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== Materials Gathering ==  
== Materials Gathering ==  


Urbanite (concrete chunks): construction sites, calling local construction companies, looking for piles around town (ask before you take). Not much preparation is needed after you've harvested your urbanite. You may want to get a small sledge hammer to shape the chunks just the way you want. More about Urbanite in the Foundation Section.
'''Urbanite''' (concrete chunks): construction sites, calling local construction companies, looking for piles around town (ask before you take). Not much preparation is needed after you've harvested your urbanite. You may want to get a small sledge hammer to shape the chunks just the way you want. More about Urbanite in the Foundation Section.


Clay: It is best to dig on site.  You can dig up some clayish soil from below the topsoil in your yard and give it the "E-Z soil analysis test" here. Grate the clay on a heavy duty screen (1/2") to help break it up before it is mixed in with your sand or straw. Raw clay is easily mixed with a paint mixer after it has been soaked in a bit of water for a while (though our mixing attachment did break after a while).  
'''Clay''': It is best to dig on site.  You can dig up some clayish soil from below the topsoil in your yard and give it the "E-Z soil analysis test" here. Grate the clay on a heavy duty screen (1/2") to help break it up before it is mixed in with your sand or straw. Raw clay is easily mixed with a paint mixer after it has been soaked in a bit of water for a while (though our mixing attachment did break after a while).  


Sand: It is very important that you use river sand for construction with cob, beach sand is too smooth. You can get sand directly from the river, however try to minimize your impact to the river bank. You can also buy sand from construction companies that sell gravel. You want about 1/4 inch or smaller. We used varying sized screens (as seen below) to filter the sand to different sizes we wanted. Larger pieces of sand and gravel are welcome in the foundation to be used as fill. Medium pieces are good to mix with your clay. The finest sand is used for laying the fire brick and for a fine plaster coating.
'''Sand''': It is very important that you use river sand for construction with cob, beach sand is too smooth. You can get sand directly from the river, however try to minimize your impact to the river bank. You can also buy sand from construction companies that sell gravel. You want about 1/4 inch or smaller. We used varying sized screens (as seen below) to filter the sand to different sizes we wanted. Larger pieces of sand and gravel are welcome in the foundation to be used as fill. Medium pieces are good to mix with your clay. The finest sand is used for laying the fire brick and for a fine plaster coating.


Straw:  Usually you buy it unless you know someone in the bailing business. We bought organic rice straw for $5.00 a bale and needed about 2 bales.
'''Straw''':  Usually you buy it unless you know someone in the bailing business. We bought organic rice straw for $5.00 a bale and needed about 2 bales.


Manure: A good ingredient in the plaster. Get from fields with permission. Call local stables for horse manure or dairy/meat farms for cow manure. Do not trespass on fields! Manure works great as a fiber because it has already been digested into very small particles which can form long chains of tensile strength in cob mixtures. Grate the manure to break it up from its post-digested form. Consider using rubber gloves or washing your hands afterward.
'''Manure''': A good ingredient in the plaster. Get from fields with permission. Call local stables for horse manure or dairy/meat farms for cow manure. Do not trespass on fields! Manure works great as a fiber because it has already been digested into very small particles which can form long chains of tensile strength in cob mixtures. Grate the manure to break it up from its post-digested form. Consider using rubber gloves or washing your hands afterward.


Fire Brick: Unless you find it somewhere creative, we suggest buying it from a place that sells brick and building materials. The bricks will assembled into a floor for your oven so it is important that your brick resource has kept the bricks clean and free from things you may not want eat later (such as chemicals).
'''Fire Brick''': Unless you find it somewhere creative, we suggest buying it from a place that sells brick and building materials. The bricks will assembled into a floor for your oven so it is important that your brick resource has kept the bricks clean and free from things you may not want eat later (such as chemicals).


== Materials needed to make the earth oven and estimated costs ==
== Materials needed to make the earth oven and estimated costs ==

Revision as of 18:04, 26 March 2015

The earth oven is located at Humboldt State's Campus Center for Appropriate Technology, or CCAT.

What is an earth oven?

“An oven made of materials gathered from the earth. This may include clay, sand and straw or other fibrous materials (such as paper or horse manure)” These materials are mixed in varying proportions and usually in three layers. “Earthen ovens provide an even source of all three types of heat (conduction, convection and radiation) to bake to perfection. This technology has existed for thousands of years and has been used by cultures worldwide. Some ovens in England are over 500 years old; this is not unusual if protected from the elements. The ovens are made of the earth - dirt - and dirt lasts a long time”

Structure of an Earth Oven

  1. The first layer:
    • “The thermal layer is made mostly of clay and sand. It is best when created in a dome shape providing heat for the oven by retaining heat from a fire that is first lit inside”
  2. The second layer:
    • “The insulation layer is made mostly of fiber and clay. It covers the thermal layer helping prevent heat loss”
  3. The third layer:
    • “This layer is optional. The outer layer is made of clay, sand and fiber. It protects the first two layers from the elements and can be used to sculpt the oven into almost any shape”

Earthoven1.jpg Earthoven2.jpg

Materials Gathering

Urbanite (concrete chunks): construction sites, calling local construction companies, looking for piles around town (ask before you take). Not much preparation is needed after you've harvested your urbanite. You may want to get a small sledge hammer to shape the chunks just the way you want. More about Urbanite in the Foundation Section.

