The Simple Art of Making an Earth Oven
Kiko Denzer, earth oven builder and sculptor, gives a step by step gude to making beautiful, outdoor ovens.
An oven is just a hole in the ground – light a fire, heat it up, and bake! An earthen oven is about that complicated. If you made mud pies when you were a kid, you can make a beautiful mud oven. You can also do it for nothing (or next to it), and it will work as well as a custom-built masonry or ceramic model costing thousands.
Earth is not only more common, safe, cheap, and easier than brick, it's also more environmentally friendly and, I think, beautiful. Mud or brick, however, it's what we're all made of. (In Hebrew, Adam means 'red clay' and when you think about where your food really comes from, the bible phrase, "for dust thou art...," suggests a deep, but perhaps different truth.)
The three basic steps to oven-making are:
- Make a pile of damp sand, and pat into a hemisphere.
- Cover with a layer or three of mud.
- Cut out a doorway and remove the sand.
If you fire-dry it, you can bake tomorrow. To bake, fire the oven without a door cover. Then clean out the fire and load your loaves or meat or veg into the hot oven, put something in front of the door, and wash your hands for supper.
- The floor: brick is a common choice – new, old, red brick, or firebrick.
- The mud: In most places, there is clay subsoil below the topsoil. Like cement, clay holds things together. If you have a garden, you likely have good soil for building. Use it straight out of the ground, or mix it with sand. If you have no garden, anyone digging a hole will probably give you some.
- The base, or foundation: You can spend lots of time (and money) on a magnificent stone foundation. Or build on the ground, on a pile of rocks, on logs or railroad ties, old metal barrels, or even on sawhorses.
- Protection from the elements: A roof is nice, but not necessary. It takes a long time for an oven to 'wash away in the rain' (look how long the hills last...). But a wet oven takes longer to heat up. So cover it with a tarp or bend pieces of roofing tin into a vault for a simple, elegant, and cheap solution. DON'T USE PAINT OR CEMENT! An earthen oven, like a living thing, breathes. When baking, it 'exhales' steam. Cement or paint will trap moisture and destroy your oven.
Tools & Materials
- Dirt (subsoil)
- Sand to mix with mud: 'sharp'(not rounded or beach) sand is best
- Material for the oven form: sand,or anything that will make a firm, but removable form (topsoil, chunks of turf, etc.)
- Firebricks or standard red bricks, new or used, free of old mortar
- Wood for a baking door
- 20 litre (5 gallon) plastic buckets
- Tape measure
- Tarps: 2x2.5m (6x8ft) minimum. In the States, they throw away huge plastic lumber wrapper that's often stronger than the tarps you can buy
- Spoons, spatulas, dull knives, etc., in case you want to do any sculpting
- Lumber scraps
- Old clothes to get dirty in, and boots if you don't want to go barefoot
- Spirit level, two or four foot (optional)
- Your oven can be one or more layers thick. Here we'll just describe a single layer, to give you the idea. It's easy to extrapolate and improvise.
Finding Clay Subsoil
Clay subsoil is pretty easy to recognize. I take a pinch of dirt in my palm, spit into it, and mix it with a finger. Silt or organic matter feels floury or crumbly. Clay feels sticky, slippery, and a bit greasy. Wetted, it rolls into snakes between your palms and wraps around your finger. (Beware dirt that may contain glass or debris; ask permission where necessary.)
When dry, the clay should be hard, not crumbly. Test it. Make bricks and let them dry. See how much they crack and how hard they are. Try mixing dirt with sand, and make more bricks. If they show less cracking and shrinkage, good! Compare and choose the best. Remember what your proportions were.
Your Oven Floor
The simplest and easiest thing to do is simply to set your floor bricks in a 10-15cm (4-6in) bed of sand, tamped and smoothed into an even, level bed. No mortar is necessary. The bricks will be held in place by the heavy, solid oven walls.
Set your first brick carefully, level and solid. The more even and flat your bricks, the easier you'll be able to clean it. Hold the next brick level and about 2.5cm (1in) above the sand; gently 'kiss' its long side to the match-ing side of the previous one. Set it flat and firm on the sand. Don't wiggle it! Minimize cracks and gaps. When they're all down, set them lightly with a hammer handle. If one stands up a bit proud, tap it down.
Make A Sand Form - Shape The Void
The sand form is the shape of the void. Sand (or whatever) should be moist enough to pack into a ball, but not so wet that it slumps. Make a pile on the floor bricks. Make the walls near vertical at first, to give your loaves 'head room'.
The form should be a bit higher than the radius of the oven floor. Too high, and you get cooling eddies at the top of the dome; too low, and the fire won't get enough air. A 69cm (27in) diameter oven, with a 34cm (13in) radius, should be 40-50cm (16-20in) high.
Hold a straight stick level across the top; measure the distance to the floor to get the interior height of your oven. Multiply it by 63% (0.63) to determine the proper height for your oven door. Write down the number!
