Building with Mud, and How to Build a Cob Oven

Cob is an earthen building material that is made of clay, sand, straw, and water. It has been used for thousands of years to construct homes and buildings. It has been used worldwide, but has only recently started to attract interest from Western countries. Cob has its origins in millennia of traditional building, in some of the oldest permanent human dwellings. Humans have made shelters this way for so long that we may carry a genetic memory of how to do it.

The word ‘cob’ comes from an old English word meaning ‘lump’ or ‘loaf’. The wet cob mixture is used to build thick earth walls; the building technique is very similar to sculpting with modelling clay. Because cob building requires no forms, we can build our walls into any shape we choose. Curves, niches, arched windows and built-in furniture are common features in cob buildings.

Cob building requires no cement, no expensive tools or materials and is a ‘people friendly’ way of building. Once the material is dry, cob is incredibly strong. The drying-out, setting process of clay products can be reversed if necessary, so even when the building material is hard and dry, you can usually alter it by wetting down whatever you want to change. This is different from lime and concrete products, which set and become hard through an irreversible chemical process.

Often we can find the bulk of the materials on or near the building site. In natural building, we make an effort to build with materials that are found locally. Unless they are recycled materials, we also prefer to harvest these materials ourselves, with the least amount of industrialization, so that we can be gentle on the earth.

One of the most commonly asked questions about cob is “What happens when it rains?” The cob tradition originates in England and Wales — which are not dry places by any standard. These areas have whole villages of cob buildings, many of them hundreds of years old. The secret is the same as that of any building style: A good roof and a good foundation are the keys to survival in wet climates. Cob works well in all climates. If it gets too cold, it is easy to add extra insulation on the outside of a cob building. Cob houses benefit greatly from good passive solar design.

For all these reasons and more, cob is an almost ideal building material — offering new advantages and opportunities to the developed world. Building with earth and other natural materials is becoming a solution to our world’s energy and consumption problems. Cob is dirt cheap and ecologically friendly. It is easy to learn, inexpensive, beautiful, healthy, comfortable and the materials can be found almost anywhere

Besides the above-mentioned advantages over conventional building methods, cob can offer people a way out of the crazy real-estate market, an unhealthy lifestyle and a job with no joy. Many people who never considered themselves builders have created a cob house for themselves. Cob is useful for much more than houses, too. You can make cob ovens, fire places, cob furniture and cob floors.

Labour intensive

Often, when discussing the pros and cons of building with cob, or earthen materials in general, it is brought up that it is such a labour intensive way of building. Because this comes up so often I believe it is interesting to look at this statement carefully and see what it suggests about who we are as a culture in relation to work, time and building.

Labour can become so pleasurable, so health giving, so rhythmic, so full of joy, that you actually want to do it. “Labour intensive” would then be considered a positive statement. Why would you want to do less of something you enjoy! This is the overwhelming experience of the people who build with cob, assuming they have given themselves the time it takes to build by hand and foot.

The next issue is that of time. Our obsession with speed and getting things done gets in our way of living our lives. Why? Because we are never done! Once your house is done you will move on to the next thing that needs doing. “Being done” is an illusion, rather than an absolute thing. If enjoyed, the time spent cobbing a building together is time well spent!

Aside from these philosophical considerations, it is valuable to look at the practicality of building by hand and foot. By harvesting and processing our own building materials, we do the work which others pay a conventional builder to do. If those builders had to go out into the woods and make their own lumber, cob may not seem that labour intensive anymore. Many cob builders enjoy having time rather than money and use that time to hand-build their house. In this process, it is more likely that the house will be ‘crafted’, rather than ‘assembled’ (or stapled together) as is the case with conventional building. Consequently, cob houses tend to be more creative, beautiful, and loved. Craftsmanship in the conventional building world comes at a high price; with cobbing, novice builders can be craftspeople as the wall goes up.

The de-industrialization of the building process also means that we can pay closer attention to the impact we have on the earth. The harvesting of materials can be done locally and carefully. The labour intensive methods become a key ingredient toward sustainability and care for the earth. Lastly, many hands make light work. Often people imagine that they are alone in the world and therefore have to be able to do everything by themselves. This is a choice, but it doesn’t have to be that way. Building with cob is an invitation for people to connect with one another by building together.

So, the stigma that the term ‘labour intensive’ has, may not serve us well. In fact, I think that we should seek out more labour intensive activities and crafts rather than less. Labour can bring us health, beauty and community and reduce our dependency on oil and industry. Of course, with enough machines and diesel fuel, I can cob any size house in the same time as the conventional builder staples his or hers together. But why would I?

