Experimenting with earth

For the Greenlight festival, I wanted something easy and crafty to show at the LESSBIG stand. It turns out earth bricks were that thing: cheap, uncomplicated and quite a lot of fun to make. Here’s what I learned and how you can try out your own…


Building with earth has been around for millennia, with some of the structures still standing today. The first question we have to answer then, is: “What is earth made of anyway?”


This diagram (from this wikipedia page) helps us out. What we generally call earth/soil/dirt varies widely in its composition, as anyone struggling to grow veg in heavy clay soil is likely to agree. In any random spadeful of earth there will also be a bit of air, water – and most probably some odd bit of twig or rock. First things first are to clear out any obvious lumps (and worms!) and give it a good sieving / filtering.

Clay is the smallest particle of the three. The two clever things about clay are its plasticity when water is added, and that the silica particles permanently change and harden when they dry or are fired in a kiln.

Silt is the next smallest particle. If you see a muddy looking stream or river, it is mostly likely silt being washed away.

Sand is the largest of the three. Sand is basically ground up rocks and minerals, generally big enough to be seen individually with the naked eye. Away from the coasts, it’s most likely to be quartz based. This is not crucial information, but it’s kind of interesting anyway!

This diagram shows the comparative sizes of the different particles:

image: http://www.ext.colostate.edu
image: http://www.ext.colostate.edu

Clay is the ‘magic’ component that binds the rest together, so it is important we have an idea of how much clay our soil contains. There are various rule of thumb tests we could do to figure this out; pick up a handful and squeeze it, if it sticks together by itself, there’s plenty of clay in it.

A slightly more scientific way is the old ‘soil in a jar test’. Stick your mixture in a jar with some water (and a little soap), shake it up and pulverise it until fully mixed, and allow to settle for at least a couple of hours. (this site gives a good walkthrough)

image: http://www.ext.colostate.edu

Recommendations for earth bricks that I’ve seen range from 15-25% or even up to 40% clay. With too much clay, the bricks will be too brittle once they’ve dried, and with too little they won’t bind together in the first place.

From earth to bricks

Top tip – use a jar that seals really well and be prepared to let it soak and come back to shake it again to break any other clumps. Here’s the soil I’ve used in my test bricks so far after a few hours.


It actually doesn’t look like there’s a huge amount of clay in there, probably because I just used topsoil which was relatively easy to dig up rather than going a bit deeper down to soil that would likely have had more clay in it. You can definitely see the larger sand particles at the bottom though. (see this site for another example of the jar test with more clay in it)


Here we are in the shed, with some scales, our earth and a bag of pre-mixed sand and cement. The mix is 5 parts sand to 1 part cement, and was about £3 for a 5kg bag.


I’ve measured out 200 grams of each in the old kitchen scales.

Get the two mixed together dry before you add a tiny bit of water – you can always add more water.


Whoops. Saying that, I then added far too much water. With my trusty stirring stick, you can just about see the consistency here is like runny concrete. There are still fairly large lumps of clay. Maybe this consistency is actually useful in some cases, but I just added a bit more soil and cement mix. There go my oh-so scientific measurements!


Okay, this is a bit better, a bit more gritty and sticky. (If you have randomly arrived here without having reading the rest of the article… it’s not what it looks like)


I’ve quickly bashed this frame together from scrap wood when I made the larger brick with cans, so I’m reusing it for this smaller one. I’m sure there are much more accurate ways of making brick forms (also compressing mechanisms) but this is what I’ve got for now. Let’s slop a bit in.


The trusty stirring stick is brought in once again.


Oh wait, this bit of scrap wood is a better size, I’ll use that instead. (preparedness to the max)


Take away the wooden bits, tidy up with a knife or similar and leave in the sun. Drying times vary on conditions of course, but they take a loooong time to dry fully.


They can be dry enough to handle within a few hours, just for moving around or reusing a frame, but I would say at least 1-2 days to be dry ‘enough’ to actually put a load on, and perhaps as long as 2 weeks to be fully dry.

Then what?

The two samples below show what happens when you add a covering – some generic exterior oil-based paint on the one, and some ‘faux’ plaster / polyfilla on the other. Both have stuck fairly well as far as I can tell.


This big brick is the one I used the frame for initially. At this stage it has had nearly 2 weeks to dry. You can tell there is still some moisture and probably organic material in there, as there is even a bit of mold growing on it (the whitish bits).


I had heard that linseed oil (from flax seed), though traditionally used for sealing and finishing wood, could be used in a similar way for earth bricks. Maybe different applications fair differently, but I couldn’t see any of the bricks I’ve made lasting under even a bit of light drizzle, let alone proper rain. So, this boiled linseed oil (about £3 from Wilkos) might be a good natural alternative to oil-based paint and would still let us see the original texture and color of the soil, which for some is part of the appeal of doing this in the first place.


There’s quite a dramatic change in colour straight away. For one thing you can see how uneven the surface is!


But anyway, after a couple of coats and some drying time, there we are. The general surface feels more stable, not as gritty and sandy so I’d say it appears to have done the trick. However, I haven’t really tested any of these against the elements or compression yet, so stay tuned!

Some last things to add: It measures 22cm wide, 15cm high and 10.5 cm deep and weighs about 3.5 kilos. (Heavy enough to make it difficult to hold the scales up with one hand and take a photo with the other!) So that’s 3465 square cms – basically 1g per square centimeter. The two cans take away about 420 each, so 3465 – 840 leaves us 2625 sqcm. If this calculator is anything to go by, concrete is 2grms per cubic cm, and our brick is about 1.3. Is that good? In my mind, that suggests there ain’t enough clay in it, nor, perhaps have we compressed it enough.

IMG_0127 IMG_0128

Finally some other creations, including a bird and a disco hedgehog. The big white mess is lots of small bricks held together with polyfilla – again just trying things out. I have found miniature brick molds for scale model enthusiasts online. The six slightly different bricks are different mixes of ‘raw’ earth with the cement & sand mix – from 100% earth to 50/50. (left to right). None are particularly tough, all just shaped by hand.



Well, that was fun.

What are the benefits of building with earth?

1. Your raw materials could be right there, on site, ready to get started with. No expensive lorries, no transportation emissions, no nothing. Just dirt and water (potentially).

2. Unlike conventional bricks they do not require baking in a big industrial kiln which adds costs to you and the environment.

3. The finished product is non-toxic and non-flammable. Because of the sheer mass of the things, they can be pretty strong and great for keeping temperatures steady.

What are the disadvantages of building with earth?

1. Can take weeks to fully dry.

2. While their mass is in some ways a benefit, on the other hand, it means walls need to be stronger to stand up under their own weight. Generally speaking, earth walls seem to more often be used as infill than as load-bearing.

3. Manual work to dig and filter the ‘raw’ earth is not easy, let alone mixing, shaping and compressing them too. (Though there are mechanical solutions to many of these issues)

4. Methods vary greatly and essentially unstandardized from start to finish. Sure to entertain the local planning department. (But standards are being developed)

5. Non-renewable or non-environmentally friendly binders / additives may be required to increase strength or waterproofing. However, some have tried lime, fly-ash (from power stations), brick dust or volcanic ash as additives.

 What next?

– Try something out with a greater clay composition and perhaps less additives; or at least more natural additives.

– See how or if the linseed oil protects against rain and weather generally.

– Greatly improve the method for shaping and compressing bricks: make a better mold.

– And probably most important of all, try some kind of quasi-scientific stress testing for compression strength.

Stay tuned! Don’t forget to subscribe.


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