Use whatever mixture gave you the best, hardest, strongest test bricks. Prepare a pile of dry mix on your tarp. (If you're using pure subsoil, you may need to do nothing, or just add a little water so it's moist enough to build with.)
With two people, hold two corners of the tarp each – feet spread, knees bent, elbows down and shoulders back – roll the mix from side to side. Don't lift! You can do this solo by pulling one end of the tarp over the other, rolling as you go. A longer tarp is easier. Or just use bare hands and feet and muck around in it.
Add water slowly. Take off your shoes, jump in, and do the twist – seriously! As you twist and turn, your feet work like rotating pistons, breaking up the clay and pressing sand into it. Play music! Grab a partner! Dance 'til it starts to clump, like dough for pie crust.
Test it. Pack a hard ball (50-100 pats from hand to hand). Drop it from breast height. It should hold together. If not, add a little water. If it's too wet; add dry mix (harder), or use as is and allow more drying time (easier).
First, cover your sand form with sheets of wet newspaper (so that later, you'll know where the void ends and the oven begins). Smooth them down flat. Press handfuls of your mix around the base of your sand form. Use your fingers as a gauge to guide you in maintaining thickness. Make a layer at least three inches thick. You can make it thicker (it will hold more heat), but a thicker oven also requires more fuel.
Maintain a well-defined edge. As you go higher, the face of the layer should angle upwards. Cover the whole form, even where the door will go (you'll cut it out later). When it's all covered, take a flat board and pack the material 'til solid. If it squishes and/or sticks to the board, don't worry! Your mix was too damp. Rock the board firmly back and forth, up and down, or rub it as if you were polishing. Make it beautiful.
Say your dome is 45.5cm (18 in) high (interior height). Your door should be 0.63x45.5=28.7cm 0.63x18=11in) high. As for width, 30-50% of the inner diameter is good.
Scratch a line in the material where the door will be. Cut a hole just big enough to get your hand in. Dig a narrow channel into the form. If the mix was moist but not wet, you can dig out the whole form immediately. If, when you poke the thermal layer with a finger, it's firm and resists denting, go on and dig. If it's soft, or you're in doubt, wait and let it dry (this can take days or weeks depending on the weather)!
When you remove the sand, watch for the newspaper so you know you're not digging away the oven. Refine the doorway so it's smooth and even.
Air drying the finished oven can take weeks, but if you're impatient to bake, make a small fire in the oven to speed up the process. Even when they're not quick-dried, they may crack. If so, don't worry. Cracking due to natural expansion invariably occurs during firing. If the cracks don't close when the oven is cool, you can fill them (from the outside) with mud.
When I've built a quick oven and heated it up in a hurry, I've sometimes sealed it with bricks and sand, or whatever else I could get my hands on. It doesn't even need to fit perfectly, since you'll drape it with a wet cloth to keep it from charring and to add steam for baking. You can also soak it in water, or screw a sheet of metal to the inside, or wrap it in foil.
You're ready to fire your oven, and bake
Kiko Denzer is from Oregon, USA. He is a sculptor and builder who has been working with cob and earthen materials for 8 years. He has taught in Mexico and Canada, as well as the US, and often with his wife, Hannah Field, who was a baker at the Village Bakery in Cumbria.
Build Your Own Earth Oven, by Kiko Denzer is available from the Green Shopping Catalogue.
Some Notes On Minimising Heat Loss In Earth - Added to web version of article 29/11/05
A well insulated earthen oven CAN be extremely efficient, but typically only when used continuously – as in a commercial bakery, where the oven is kept hot and fired every day. Ovens in such settings often have a foot or more of insulation; some have baked as much as five pounds of bread for a single pound of fuel burned!
But any oven, when insulated around top, bottom, and sides, will hold heat longer and cook more food per pound of wood burnt. No matter how you use your oven, insulation is important. I strongly recommend it.
I became particularly concerned about this issue after seeing some reports from builders making ovens in Africa, where wood is scarce and the where loss of vegetation is replacing forests and fields with deserts. So I've written up some brief notes about insulation in the hopes that it will make for better ovens and more informed oven-makers – and less demand for precious wood fuels...
First a quick review: a masonry oven bakes so well because hot, densemasonry radiates heat evenly into the small, hollow cook space on the inside. At the same time, however, a masonry oven loses heat in every other direction.
It may be common knowledge that 'heat rises', but less commonly known is that heat rises only in a gas or liquid – air or water – where particles can flow around each other because they aren't rigidly connected in a solid matrix. In brick, stone, or earth, however, when the rigidly connected molecules heat up, they get excited. In a liquid or gaseous state, that excitement would cause them to flow (think volcanoes and lava). But as long as they remain solid, they can't move.
So what do they do with all that 'heated excitement'? Well, imagine a room completely filled with beach balls. If you were to reach through a window and start tapping one ball, immediately, you'd see that the energy of tapping would be transferred to every other ball in the room - because all the balls are touching other balls. Almost instantaneously, they'll all start to quiver. That is, essentially, the principle of conductivity at work, and that is how it moves heat in all directions.
