Here are the (free) resources that we use to help our clients to make beautiful earth/ lime finishes to their earth buildings. The idea is that our clients and also interested contractors can freely access all information needed to do the work on site, so that it will be cheap and easy to have a beautiful sustainable home.
These finishes can also be applied to cement block walls (CMU) or to concrete, so if you’re looking to change the wall finish of your existing home, you can still use these techniques. Earth plasters have the ability to absorb and release humidity and heat, which helps to keep a balanced interior environment. Previously lots of urban clients of our earth homes in Burkina Faso would opt for a cement based plaster finish on the interior. However this doesn’t have the same buffer effect, and it also compromises the concept of having a zero carbon house.
Exterior / water repellent finishes
For exterior finishes we recommend using a lime based render. The Nito Project made an excellent instruction video on the technique:
Below the video is a detailed instruction on the quantities
Most text from The Nito Project Youtube channel
This is a how-to for a polished lime plaster, sealed with soap, similar but more simple than tadelakt (a traditional Moroccan plaster, see more info below).
The lime plaster shown here is also similar to Venetian plaster, but without the marble dust. The use of cheap and commonly available materials is what makes this finish so great and its versatility makes it suitable for wet areas, backsplashes, showers, exterior walls and so forth, especially as a final plaster on your natural home. It is a good alternative to cement plaster as it has less embodied energy and absorbs carbon dioxide in the setting process.
To make lime, limestone is burnt, releasing CO2. Over time, lime plaster reacts with CO2 in the air and becomes limestone again. This is why a lime plaster has a much lower CO2 footprint than a cement based plaster.
Lime is more permeable/breathable, more flexible and less prone to cracking.
2-3 parts plaster sand, screened 1/8”-1/4”
to 1 part Type-S lime,
to 1 part water or as needed.
1 part Type-S lime to 3/4 part water or as needed, use pigment for color (make sure you use a lime fast pigment).
Soap: 1 part Olive oil soap to 20 part water. (experiment with type of soap (lye) and strength as other source says 1:6 for first and no water for second layer. The soap chemically reacts with the lime plaster, forming lime calcium soaps. Calcium soaps are insoluble in water, and fairly hard. They are familiar in areas with calcium-rich (“hard”) water, as deposits in bathtubs, sinks, and showers; when soap is mixed with the water’s dissolved calcium carbonate/lime, calcium soaps form. 2 C17H35COO−Na+ + Ca2+ → (C17H35COO)2Ca + 2 Na+
The timing between coats is very important, as explained below.
Substrate/existing wall should be something lime bonds to, such as any unsealed lime or cement plasters, cement board, brick, clay or earth walls.
Make sure it’s well keyed, having a texture for the lime to grip to. Using concrete bonder can help bonding and decrease wicking, giving you more time work your base coat.
Base Coat: is applied 1/4” (6mm) thick but can be applied in two passes. It shouldn’t go any thicker than 1/2” (13mm) in total. Floating is optional, the main goal here is to leave a smooth, fresh base for the putty coat.
Putty Coat: this has to be applied when the base coat is firm but still wet, which varies a lot by weather and how much the substrate is wicking water from the base coat. Typically on a hot day, out of direct sun this would take 1-2 hours. If you apply concrete bonder to your existing substrate and cover your base coat well with plastic, you can extend this window over night. This is usually preferred because the slower the lime drys the stronger it will be. The lime putty needs to applied very thin so as not to crack, you can listen to the video for how it sounds when you’re spreading it thin. It is also very important to keep your trowels very clean during this step, wiping it with a rag after, as extra build up on your trowel can lead to it sticking and could pull off your putty coat. Make sure to apply on an angle, because a flat trowel will stick to the putty coat.
Soap: This step is optional as the soap is to waterproof your plaster. This also needs to be done while your putty layer is still wet, ideally as soon as possible but dry enough so that you don’t smudge your surface and your polished, shiny luster which you work so hard at. A good way to test if you’re ready to soap is to wet your finger and run across the surface without it smudging the finish. Your plaster may not dry evenly so should check multiple places. After soaping I recommend washing the wall with water.
