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project intro 
using natural finishes. Lots of great information on lime and earthen plasters. (click for larger image) |
July '09. My plans for the type plaster I will use have changed dramatically.
Originally I had intended to use a fairly traditional three-coat lime system (detailed below), but recent research has convinced me that I should look into using an earthen (clay-based) plaster for my scratch / brown coats, with a lime plaster top coat.
I admit that I had regarded earthen plasters as a 'poor man's' lime until I started to learn about it.
More research is necessary, but it seems that clay actually is a very appropriate plaster for walls - and particularly 'soft' walls such as cob or straw bale - as it has the ability to 'control' moisture very effectively. Combined with a top coat of lime to protect the clay, this system replicates the traditional plasters used successfully in England and Scotland for hundreds of years.
Originally I had intended to use a fairly traditional three-coat lime system (detailed below), but recent research has convinced me that I should look into using an earthen (clay-based) plaster for my scratch / brown coats, with a lime plaster top coat.
I admit that I had regarded earthen plasters as a 'poor man's' lime until I started to learn about it.
More research is necessary, but it seems that clay actually is a very appropriate plaster for walls - and particularly 'soft' walls such as cob or straw bale - as it has the ability to 'control' moisture very effectively. Combined with a top coat of lime to protect the clay, this system replicates the traditional plasters used successfully in England and Scotland for hundreds of years.
Applying the final 'finish coat' hydraulic lime plaster during a strawbale building workshop in Oregon in 2009. (strawbale.com)
Contained below is a brief discription of why earthen plaster is so effective.
Clay is hydrophilic and tends to absorb moisture. It draws water out of whatever surface it is in contact with, thereby drying it. This moisture passes through the clay and evaporates into the atmosphere. However, due to its molecular structure, if you wet the surface of clay (with rain, for example) it expands slightly and in doing so blocks off, or significantly slows, the passage of water through the rest of the layer.
In this respect clay acts somewhat like a waterproof but breathable fabric, preventing the passage of large water droplets, but allowing small molecules of water vapour to migrate through.
Another advantage with earthen plasters is the fact that clay-rich soils can be found almost everywhere, making them a very environmentally-friendly option since there is often little or no transportation required, and almost no processing.
Furthermore, since they are unprocessed, they can be returned to the earth and recycled in the most natural and continuous way.
I will add more information on earthen plasters as my research continues, but for the moment I will leave all the information about lime plaster on this page.
Lime Plaster...
Over the last few months I have done a considerable amount of research into the properties and use of lime. I've had the opportunity to talk with some experienced lime plasterers and in May '09 I attended a three-day plastering workshop in Oregon. This accumulated knowledge is contained in my lime info page.
The Method...
(this information is partially out of date, but remains here for reference if you are considering a lime plaster system).
Based on this research and my own experience slaking quicklime into lime putty, I plan to use a combination of different types of lime for the interior and exterior walls. The plaster will be built up in three coats in a form that has been used in Europe for hundreds of years and stood the test of time in very harsh environments.
Originally I had intended to only use 'pure' (high-calcium) lime putty for all my plaster and to slake it from quicklime myself. However, my introduction to slaking convinced me that making all the lime putty I would need would be unrealistically time-consuming and laborious.
Consequently, I will be using lime putty only for the finish coats. For the scratch and brown coat I will probably use hydrated Type S and hydrated pure lime. This will be bought much closer to the time of plastering and does not need to be slaked like quicklime.
The benefits of using lime are listed on my lime info page, but the main ones are: it is more environmentally-friendly, contributing significantly less greenhouse gasses to the atmosphere; it has a degree of flexibility that allows it to absorb small movement without cracking; it can 'cure' small cracks naturally; and it allows the walls to transmit - and thus regulate - moisture in a way that can significantly improve the air quality in a building.
Spray guns will be used to apply the first and second coats, thereby adapting a modern technique to replicate the traditional method of throwing or 'harling' the plaster onto the walls. Harling creates excellent penetration and adhesion of the plaster, and helps to expell excess air from the mix.
The Scratch Coat...
