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How to make a clay or earth oven

The notes here are a mixture of my own experience and notes plucked from the internet.
By the way, our own oven has a few dimensions you may like to note. It works really well, and an Italian friend was impressed with how well it cooked pizza.
From back to front- internal it measures 1 m on the floor. Internal width at widest point is 800mm. Its highest point in the internal height is 420 mm. The door is 450 mm wide at base and at highest point is 240mm. The door is shaped like a half circle, with the flat, rather obviously on the floor.

I believe the notes here are about as good as you will get. Check them out. I have added further thoughts below.

This site seems useful

I am trying to provide information for people in the tropics who are really limited in the dollars they have to spend.

These notes are not finished. That comes later. Kiko Denzo has written the guide for building earth ovens. The website these notes were taken from was based on his book. I have modified again, for the tropical situation.

Mud oven Project – technical notes.

* Basic oven is a simple, dome shaped shell of mud and sand
* we built a simple one layer oven – however you can build up to 3 layers-
– dense thermal layer
– less dense layer with straw
– finish layer
* to figure how big to make the oven – start off deciding how often and for what you will be using it. lay out loaf/pizza pans to see how big a circle you need to contain it – thats how big the oven needs to be.

There is a good chance that food drying may be a part of the reason for building an earth/clay oven in Papua New Guinea. Make it big enough!  1 m diameter circle for the inside of the oven makes a good large oven. Experiment with different sizes as larger ovens may tend to collapse.
Where should the oven be? Think about:
– wind – direction
– wood – storage
– water – having water close by is always good- and assists with safety. Remember young children get burnt really easily.
– roof – protection – this is a must.
* materials list:
– water
– dirt – subsoil – amount will depend on clay content (don’t use top soil)
– sand to mix with mud – sharp sand not round
– sand for oven form – needs to be able to hold its form
– straw if doing an insulating layer
– foundation – use bricks, wood, concrete, urbanite (old broken up concrete) etc – height determined by how low you want the door
– floor of oven – firebricks (expensive), standard red bricks (approx 28) new or used but must be smooth and flat.
o wood for door
o newspaper
* note on waterproofing oven: Denzer says an earthen oven is like a living thing and must be able to breathe. When baking it exhales steam, by putting a non breathable finish on like paint or concrete on top of the oven it is like putting on a rubber suit – it traps moisture. and the steam will condense and soak back into the oven – possibly cause collapse. the best sort of water proofing is a roof – leave at least 3inch ventilation gap.
* Mud: dig down to where the subsoil layer is – hard solid mass of earth. recognising subsoil – its hard, it doesn’t crumble. add water and you can mould it, when dry it is hard and not crumbly, it feels sticky and greasy when wet.
o final cob mix needs a mix of 75% sand and 25% clay
o give mud/cob mix test to see if it is right consistency – dampen mix lightly – not paste, listen to it crunch – squeeze and hear grinding. add more sand if it doesn’t.
o then the snow ball test – make it dry not too wet, pack it into a firm ball – this might take a few minutes. drop it onto the ground and it should hold its shape if it crumbles add more clay, if it goes flat add more sand, if it holds its shape – perfect.
o more technical details in the book about shrinkage and strength, which we didn’t really bother with.
* foundation: must support the weight, be at the correct height, protect against the elements.
* see our post on building the foundation out of wood and using prefab concrete at a base.
* read the book for urbanite solution
* about the sand form for the oven void: draw the circle around the brick floor – and fill with sand – it should be moist enough to form into clumps and hold its shape, shape it by walking a wooden board around and moisten with a light sprinkle of water. till it is smooth and firm.
o measure height of oven, the door must be 63% of height. – why 63% research of traditional clay oven found that was the optimum ratio for perfect baking.
o dome height is 60-75% of dome diameter
* making mud: mix ratio of sand and clay to form fine granules – get all the clumps of clay out. put dry mix on a tarp and stamp and twist on it, working the clay and sand together. roll the tarp over the mix back and forth until it looks evenly mixed. add water slowly adn mix with tarp again, and then jump in and twist about breaking up the clay.
* apply mud in layers around sand form. press handfulls around the base and press down with thumb and fingers – being careful not to damage the sand form – at least 3 inches thick – use your hand a guide – thicker oven requires more fuel but will stay hot longer.
* when its all covered – take your flat board and whack or tamp it until smooth and solid. to add another layer rough this one up first. then add straw mud layer and then sculpt.
* cut door and remove sand – remember your dimensions.