You can apply any additional masonry sealers or wax.
All round trowels One of my favorite for applying and leveling is this marshalltown https://amzn.to/34oaJJX Another very good all round trowel is this 0.5mm rigid coarse Carbon Steel http://www.japanesetrowels.com/ Another link Aburayaki Square Application Trowel https://japaneseplastering.com/japane…
Stainless-steel flexible trowels for applying your putty coat are pretty easy to find on the websites above, or on amazon: https://amzn.to/38L7e3t Plastic trowels can help polish lime and bring out the gloss, and rarely leaves burn marks as compared with steel trowels https://amzn.to/2PRlSh1
Text from The Nito Project (contact firstname.lastname@example.org) Some of the above links are affiliate links, which means The Nito Project receives a small commission with each sale at no extra cost to you.
What is a good alternative for olive soap?
answer from Bill Steen (edited by ndfk): For sealing the plaster many soaps will work, for example common bars of a yellow soap in combination with potassium alum to seal walls from water. After all the black soap (olive oil soap) is simply nothing more than a household soap. Pure olive oil is often used in Italy, ghee in India, the use of natural oils to seal plaster is something with a great deal of history behind it. When we were working more in Mexico we got the alum from a hardware store and the common yellow bar of soap from the grocery store.
answer from Luís Carlos Gulias (edited by ndfk): Traditionally, tadelakt is made with ‘Moroccan black soap’ (savon noir or Beldi soap), which is made with KOH and olive oil. It is because this type of soap is common in the region and not because it is the best. In fact, what you need is a fatty acid. Any fatty acid will do, but lauric acid will result in a harder surface.
Answer from April Prichard (edited by ndfk): KOH (potassium hydroxide) is used to make liquid soaps! NaOH (sodium hydroxide) will make solid soaps – that is the difference. Castile soap can be either liquid or solid soap, because traditional castile soap uses only olive oil. Modern versions mix small amounts of other oils into it, but real castile soap will say that it only has olive oil in it. The cure time for traditional olive oil soap is a bit longer. I think in this case the lime is reacting with the sodium olivate (saponified olive oil) to make calcium stearate. This should work with any real soap – just make sure the package doesn’t say something like “deodorant bar” because those are not real soap. Those type bars are solid surfactants that have been melted and molded into bars.
Is there no need for a stone to finish it, like with tadelakt?
answer from Luís Carlos Gulias (edited by ndfk): The “special stone” used in tadelakt is to generate heat and speed up the chemical reaction. I think a heat gun will do the same.
answer from Bill Steen (edited by ndfk): Stones for shining can be used, but are not needed. The stones do work well on curved surfaces and until recent times were used commonly in Mexico when applying plaster to domes on roofs.
Other earth wall finishes
The NGO ‘la voûte nubienne’ from Burkina has been experimenting with various other exterior finishes, to waterproof the plain adobe sun dried blocks. One method is the use of tar mixed in the final finishing plaster. This is not an ideal material to work with, and it will still need to be reapplied every 3 years or so.
Another possibility is the use of earth blocks with small rocks embedded in the external face of the block. Then , to apply a regular cement bonded plaster to get a water repellent seal.
Currently the most commonly used method for our projects in an urban setting (Bobo Dioulasso and Ouagadougou) is to have all exterior facing walls made of laterite blocks. Laterite blocks are a great and beautiful natural material. But it does weather over time. A coat of natural oil (for example linseed oil) can be applied to increase the durability.
Detailed and well made video by The Nito Project (in English).
Learn by doing
We have a network of craftsmen that assist with workshops to learn these techniques hands-on. However, these techniques can also all be learnt by simply doing, following the instructions of the Youtube video’s and guides on this page.
Contact us if you want us to organise a workshop, or if you have other questions about earth finishes.
Austrian lime and earth plaster artisan and workshops smooth lime (Kalkglätte)
Germany source of products for earth finishing soap and natural colours: https://www.kreidezeit.de/
more info on earth floor installation: https://theyearofmud.com/2017/08/27/large-earthen-floor-installation/