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 with 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.
• location of site - you need to think about:
• wind - direction
  
• wood - storage
  
• water - proximity
  
• roof - protection
• 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
• wood for door
• 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.
• final cob mix needs a mix of 75% sand and 25% clay
• 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.
• 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.
• 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.
• 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.
• 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.
this can be purchased through Amazon
More books by Kiko can be found at Handprintpress, he has some new books coming out soon. He not only writes about mud ovens but about making art out of mud, making bread art and making beautiful things with your hands. Check it out. Hand print press.

Some pics and links to mud oven projects:
Maya - Kiko's huge mud oven
Cob cottage - info on workshops and techniques
Cob cottage pics - lots of pics, not only for mud ovens but for houses etc
Earth oven faq -