From “how to build your own earth oven: a low cost, wood fired mud oven; simple sourdough bread;perfect loaves” by Kiko Denzer printed in 2004 by hand press, USA.

Getting started- things to think about before starting on the oven build.
Choose a site for the oven.
Close to home is good. You may dry food or seeds, even rice in the oven and you don’t want thieves to have it too easy.
Wind direction- smoke getting in the way.
Protection – a full iron roof over the clay oven is a must. If you let rain on the oven, you can say goodbye to your oven- it will simply wash away. Build the roof before you build the oven!!
The roof could be made of saksak or kunai grass, but remember those rooves will start to leak before the oven is ready for replacement.
Benches nearby- very useful. Build these into the structure of the oven protection roof.
Running water/sink- a real bonus if you can manage it.
Shade- really useful.

Build a base.

Dig fire clay out of the ground or from the side of a hill that’s eroded away. Fire clay can easily be found right below topsoil. Light in color, it is sticky when wet and has little or no organic matter in it.

Suggestion for NARI- given that NARI intend to make an oven that will be a demonstration for many people- source two different types of clay and make two ovens- one using each kind of clay.

#
Step 2

Build a base for the earth oven. Large rocks and clay can be used. This will be a solid base designed to lift the oven up high enough that you can see into it, without always bending down. It also helps to keep very young children away from the danger part of the oven. They get very hot inside, but remain reasonably warm on outside when fired.

Construct it out of six-inch-by-six-inch treated posts and two-inch-by-six-inch treated boards. Make it high enough so when pizza or bread is slid into the mouth of the oven, it’s at a comfortable working height. Set a slab of pre-fabricated concrete on top of the wood pedestal.
#
Step 3

Lay out the fire brick on the surface of the concrete slab. Move the bricks around until they are tightly butted up against one another. Draw a circle on the top of the bricks for the inside diameter of the finished oven. Number the bricks so they can be put back in the same position before taking them off the concrete slab.
#
Step 4

Build a two-inch high clay wall along the top outside perimeter of the pre-fab concrete base. Make it from a mixture of builder’s sand and fire clay. Mix three parts sand to one part fire clay and add water. Fill the inside of the wall with a layer of road base, a mixture of silt, gravel, clay and sand, purchased from a gravel pit. Smooth it out and add a layer of regular sand over it. Use a length of two-by-four-inch board to smooth the surface of the sand.
#
Step 5

Set the bricks back down on top of the sand in the same position as before. Get a wheel barrow full of sand. Wet it if necessary so it can hold its shape while forming a dome of sand. The finished dome will create the inside shape of the fire clay oven. Keep packing the clay and wetting it with a spray bottle of water. Tap it into shape with a piece of scrap board. The finished height of the dome must be approximately 75 percent of the diameter of the base.
#
Step 6

Mix builder’s sand and fire clay together on a tarp. Start with three parts sand to one part fire clay. Begin by mixing them together dry on the tarp, breaking up any clumps of fire clay. Then add water slowly. To judge if the mix is correct, grab a handful and pass it back and forth between your hands until it’s densely packed. Next, drop it to the ground. If the ball of clay hits the ground and becomes deformed, but doesn’t crack the mixture, it is correct. If it cracks, add more fire clay. If it’s too sticky, add more builder’s sand.
#
Step 7

Cover the finished sand dome with wet newspaper. Starting at the base of the dome, start packing a wall of cob, the fire clay and sand mix, around the dome of sand. Never press into the sand dome. Pack the cob against the dome with one hand while pressing it down on top with the knuckles of a clenched fist. Make the wall four inches thick over the entire surface of the domed sand.

Step 8
Let the cob harden for four hours before cutting out the oven door. The door height should be 63 percent of the total height of the dome of sand. After cutting open the oven door, carefully remove the sand and newspaper. Using crumpled-up newspaper and very dry, fine kindling, start a small fire inside the oven. Since the oven is damp inside, start the fire closer to the mouth of the oven. Once it gets going, push the burning wood back toward the rear wall. Build another fire in front of it. Watch how the smoke exits the mouth of the oven. If its pooling up inside the dome, take the knife and cut the doorway bigger until the smoke starts flowing out.

Step 9
Build a wooden door for the final opening of the oven. Cut it to the shape of the doorway. The door is only used after the oven has been heated up and the hot ash scraped out using a scraper and a shovel/spade. The door helps to hold the heat inside the oven before you slide in the bread dough or pizza.