The first ('scratch') coat is the thickest and strongest of the three coats on the wall. Typically about three-quarters of an inch thick, it will have added fibers (horse hair or chopped straw) and the highest ration of aggregates to lime thereby making it the strongest layer. For the exterior walls I intend to use Type S with pozzolans, and use hydrated pure lime for the interior.
For the scratch coat I will probably work with a ratio of 1:2.5 lime to aggregates, however this will be decided after doing a void test once the aggregate is on site. (see the glossary for a description of the void test)
It is called the scratch coat because the surface is 'keyed' or scratched with a large comb or brush to provide a rough surface for the next coat to adhere to.
However, just before it is scratched, the plaster is 'pressed-back' with trowels. This compresses it, pushing it deeper into the underlying bales and simultaneously forcing out any small air pockets which might weaken the plaster as it dries.
The Brown Coat...
The second ('brown') coat is usually about half an inch thick and serves two main functions. It adds further strength and at the same time levels out unevenness in the surface of the scratch coat. It is sometimes called the 'straightening' coat.
Like the scratch coat, I probably will use Type S with pozzolans on the exterior, and hydrated pure lime for the interior. The ratio of lime to aggregates in the brown coat is slightly less than for the scratch (probably 1:2), making it marginally 'softer'. Before it sets, the brown coat is scoured so as to create a slightly rough surface for the last coat to adhere to.
The Finish Coat...
The final ('finish' or 'top') coat will be made from aged lime putty and is a thin veneer of plaster which provides colour, texture and the final barrier against the elements. About an eighth of an inch thick it is the softest of the three coats with the highest percentage of lime and the finest aggregates (about 1:1). 10% brick dust pozzolans may be added to the external finish coat to speed up the set.
The colour in the finish coat is usually added in the form of powdered pigments. A wide selection of very natural milk paints are available and can provide a beautiful finish.
Making each layer thinner than the previous one improves the penetration of CO2 which is needed for the continued carbonation and strengthening of the plaster.
Furthermore the finish coat is, to some extent, a sacrificial layer designed to erode sooner than the brown or scratch coats. If the finish coat were the strongest, it might mask deterioration of the inner layers until it is too late.
Lime Wash...
Although not necessary, I may apply a lime wash finish to the walls. The wash is a very dilute solution of lime putty into which can be mixed pigments and other additives such as caseine. The caseine adds more durability and water-resistance to the wash.
Five or more layers are applied to the external walls which might be lime-washed every two or three years depending on their condition.
The Aggregates...
The best aggregates to use for the scratch and brown coats should have a good variety of particle sizes ranging from a quarter of an inch, down to minute 'fines' that are little more than dust. This variety is important as the smaller particles fill the gaps between bigger ones thus preventing voids which can weaken the mix.
The particles also should be rough or sharp in texture, so that they bind tightly with each other. Typical 'cement sand' from a gravel merchant usually works well for brown and scratch coats. Finer plaster sand should be used for the finish coat.
For the exterior coat(s) I anticipate adding brick dust pozzolans to the mix in order to speed up the set of the plaster and increase its durability. (More information on pozzolans can be found in the glossary). Research on which pozzolans are locally available continues...
One other point: when mixing the ingredients for a particular coat it is important to maintain the same ratios. Each mix of plaster should contain the same carefully measured quantities by volume. This is called 'batching' and failure to do this can result in differential weaknesses in the plaster, leading to cracks and delamination.
I will be slaking the lime putty from quicklime myself, allowing it to sit and 'mature' for about a year before use in the summer of 2010. This process will begin in the latter part of June 2009.
If you are interested in participating please contact me so that I can add you to my mailing list for events.
Although the use of natural hydraulic lime (NHL) is reasonably common in north America, I want to avoid it because it is imported from France via California. This is an un-acceptable carbon-footprint to me.
Links... (hover over links for info)
The Living Building Challenge
Fotoprint
John Gower - Gower design group
Chelsey Braham - C D B Design
Earth Futures
BCSEA
SIPDistribution.ca
Chemicallime.com
Perlite.com
Strawbale.com
HomesteadHouse.ca
Eco-Sense.ca
Building Limes Forum
Straw bale fire test movie
Straw bale earthquake test movie
Susanne Dannenberg - visual artist
Goodshepherdwool.com
Elke Cole
