Cob Re: Plastering

[M J Epko]

At 09:18 AM 12/20/97 +0000, Patrick Newberry wrote:
>I've heard cactus juice boiled and added to the final coat works
>well.I have a source near here and will attempt to use that method.
>That requires some boiling of the cactuses but I think you only need to
>deal with it on the final coat. 

	Pat and all, here's a bit of discussion on that which I had no part of &
is thus not suspect.   :)

>From:	absteen at gateway.dakotacom.net
>Subject:	Re: Mission: Cactus Juice
>
>The gel from the prickly pear cactus was used in conjunction with lime
>plaster at the San Xavier del Bac mission.  It has a long history of use in
>combination with lime in Mexico and possibly other parts of the world.
>Basically, the cactus gel is high in pectin and acts like a glue.  It helps
>the lime set more quickly and improves water repellency.  In our opinion it
>markedly improves the characteristics lime plaster.  Traditionally this
>plaster was used over both earthen (adobe) and brick walls.  When using it
>in conjunction with straw bale walls, we apply 1 to 2 coats of earthen
>plaster to the straw bales before the lime.  When something more weather
>resistant than earth plaster is required, it is an excellent solution.  The
>species of prickly pear we most often use is called opuntia ficus indica
>and is relatively spineless and very easy to work with.  There can be a
>large degree of difference in pectin content between different prickly
>pear, some with very low amounts.  This particular variety has a
>sufficiently high level.  Cholla cactus is also used and in some cases
>species, the pectin content can be higher than prickly pear.  Aloe Vera is
>another option, but typically it is less in its glue-like characteristics.
>
>The same gels have been used with earthen plasters as well.  We always use
>it in conjunction with exterior earthen plasters as it improves them
>greatly when it comes to water resistance.  In a series of tests conducted
>by the Catholic University of Peru in Lima, earthen plaster stabilized with
>prickly pear gel performed equivalent to earthen plasters stabilized with
>4% (by weight) liquid asphalt.  The prickly pear stabilized earthen
>plaster showed only light damage when subjected to 20 cycles of simulated
>rain, each 3 hours long.  Our experience concurs.
>
>The question, as is the case with many natural plant products, remains how
>to prepare this substance for use in plasters.  I didn't see the piece on
>the mission, but if they were preparing it as they have in the past, they
>could have been boiling it and then squeezing it with a mop press to
>extract the gel.  I don't know if they have made any changes in that
>process.  In contrast, it is commonly prepared by chopping the pads of the
>cactus into small pieces and soaking them in water for 1 to 2 weeks,
>depending upon the climate.  That gel is then diluted with water, 1 to 4
>and then used as the water to mix the plaster.  You should be warned that
>it will smell horrendous after this period, but it is considered by many to
>be a much superior product than the substance that has been just boiled as
>it has become a more complex compound during this aging process.
>
>For a more complete description, refer to pages 209-210 in our book, The
>Straw Bale House... If you have any further questions please let us know.
>
>Best wishes,
>Bill Steen
>
>>hollen at vigra.com writes:
>>>There was an episode of "This Old House" not too long ago (at least
>>>here in SoCal) that Steve Thomas visited a mission in the process of
>>>being restored.  In the past, some restoration had been done using
>>>cement stucco and its impermeability had caused the moisture in the
>>>building to be trapped in the wall and even eventually migrate back
>>>towards the interior and begin to destroy some of the frescoes on the
>>>interior walls.
>>
>>It was also destroying the integrity of the Adobe walls themselves.
>>
>>>The current restoration was being done with stucco materials using
>>>prickly pear cactus mucilage and ??????
>>
>>>It was pointed out that the "old world stucco recipes" far outlasted 
>>>modern cement stucco and were breathable.
>>
>>I believe this was Mission San Xavier Del Baq (sp?).  Can't remember if it's
>>New Mexico or Arizona, but I'm sure someone else on this list will know.  It
>>was also written up in Progressive Architecture (now defunct, but back
issues
>>probably available at your university architecture library) some time
between
>>'89 and '94 (sorry, I did a quick look and didn't find it.  And I'm not
going
>>to dig through 6 years of PA right now.)
>>
>>The problem was this:  The original natural earth stucco that was used
>>breathed.  The stucco would allow some moisture into the walls.  Not a big
>>deal, as rainstorms in the SW desert don't last long, aren't frequent, and
>>there's plenty of time between them for the walls to dry back out on their
>>own.
>>
>>The problem came when non-breathable stucco (cement early in this century,
>>and later even elastomerics if I remember correctly) was applied as
repair to
>>some areas that needed maintenance.  Now, we all know that no matter how
hard
>>we try to "seal" something, eventually it will crack, puncture, or otherwise
>>leak in some way - even "invisible" hairline cracks count.  This of course
>>happened at the Mission, and water got back into the walls again.  Of
course,
>>it is now sealed IN by the lovely non-breathable surface, and the walls can
>>no longer dry out properly.  So the walls start to degrade MORE quickly than
>>before the "repair".
>>
>>The smart people who were restoring the place in the 80s or so figured this
>>out, went to Mexico, and got some artisans who still used the old
techniques.
>>They add squeezings from boiled prickly pear cactus to the stucco as a
sort of
>>breathing water barrier, then finished the stucco by rubbing it with smooth
>>river stones to reduce the size of the pores.
>>--
>>
>>Bill Christensen
>>billc at greenbuilder.com
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>
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   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      M J Epko        duckchow at ix.netcom.com
      almost Wyoming, north of Nebraska, USA
               by way of New Mexico
      (not soon enough) - for now, Minnesota
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    You may drive out Nature with a pitchfork, 
    yet she will always hurry back.
                            - Horace, Epistles