Step 1
Mix your mud, made from clay soil and water, until the texture of the mix is slightly wet but workable. It is easiest to mix your mud on a tarp. Bare feet, enjoy! (Wet the clay before hand- at least two days to allow dry lumps to be thoroughly wet before you start puddling the clay)

Step 2
Get a helper to hold two corners of the tarp while you hold the other corners and roll the tarp different directions to mix the mud. The purpose is to break the clay into small pieces and mix it with sand.

Step 3
Make a hard mud ball, and drop it from chest high to test your mud mixture. It shouldn’t break when it hits the ground, and if it does, just add more water or sand and clay mixture as needed; if your mud is too moist, give it more drying time.

Building the Oven
Step 1
Cover the sand form with sheets of wet newspaper and smooth them until they are flat. The newspaper prevents mud from sticking to your sand form.

Step 2
Apply the mud mixture over the wet newspaper in a layer that measures 3 to 4 inches in thickness. Use your hand’s width as a measuring gauge.

Step 3
Angle the mud mixture top inward as you build it upwards, and keep the sides neat, clean and squared. Make sure to press the mud into itself and not into the sand form.

Step 4
Pack the mud with a flat board solidly against the sand form. Add several more layers of mud, but make sure each layer is dry before you add the next one.

Finishing the Oven

Step 1
Scratch the doorway line into the mud. Remember to make the doorway height 63% of your interior oven size (see section 2). The width of the oven door should measure 1/3 to 1/2 of the oven’s inside diameter.

Step 2
Make a fist-sized tunnel into the bottom of the sand form. Use a sharp knife or small saw to cut out and remove the sand from the inside of your earth oven.

Step 3
Carve out the rest of the door opening and finish removing any sand from the inside.

Step 4
Rub the doorway until it is smooth and even using a piece of wood, a spoon or stone.

Tips & Warnings

Standard red bricks can be used in lieu of firebricks, which are far more expensive. They have to be smooth and flat, as they’ll be forming the floor of the oven.
A second insulating layer can be added over the first layer of cob if you want a more heat efficient oven.
If the walls of the clay ever crack just fill them in with more cob.
You can bake anything in a clay oven that you would bake in a traditional kitchen oven.
Don’t worry if all the newspaper doesn’t get pulled out of the oven after removing the sand. It will burn out when you build fires in the oven.
The door is never put in place when there is a live flame burning inside the oven.
Don’t weatherproof your oven by putting a layer of concrete or stucco over the clay walls. Clay ovens need to be able to release steam through their walls when food is baking inside. In other words, a clay oven needs to breathe.

4 comments to How to make a clay or earth oven

  • dave

    Hi Rosemary, Nice to hear from you. not sure quite what you mean by double decker. We ‘cheated’ with the NZ unit. I used some fire cement / concrete that tolerates heat. I made a unit in PNG, out of sand and clay – you add a surprising amount of sand to the clay. It wasn’t real successful, but hassle was I wasn’t there to help solve problems. there’s a great book on the real earth ovens. google denzo and earth oven – I think you’ll find it. I have ideas that in PNG where I work people could dry food for storage in ovens like this. PS. Work on an island north of East timor – called Wetar. I’m writing a training manual for sustainable village agriculture – ag is my background. best wishes. dave.

  • I’m working on our first earth oven now. Hope to have it be double-decker by using a store-bought fire place grill and bake rack. I’m seeing that I’d like a chimney arrangement. Any tips? What part Indonesia? I’m heading to Bali in December. Spent 5 months there last year and some in Thailand. I volunteer in schools and work with women in villages. I think this technology has great possibilities in third world countries.
    Cheers, Rosemary

  • dave

    Sorry, for now the answer needs to be really short- I am working in Indonesia presently. Heat the oven up really hot, by having fire right inside. Takes mine 2 hours to heat.
    Then, push last of embers/fire to sides and back. Sweep out centre. I use a small green shrub as std brush/brooms get scorched and catch fire. Once clean enough, I slide my pizza or naan bread off a paddle straight onto floor. Paddle is really rough and basic- just a piece of flat steel, screwed to a branch of tree. I use semolina or flour to ensure pizza doesn’t stick to paddle. When it does – it is a pain.
    For bread- that comes later, when heat has gone out of oven. Bread cooked on floor is fine, or in a camp oven or whatever that is heat resistant. Most of all- enjoy. A fantastic lot of fun we have iwth it.
    Naans are cooked on floor, and that is great as you can put the butter/garlic mix onto the dough before cooking. Yummy. enjoy and who knows I can try and answer more questions another day.

  • Hannah

    Those are great instructions.
    But how do you bake in the oven?
    Where does the fire get placed? Does the bread rest on the floor of the oven?

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