Clay: It is best to dig on site. You can dig up some clayish soil from below the topsoil in your yard and give it the "E-Z soil analysis test" here. Grate the clay on a heavy duty screen (1/2") to help break it up before it is mixed in with your sand or straw. Raw clay is easily mixed with a paint mixer after it has been soaked in a bit of water for a while (though our mixing attachment did break after a while).

Sand: It is very important that you use river sand for construction with cob, beach sand is too smooth. You can get sand directly from the river, however try to minimize your impact to the river bank. You can also buy sand from construction companies that sell gravel. You want about 1/4 inch or smaller. We used varying sized screens (as seen below) to filter the sand to different sizes we wanted. Larger pieces of sand and gravel are welcome in the foundation to be used as fill. Medium pieces are good to mix with your clay. The finest sand is used for laying the fire brick and for a fine plaster coating.

Straw: Usually you buy it unless you know someone in the bailing business. We bought organic rice straw for $5.00 a bale and needed about 2 bales.

Manure: A good ingredient in the plaster. Get from fields with permission. Call local stables for horse manure or dairy/meat farms for cow manure. Do not trespass on fields! Manure works great as a fiber because it has already been digested into very small particles which can form long chains of tensile strength in cob mixtures. Grate the manure to break it up from its post-digested form. Consider using rubber gloves or washing your hands afterward.

Fire Brick: Unless you find it somewhere creative, we suggest buying it from a place that sells brick and building materials. The bricks will assembled into a floor for your oven so it is important that your brick resource has kept the bricks clean and free from things you may not want eat later (such as chemicals).

Materials needed to make the earth oven and estimated costs

1-27.PNG 27-44.PNG


Things to consider before making an earth oven

Kiko Denzer suggests people to consider:

  • If they like to work in the elements, or how will they protect their oven from the elements?
  • Will they build a roof over it?
  • Can it be a well-lit area?
  • Does it face away from the wind to carry smoke away, or will smoke be blown in an undesirable location?
  • Do they need a permit to build it there?
  • How will it affect their fire insurance?
  • Is it near food preparation areas?
  • Will it be waist level, or will people have to sit on the ground to use it?”


Turtle cob oven (2005)

A group of HSU students completed the Turtle Cob Oven by adding to the earth oven located at CCAT. They gave the turtle feet, fixed cracking, and painted the Turtle's Shell. Since they were adding to the previously built cob oven, they needed to find a way to attach more cob to form the feet. They did this by drilling into the plaster and cob with a one inch drill bit about one and a half inches deep (as far as possible due to the foundation). They cut seven inch pieces of bamboo and placed them snugly inside the holes, and also scraped the surface of the plaster with a screwdriver in order for the cob to stick into the groves.

Earthoven3.jpg Earthoven4.jpg


Cob protection and beautification (2009)

A second group of HSU students took on the job of protecting and beautifying the previously made earth oven (aka cob oven) and cob bench already found at CCAT. Their goal was to find a way to protect and beautify the cob installations. “While cob is a very durable material in some climates, like Humboldt County's, it must be sealed. Weather plays a large role in the degrading of cob and without a proper seal, like a plaster, the cob will not last very long. Since being built, signs of weathering were already apparent in the oven”

Testing the plaster: A variety of plaster mixes with a variety of ratios were tested in order to find the best plaster for the project. Each plaster was tested for scratch resistance, the amount of dust given off, compressive strength, and water resistance. Each plaster was also observed for cracking during the drying process.

The final project: “A two layered plaster design was chosen. The first aspect of the design is a base coat made of one part clay and soil, one part sand and one part sawdust. This layer is applied relatively heavily to reshape in minor ways and to fill in holes and cracks from wear and tear. The fill coat also provides a clean smooth surface to apply a finish coat to. The second coat was applied for both aesthetic purposes and to provide long lasting durability. This plaster mixture uses a ratio of 1 part mixed hydrated lime and water to three parts sand. This mixture is similar to one used by CCAT before, and it is recommended in Green Building”

Earthoven5.jpg


Similar projects to consider

October 2013 Update

The cob oven appears to be in great shape. The living roof is still doing a good job of protecting the oven from the elements, which has helped the oven last over the years with little damage from weathering. The main visible damage the oven has experienced is the char in the front of the oven—beginning at the opening in the front and extending towards the top of the oven. This is as a result of firing up the oven during each use, and is a merely aesthetic component of the oven’s presentation that does not affect the oven’s proper functioning.

Earthoven6.jpg

Resources

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