In your oven, conductivity moves heat into every material in contact with it, including the earth under your feet. Theoretically, if you could fire your oven long enough, you'd heat up the whole world! Of course that doesn't happen – but what does happen is that you may end up burning LOTS of wood in order to get your oven hot enough to bake a little bread. This causes problems, whether you live in a land suffering from over-harvesting, over-grazing, and desertification, or whether you live in a fuel and forest rich area. We all depend on trees and forest for the air and environment that make life possible.
So I hope you might reflect on your relationship to the forest and make some important strategic decisions about efficiency and fuel consumption – and perhaps move closer to the precious and sacred sources of all life.
But back to minimizing heat loss: be they earth, brick or stone, all masonry ovens will absorb heat as long as there's heat to absorb, and at the same time, they will lose heat as long as their insides are hotter than their outsides.
This radiant property of heat is what makes masonry ovens effective, but it can also make them very inefficient. It takes a lot of wood to heat up that masonry, but if you only cook a single loaf of bread, most of the heat will be lost and wasted. So the trick is to take specific steps to maximize your use of the internal heat of the oven, and to minimize external heat losses. If you do take those steps, and if you bake every day, you can bake extremely efficiently.
On the other hand, if your oven is only used occasionally, and cools down between bakes, efficiency will diminish significantly. But you can still improve it by the same principles of maximizing use and minimizing loss of heat.
The key is to surround your hot, heavy oven with some kind of barrier or 'thermal break', to slow down the rate of conduction and insulate as fully as possible.
Fireproof insulation is easy to make, simply by mixing a 'slip' of clay and water, beaten/mixed until smooth, free of lumps, and about the consistency of heavy cream – so your fingers and fingerprints are coated. Then you mix the slip with some kind of coarsely chopped, dry woody material. The book mentions using straw in this application, but I've since found that material of smaller particle size is easier to work with: sawdust, woodchips, chopped straw, the chaff from harvesting grain, perhaps dry leaves – I suspect just the thing is around your building site somewhere.
Add enough slip so that you can pack your 'dough' into a firm ball – but remember, the more clay, the more conductivity, and the greater your heat loss. So make a test ball. When it dries, it should be fireproof lightweight, and it should stay stuck together.
If it gets hot enough, the woody material will char and eventually burn out, but if you've packed it tight enough with the clay slip, it won't get enough oxygen to burst into flame.
A very good additional experiment is to fire your test ball as hot as you can, either in a wood-stove, an oven, or a campfire. You should be left with what is essentially a non-flammable, very lightweight (and fragile) clay 'foam'. Indeed, this is essentially how insulating fire brick is made. If you were so inclined, you could make such bricks and use them for insulation. But in general, I use the 'sawdust-slip' mix to surround the whole oven with a layer at least 3" thick, preferably more, even up to a foot or so of thickness, depending on the situation.
Packing the insulating sawdust-clay 'dough' around the walls of the oven is easy enough, but when putting it under the floor it helps to let the material dry completely before you build on it. In addition, if you're building a very big oven, perlite, vermiculite, or other mineral insulants will better support the weight of a big oven. And if you have a supply of (recycled?) aluminum foil, you can wrap a layer of that around the oven, prior to insulating, to help reflect heat back into the oven and further reduce heat loss. (Just to prove to yourself the value of the foil, put your hand as close as you can stand to a burning candle. Then put a piece of foil between your hand and the flame. How much closer can you go?)
Alan Scott, the grandfather of modern brick oven-building, makes very sophisticated hanging slabs of insulated concrete under his ovens. They further reduce heat loss by using a cleverly engineered air gap to isolate the floor of the oven from the foundation and the ground. (For more info, see The Bread Builders, by Scott and Wing)
In addition, there are groups and individuals working on fuel efficient, low-mass, non-radiant ovens and stoves for use in fuel-poor countries. One of them is Aprovecho Institute, in Oregon, which has some good things posted on their website, aprovecho.net.
I agree with the addendum about insulation except for the part about aluminium foil.
An earth (or clay, or cob) oven needs to breathe. This means that it releases and aborbs moisture as it heats and cools, respectively.
Dough, if you think about it is basically flour and water - by the time you have bread most of the water is gone via evaporation. As the oven cools some of this water will be absorbed into the walls and ceiling.
Now, if you clad your thermal layer with aluminium foil, this creates an impermeable layer that water can not penetrate, meaning that water is likely to accumulate on the underside of it which over time can lead to collapse of your dome structure as eventually the thermal mass liquefies.
This is not an issue with ovens built using bricks or concrete - but then those ovens have a very large ecological and carbon footprint compared to the humble earth oven.
Another point worth mentioning is a well fitting door, ideally insulated itself will greatly increase the efficacy of your oven.