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Re: GSBN:Re: Lime plaster problems

I'll plead busy as well and before I made any comments I wanted to make sure they would be somewhat thorough. There are a few things that have become evident to me over the years when it comes to lime over clay plaster. In mho I would have to say that results are unpredictable and undependable. I'm not saying it can't or doesn't work, but rather that outcomes vary widely. David Bainbridge mentioned in his earlier post the Getty people in Los Angeles as a resource. I've made it a point to talk with them every few years about lime plaster and in particular their work with prickly pear cactus gel. Anyhow in one of the last conversations we had they told me that there was a growing sentiment amidst the conservation community that the use of lime plaster over earth surfaces was not such a good idea. They mentioned that although lime had better permeability than cement and allowed the earth to dry better when wet, it was still too different from an earthen substrate for it to work dependably.

My experience basically says the same thing. For me the whole issue comes down to what type of earth plaster is underneath. Apparently some are much better than others and just what all the variables are I can't say. I think it will take some time to get a better grasp on it. Clearly I would suggest that if the substrate has a lot of clay there is a likely possibility that if water is absorbed through the lime plaster and passed on to the earth substrate then obviously the clay will expand and have a tendency to push the lime plaster away therefore causing the two to separate. If the earth plaster is weak then the connection will break down over time. So in short I guess what I'm saying is that the earth plaster has to be of a really good quality for any chance of success. It needs to be strong and not have an overly high content of expansive clay. Another very important piece is that the earthen plaster substrate must be thoroughly dry as earth plasters shrink when drying. If that process is not complete the earth will pull away from the lime plaster and leave it hanging in the air. And as others have pointed out the surface needs to be extremely well keyed for the lime to adhere. I don't think you can under-do that aspect. Additional measures like that of placing sharp jagged stones in the plaster would also help such as the practice of "rahuela" used in Mexico where stones are placed in the mortar joint between adobes.

I think it was David that also mentioned the Japanesete We have worked closely with a pair of Japanese plasterers over recent years and it is important to note that the Japanese approach relies heavily on use of a glue when mixing what they call "Shikkui." Traditionally rice flour was used but was replaced with seaweed gel during a time of food shortages. In general I would say that their plastering practices are far more meticulous than anything I've seen in this part of the world and in the words of the best plasterer from there that I can think of, "there is great difficulty in the connection between the two coats and at best it does not last for a very long time."

Having said all this I will say that my comments apply primarily to exterior finishes and I've not yet experienced any problems with interior or protected locations. But I should add that we also typically include a percentage of some glue material in the lime plaster.

John Glassford suggested the use of lime/clay as an alternative and I think that it can be a very good one. Much here depends upon the reaction between the clay soil and the lime. Over the years I've watched varying reactions, some soils produce exceptional results, others marginal. I recently cornered Harry Francis who once worked for the American Lime Association while we happened to be together recently in Washington DC. I told him that the using pH as an indicator of the right type of mix was not proving true all of the time. Like he always does Harry went home and searched his extensive resources and just emailed me a paper that addresses some of those variables particularly soils with high levels of potassium and sodium carbonates. I haven't had time to really absorb the content, but if any of you want a copy I would be happy to forward it.

I've already said more than I had intended, but to sum it up I would not suggest that we recommend the use of lime plaster over earthen substrates as a general practice. I would suggest that it be adopted only after some testing and experimentation has been done and with a warning that the results are always guaranteed.

On Oct 7, 2007, at 7:10 AM, Rikki Nitzkin wrote:

Sorry to chime in so late...been busy. Its just that I haven't seen anywhere that in the responses to remind Andy that it is very important to wet down as well as scratch the base layer of plaster before applying a new coat, and if the material is different (lime on clay in this case) really rub the
first bit in so it binds well.

Hope your problem has been solved.

Rikki Nitzkin
Aul?s, Lleida, Espa?a
(0034)657 33 51 62
www.casasdepaja.com (Red de Construcci?n con Balas de Paja)

-----Mensaje original-----
De: GSBN [<a target="_blank" href="mailto:GSBN@...";>mailto:GSBN@...] En nombre de Brian Hodge -
Enviado el: jueves, 27 de septiembre de 2007 22:20
Para: 'GSBN'
Asunto: RE: GSBN:Re: Lime plaster problems

Hi Andy,
I agree with John. We have been doing a very similar thing for about 5 years and have not found it necessary to add the lime putty top coat. We
have even done it without chaff with a great deal of success.



Anvil Straw

-----Original Message-----
From: GSBN [<a  target="_blank" href="mailto:GSBN@...";>mailto:GSBN@...] On Behalf Of John
Sent: Thursday, 27 September 2007 7:53 AM
Cc: Andy Horn
Subject: Re: GSBN:Re: Lime plaster problems

G ' day Andy

My two rands worth mate.

You will need to remove the lime coat and scratch up the earth as others
have said.

I would then apply a mix of earth/lime/sand and chaff or chopped straw.

All depends on the clay content of your earth but I would look at
something like this 3 parts soil 2 parts lime 3 parts sand 1 part chaff.
I use buckets for this mix.  It works well and binds well with the
earthen render.  Then if you want you can apply a finish coat of lime
render however I have not found that necessary in most cases except for

You can see a house we renedered with earth, then earth/lime/sand/ chaff,
then lime/sand, here:

<a  target="_blank" href="http://glassford.com.au/Jumbo.htm";>http://glassford.com.au/Jumbo.htm</a>

No delamination no cracks and it is nearly 6 years since we finished

Was in Cape Town a few weeks ago but did not have time to see much it
was a Rotary trip with Hout Bay.  We will be back next year for an
extended visit, see you then.

Kind regards
The Straw Wolf
Huff 'n' Puff Constructions
<a  target="_blank" href="http://www.glassford.com.au/";>http://www.glassford.com.au/</a>

61 2 6927 6027

Mount Kilimanjaro Climb 28/8/07 <a  target="_blank" href="http://www.coolamonrotary.com/kili/";>http://www.coolamonrotary.com/kili/</a>
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