clay plaster falls off the walls after
a year or two. </div>
<div><br>
</div>
<div>I have heard several theories on why:</div>
<div>
<ul class="MailOutline">
<li>badly applied (improper bonding)</li>
<li>wrong kind of clay or lime</li>
<li>in humid areas, the water penetrates the lime, reaches the
clay and the clay expands- pushing off the lime...</li>
</ul>
<div><br>
</div>
<div>The last theory may have more validity, because we have
noticed that the lime stays on the clay longer in dry areas.</div>
<div><br>
</div>
</div>
<div>If anyone has done any research on this issue, or has
experiences to share, i am very interested.</div>
<div><br>
</div>
<div>Here in Spain we have been experimenting with using "bridge"
layers of a clay-lime plaster (1/8-1/2 part lime for each part
of clay) to stabilize the mix and make the final lime plaster
stick better. At the moment it has worked quite well, although
it is too soon to give conclusive results. In many cases the
"pure lime" plaster finish has even been unneccesary because the
clay-lime plaster is quite strong and water resistant.</div>
<div><br>
</div>
<div>Has anyone else tried this?</div>
<div><br>
</div>
<div>take care, </div>
<div>Rikki</div>
<div><br>
</div>
<br>
</blockquote>
<br>
<div class="moz-signature">-- <br>
Jacob Deva Racusin <br>
Co-Owner<br>
New Frameworks Natural Design/Build<br>
<br>
Author, The Natural Building Companion<br>
Chelsea Green Press, 2012<br>
<br>
(802) 782-7783<br>
<a class="moz-txt-link-abbreviated" href="mailto:jacob@newframeworks.com">jacob@newframeworks.com</a><br>
<a class="moz-txt-link-freetext" href="http://www.newframeworks.com">http://www.newframeworks.com</a></div>
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