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Do You Know Your Plaster?

15/5/2018

5 Comments

 
Distinguishing earthen plaster, lime plaster and lime wash

Earthen or clay plaster, lime plaster, and lime wash: these are three things people often confuse. And it pays to be able to distinguish them, because each behaves differently on a wall. Here’s a quick clarification.

Earthen plaster (possibly with lime added)
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Earthen plaster uses clay as a binder
Classic clay/earth plaster is a natural wall render usually made from a mixture of clay, straw, and sand. Sometimes, depending on the clay and climate, folk add lime to this mixture. This doesn’t make it lime plaster though. That is something totally different. Clay plaster is a flexible, breathable render. If you want to learn more about making it, sign up for my free earth plaster PDF and email course.
 
Lime plaster
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Lime plaster is harder and less flexible
Lime plaster is simply lime mixed with sand. Sometimes people will add a pozzolan like ash to create a more cement-like result. But that’s basically it. Like earthen plasters, lime plaster breathes and can be used as a natural render. It also has other benefits; lime is both a fungicide and an insecticide. BUT, beware of using lime plaster to cover cob or earthen buildings​, as it may crack.
 
Why might lime plaster crack? If your walls are mud (adobe, cob, earthbag) when the rains of winter begin, the clay in your wall will expand as it gets wet. In summer that same clay will shrink back as it dries. The trouble is the lime plaster doesn't expand or shrink alongside. Unlike earthen plasters, lime plaster is more cementitious and rigid, so what tends to happen (especially in climates with monsoons or heavy rains followed by dry weather) is that the lime plaster eventually cracks and pulls away from the wall.

Lime wash (using lime as a whitewash or paint)
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Lime wash works great as a finish for earth buildings
​Lime wash is not a plaster. It’s a final paint-like finish made from lime, water and a binder such as casein (more on how to make it here). Lime wash can be used much like paint on cob or earthen buildings, and this breathable finish will protect the plaster underneath very successfully. Unlike lime plaster, lime wash won’t crack (or at least only slightly) because it is only a thin lime layer, as opposed to a render. When your lime wash begins to see wear over time it’s very easy to repair, simply by painting more lime wash into the cracks. 
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This was my mud home in Turkey. The earth plaster is coated in a lime wash (except the sills and sculptures, because I wanted them to stand out).

I hope that’s unmuddied the waters a little.
​                                                                              ***
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5 Comments
Cath Coffey
18/5/2018 02:46:55 am

OMG!
Was in the middle of agonising about the amount of money I would have to spend putting lime plaster over my first coat of earth plaster.....you saved me! Phew...Thanks again. I can sleep now.

Reply
Atulya
18/5/2018 01:23:16 pm

Phew! So happy you didn't do that. Your earth plaster may well be enough, but if you want extra protection a lime wash is perfect.

Reply
LS
14/2/2020 11:53:33 am

Hi there,

What would you recommend to cover a cob dome oven with to make it more resistant to rain please?

Reply
Atulya
19/2/2020 12:29:39 pm

Well normally cob will deteriorate if it sees a lot of heavy rain, so best to cover it some how. But several coats of linseed oil will help.

Reply
Rob'O O'Donnell
14/2/2020 02:39:45 pm

I am very intetested in your information.

Reply



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  • Home
  • Building
    • Earthbag
    • Living Roofs
    • Gravel Foundations
    • Off-grid how to >
      • Off-Grid Prep Course
    • Earth Plaster
    • Mud as Mortar
    • Wattle and Daub
    • Lime; Hydrated, Hydraulic, and Putty
    • Mud Building Blog
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    • Mud Ball
    • Mud Mountain The Book
    • An Earthbag House in 7 Days?
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    • Contact
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