[GSBN] lime over hessien
Jacob Deva Racusin
buildnatural at googlemail.com
Tue Apr 22 09:45:39 CDT 2014
"Lime over clay" is a topic just about as heavily-debated as "drainage
under bale walls", it seems...
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
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.
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
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.
On 4/22/14, 7:16 AM, Rikki Nitzkin wrote:
> Hi all,
> 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.
> 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.
> I have heard several theories on why:
> * badly applied (improper bonding)
> * wrong kind of clay or lime
> * in humid areas, the water penetrates the lime, reaches the clay
> and the clay expands- pushing off the lime...
> The last theory may have more validity, because we have noticed that
> the lime stays on the clay longer in dry areas.
> If anyone has done any research on this issue, or has experiences to
> share, i am very interested.
> 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.
> Has anyone else tried this?
> take care,
Jacob Deva Racusin
New Frameworks Natural Design/Build
Author, The Natural Building Companion
Chelsea Green Press, 2012
jacob at newframeworks.com
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