How to find and recognize suitable clay

One seldom needs to use clay in a pure form, like potters often do. Rather, we can dig it up from near the building site, where it is often found with other materials mixed in: small rocks, sand, silt and organic matter. This is totally fine for building purposes and can even be an advantage in some cases, as long as your ‘clay soil’ has enough clay content to provide the stickiness that you need for whatever you are building.

So how can you be sure that you have clay in your soil? Well, grab a handful, mix it with a little water and kneed it until it has the consistency of modelling clay. If it tends to stick to your hands it probably has clay in it. Roll it out to the thickness of a pencil and wrap it around your finger. If it can do that without too much cracking, you have clay! If you are still not sure, roll it into a little ball and let it dry. (You can use your oven if you are in a hurry.) If you squeeze this dried ball between thumb and index finger, it should be hard and stay together; if it crumbles there is probably not enough (if any) clay in it.

Wherever you find and mine your clay, always consider what impact you will have on the local eco-system. Will your hole turn into a pond and do you want that? Can you dig so that you create a useful flat area for something else?

Clay can come in various colours such as yellow, grey and blue, but is often found being anything from dark red to light orange. It is not unusual to find clay right below the topsoil. Here in South Africa, where I live, I usually don’t have to dig more than 30 to 40 cms before I hit clay soil. Because clay becomes quite waterproof when wet (it expands or ‘swells’), one can often also find it around ponds, puddles and springs (as a vein of clay pushes the water up). Whenever you see cracks in the earth it is often caused by clay in the soil; as it dried out it shrunk so much that it caused the cracking. When hunting for clay, hang in there. If you don’t find it where you are digging, there might be beautiful clay just a couple of meters away from you. It can be somewhat random: There may be pockets of clay rather than a solid horizontal vein.

When digging up clay soil for your building project, you are usually working with a material that can be anywhere from hard and rocky to soft and moist. If it seems too hard to dig up, soaking it a bit before you dig can be very helpful. If there are too many hard rocks or stones in your clay you can sift it through a frame covered with chicken wire.

Preparing clay for your building project

Once you have your clay soil dug up, it is time to determine how moist you want the clay soil to be for your particular project. There are no hard and fast rules for deciding this, and usually it differs a bit from person to person.

Here are the basic consistencies that you can work with:

Cob: Is usually mixed by foot with sand and straw to get a stiff material and then used to build and sculpt walls which are load bearing as well as ovens, fireplaces, built-in furniture and sculptural details.

Soft-clay: Soaking clay in a tub, bucket or trough overnight (or even just for a few hours) will generally make it soft and hydrated. That is your starting point. From there on you can use it for cobbing or turn it into softer, wetter mixes.

Clay-slurry: Mixing the clay with water and a hoe in a wheel barrow will create a cake batter type consistency. This can be used for cobbing as well as for straw-clay plaster and earthen floors. For earthen floors you want to make sure that you don’t have any rocks in it. You can screen the dry clay before you add more water, or send the ‘cake batter’ through a screen (usually a little harder).

Clay-slip: This used as a ‘glue’ to adhere already dried cob to wet cob and to prepare straw-bales for its rough (straw-clay) plaster.

Clay paints: Soupy clay mixed with flour paste becomes a great, non-toxic and easy to work with paint — especially if you buy or find light coloured clays and added pigments, which can create a wide variety of beautiful soft colours.

One may choose to make these clay-soil and water mixtures on the wet or on the dry side. There are no hard and fast rules for this, and a lot of it depends on a variety of circumstances:

  • The amount of clay in your clay-soil. Different compositions of your clay-soil will react differently to the same amount of water. Not all clays are the same!
  • What are your drying conditions? In hot dry weather I like to work with more moist materials than when it is humid and cold. Also, if you have a lot of help and you are building fast, you usually want your material to be on the dryer side. That way things will set sooner so you can continue working.
  • How you like to work! Each person has different preferences and uses different tools for any of the natural building techniques.

Whatever your conditions and preferences are, remember that these materials are very forgiving and they are easy to experiment with and, to be sure, you can always do a test before you start working on the real thing.

How to Construct a Mud – Cob Oven

I think building a cob oven is a great idea for a first project using cob. There are some other technical details that have to be paid attention to compared to other projects, but there is overall less cob that has to be mixed.

Selecting the Site:

A cob oven can be built inside or outside depending on where you want it. There are some things to take into consideration in choosing a site and if you are building it indoors you have to take into consideration where your chimney will be safest.

The site selection has to be a permanent decision as you can’t really change your mind after the fact. This oven does not move!

