Clay plasters have been used in buildings in the UK and all over the world for thousands of years, and a lot of people do not realise that there are probably over a million buildings throughout the UK alone that has clay-based materials in their structures.
Clay plasters worked exceptionally well for historic buildings for many reasons, and I will go into them a lot later on in this article. However, what a lot of plasterers don’t realise is that clay plasters have a place in modern construction as well.
What Makes Clay Plasters So Good?
Clay does not have a chemical set, and what I mean by that is that it does not need chemicals to make it go hard it requires compression either through a plastic trowel or simply by circular motion using a sponge. An added advantage when clay dries inside a bucket it can be rehydrated again by adding water which means there is less waste like gypsum on average 20%, even if clay goes hard it does go to waste simply store and rehydrate.
When I was doing my research into clay plasters “breathability” and “flexibility” kept on cropping up and in fact whenever clay plaster was mentioned, these two words were always present.
So on that note, let’s not buck that trend and have a look at Breathability.
Clay plasters prevent excess water in buildings. Let me explain; this is because clay plasters are hygroscopic, they buffer temperature and moisture allowing vapour to be absorbed if the humidity is high, allowing the clay plasters hold that moisture without causing any damage.
However, what makes clay plasters really cool (literally) is that when the humidity drops, they release that water back into the air. This process maintains constant humidity and reduces damp, and this, in turn, reduces moulds caused by condensation. Clay should never take the place of a properly vented space, especially a bathroom. What the clay helps to eliminate is physical condensation of vapour on a wall, in a bathroom. Once the space humidity drops, natural movement of the vapour continues, rapidly, allowing the wall and the space to reduce the propensity for mould growth.
If you compare gypsum-based plasters to clay plasters then gypsum absorbs the moisture, but it gets damaged in the process, moulds grow at the very least and in some situations the plaster can blow and fall off.
Clay and lime are very similar when it comes to flexibility as it is soft and pliable and it can do this without cracking which is a huge advantage when it is needed in situations where there is some movement, and this is why clay is so awesome in old properties as it moves and flexes with the building.
Gypsum, however, is not flexible in any way, shape or form and will crack at any sign of any minor movement.
Clay has a chemical attraction that draws the moisture in and because clay draws the moisture in at a faster rate than say timber it protects the timbers of a building from damp and without the damp it reduces the chances of the timber rotting.
Clay absorbs moisture, and you must be thinking now that it can’t always be good, however, clay seals and prevents water from getting all the way through into the structure of the building. The moisture in the clay plaster is released once the air around it dries.
An excellent example of how clay copes with moisture is that it can be used to line a pond. Clay will only let so much moisture in then it will provide a barrier effectively when it is saturated, and then that clay becomes watertight and becomes a perfect Pond liner that is 100% natural.