When to Use Clay? And When to Use Lime?

As you probably know, I’ve renovated two tiny stone huts this summer. They began the year as abandoned, poop-filled, caves. Now? Aha... The transformations were surprisingly inexpensive too, because a) I did all the work myself, and b) I'm predominantly using two very inexpensive materials: clay and lime. But can you use either in any situation? And why did I choose lime in some places, and clay in others? 

Before.

After.

As far as the cost goes, both are inexpensive. Earthen plaster is almost free to make. The only cost is the sand, which is about 4 euros for a 25kg bag. Lime costs marginally more (6 euros for a 25kg bag, and that goes a long, long way). Both have other pros and cons. Having worked with the two extensively now, in both hot, dry Turkey and cool, wet northern Spain, I’ve learned when to use which, and why.

Earth plaster window frame.

When to use lime
 If you have a lot of damp to mitigate, mould issues, or some burrowing termite-type trouble, then lime is unbeatable. Even the resourceful, genius of mice teeth hesitate before lime render. Lime is potentially harder than clay plaster, but interestingly, it’s not necessarily stronger.
 
Lime also works better (in my opinion) if you have a pre-existing stone structure and need to refill the mortar. It’s hard to get a decent earth plaster mixture in and out of a mortar gun.
 
Lime Pros:
1. Lime can handle plenty of damp.
2. It’s an insecticide. Lime deters termites and fire ants.
3. It’s a fantastic fungicide.
4. If you have damp issues, lime is the way.
 
Lime Cons:
1. Not as easy to sculpt (though you can etch it into shape).
2. It’s caustic, so less of a joy to use than clay (but still much nicer than Portland cement).
3. Lime isn’t hugely load-bearing (I wouldn’t make a supporting wall out of limecrete, for example).

Lime render.

When to use clay
 Clay plaster is much, much better for sculpting. It has a wonderful elastic quality that holds its shape beautifully, and once dry is ridiculously tough. I had one heck of a job bashing down a bit of my cob spice rack, for example. (Video) Earthen plasters are also wonderful to work with. Everyone feels happy when they build with mud.
 
Earthen plasters feel warmer too, and although earth is not an insulator, it's still better than Portland cement or stone (Cob has an R value of around 0.5 per inch compared to 0.08 for poured concrete). If you make your plaster very straw-heavy, that R value can increase. Yes, I know half the natural building world will disagree with me, because they are all obsessed with the fact that earth has thermal mass. But in my experience, it ain’t quite that simple. I’m not the only one who has noticed that earth-plastered walls are way warmer than stone or concrete ones. especially in the damp. Mud is magic.
 
Clay Pros:
1. Lovely to work with.
2 Cob has structural strength.
3. Clay plasters are warmer to the hand, eye, and soul.
 
Clay Cons:
1. Clay doesn’t love the damp, and can sometimes go mouldy.
2. Clay and earthen plasters usually take longer to dry out than lime in wet climates (though some have had the opposite experience).
3. Clay expands and contracts when it gets wet or dry, which can cause issues.

So what did I do in my huts then?
1. My bedroom hut is semi-submerged in the ground. In fact, one side is built out of the rock of the land itself! I could see there would be damp in one corner, so I used lime render and lime paints in those areas.
2. I used clay plaster on all the other areas because it felt warmer. I used clay plaster for sculpting sills and shelves, inlaying mirrors, and embedding windows.
3. I used limecrete on one floor because again, the flagstones were directly on the ground and prone to damp.
4. I used lime mortar between the stones, but earthen mortar to build a wall from scratch.

Lime and clay performing together.

So there you have it. We live in an adversarial world where things are idiotically pitted against one another. But it’s not lime versus clay. As with most things in life, the two actually perform beautifully when they’re doing a cha-cha.

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Remember lime is caustic

Lime is a natural substance, but it is caustic and can burn. Make sure you are protected properly before using it. Read the health and safety guide to find out more.

How to Use Lime Safely

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Links

My flagship digital earth plaster course
http://www.themudhome.com/earthplastercourse
The Mud Home Amazing World of Lime Course
https://the-mud-home-courses.teachable.com/p/the-all-about-lime-course

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