Distance from the house itself: Having a giant fire right near your abode is risky. You need it to be a reasonable distance from the house — too far for the wind to carry sparks, for example — while still being as conveniently close to the house (kitchen) as possible, since you will likely assemble pizzas indoors before bringing them out to cook. You also have to take the wind into consideration as the fire tender or cook will be standing out there for at least half an hour.

Level ground: I guess you could build on sloping ground if you had to, but it’d be a lot harder to figure out how to stack the rocks that form the base of the oven.

Whether building indoors or outdoors you need a platform that is just below waist level, which can be built out of mud and rocks or stones. The inside of the oven (the cooking area) is dome-shaped, so you need to know the diameter of the smallest circle that could contain your ring of fire or a ring of hot coals and your pizza or one or two bread loaves. The big advantage to keeping the oven as small as possible is that smaller is much better in terms of heat efficiency and consistency. Once you know the size of that circle, you can estimate the depth and (to some extent) the height of the cooking area and surrounding mass. The overall height of the oven is affected by the rock pillar you’re about to build; you’ll want the base of the cooking area to be about waist-high.

Once the size is determined you dig a circular hole of approximately 20 cm deep and then you can build a ring of stones and then fill the ring with empty wine or beer bottles from your recycling bin. The bottles should be laid on their sides, with the necks in the centre and the bottom of the bottles against the circumference of the ring. It is best to use 1.5 litre wine bottles as it fills the ring quite efficiently. This bottle-filled ring will act as an insulating layer below the oven floor. The purpose of the glass bottles is to form pockets of air between the oven floor and the stone base; when the adobe in the insulating layer dries, the air pockets will remain, regardless of what happens to the glass. The air pockets form an energy barrier — this insulating layer prevents the stone base from stealing heat from the oven.

Your stones get stacked in a ring, preferably without any mortar. When approximately 15 cm below waist height you mix some clay with water and you make a platform that will cover the bricks and the bottles inside. This will be the floor of your oven.

Once this is settled and dry, which may take a few days, depending on your climate, you can construct the dome of your oven. Mix a large amount of sand with a small amount of mud to hold it in place. When this is mixed in well with some water, it gets placed on top of the platform in the centre leaving 15 to 20 centimetres on the outskirts for the dome’s walls. This mud in the centre gets placed as high as you want your oven to be in a dome shape. Let it dry and settle somewhat, but not entirely, and then cover it with some plastic to keep the shape.

Now you mix clay into a consistency that will hold together but is pliable enough to mould into balls or sausage shapes. This is the outside wall of your dome. Take care to get the mud to squish and blend with the rest so that it becomes uniform and smooth. You can put a layer of slightly curved hunks all the way around the base of the dome, taking care to mush each hunk into its neighbouring hunks, so there aren’t any seams visible between hunks. You can smooth it with your hands and afterwards when you have filled in any little cracks you can make a clay-slip and get a really smooth finish. Remember to leave an opening at the top for your chimney and an opening in the front for the door.

Opening the Front of the Oven:

Don’t be too hasty to undertake this next step. Your dome has to be solid, settled and dry before you move on. Give it at least a few days. Break open the plastic covering and scoop out the sand/mud mixture until you have a hollowed-out dome. Clean it out and remove any plastic.

Now build your chimney either out of mud as well or insert a metal or iron chimney. Naturally if you are building indoors this has to go through the roof and safety precautions against fire have to be carefully adhered to.

Put some kindling in your oven and start your fire. Build your fire up slowly and keep feeding it so that it burns for at least 6 to 8 hours. This bakes the mud and hardens the oven. This is also a good time to check if your chimney is high enough to give your fire a good pull, and if not build onto it.

So that is the basics of building a mud oven, but of course as you undertake this project you will come across things that you have to solve yourself and you can also be really creative and create a work of art and not just a functional oven. The possibilities are endless.

It is not advisable to put wooden beams or wood of any kind into the structure of your oven as the heat of the fire can eventually cause some burning…. Rather, stay with iron or metal of you want added structure.

Further Reading:

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5 thoughts on “Building with Mud, and How to Build a Cob Oven

  1. Great tips. Thank you! We will be using Fermanagh Blue Clay which we hope will be a good material. I made a ‘green man’ with this clay 5 years ago and dried it in the sun. It’s been out in the Irish weather since and still fine, so pretty good stuff!!

  2. Lois!

    I’ve been searching for days to find information on non firebrick hearths. Yours is one of the first sources I’ve found that suggests a clay hearth – which is exactly my preference! Can you tell me any more about getting a strong clay hearth? Mix ratios? Any insight at all is helpful❤️🙏🏽

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