Do You Know Your Plaster?
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)
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 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)
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.
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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18/5/2018 02:46:55 am
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.
14/2/2020 11:53:33 am
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.
14/2/2020 02:39:45 pm
I am very intetested in your information.
11/3/2022 02:41:21 am
Thank you for making it so clear! We built cob furniture in our strawbale home. For the dining bench, we are thinking lime plaster as render because it is more rigid, but would you say a clay plaster would be durable with such traffic of people sitting and kicking heals into it?
26/4/2022 01:43:36 pm
I liked how this post shared that lime plaster is a mixture of lime and sand. My brother wants their property to get plastered. I think it's best to turn to a company that offers plaster supplies services.
8/11/2022 05:50:22 am
Thank you for revealing. Reading your article about clay and lime plaster was enjoyable. I'm going to bookmarked this article.
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