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/clay plaster (possibly with lime added)

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. Clay is the binder here. Sometimes, depending on the clay or the climate, folk add lime to this mixture. This doesn’t make it lime plaster though. Lime plaster is something totally different. Clay plaster is a beautiful, malleable and tough natural render. If you want to learn more about it, have a look here.
 
Lime plaster

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)

Lime wash works great as a finish for earth buildings

​Lime wash is not a plaster at all. 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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