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|File:DSCN8753.JPG|Stomp around to make cobb/thermal mass |2 | Place your ingredients on a tarp. Jump around on it and fold it burrito style so that you get a homogeneous mixture. Pack it in to orange sized balls and place on your oven for thermal mass. Make grapefruit sized balls for cobb.
 
|File:DSCN8753.JPG|Stomp around to make cobb/thermal mass |2 | Place your ingredients on a tarp. Jump around on it and fold it burrito style so that you get a homogeneous mixture. Pack it in to orange sized balls and place on your oven for thermal mass. Make grapefruit sized balls for cobb.
 
|File:20140402_142855.jpg|Orange sized balls ready to go for application |7 |Orange sized balls of thermal mass ready to go for application on the oven.
 
|File:20140402_142855.jpg|Orange sized balls ready to go for application |7 |Orange sized balls of thermal mass ready to go for application on the oven.
|File:DSCN8753.JPG|Stomp around to make cobb/thermal mass |8 | Place your ingredients on a tarp. Jump around on it and fold it burrito style so that you get a homogeneous mixture. Pack it in to orange sized balls and place on your oven for thermal mass. Make grapefruit sized balls for cobb. }}
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|File:thermal_mass_done.JPG|The finished cobb layer |8 | Here is what the finished cobb layer should look like. The mixture for cobb is exactly the same as the thermal mass layer except with added straw/other tensile material (most commonly straw). This layer will not be necessary for every oven. For our oven our base was not built large enough to support the eventual thermal mass layer and finishing layer, so the cobb layer was built purely for structural support of these future layers. }}
  
 
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Revision as of 18:04, 27 April 2014

Engr305 Appropriate Technology page in progress
This page is a project in progress by students in Engr305 Appropriate Technology. Please do not make edits unless you are a member of the team working on this page, but feel free to make comments on the discussion page. Check back for the finished version on May 15, 2014.

Working on constructing the thermal mass layer

Abstract

Short abstract describing the project from background to conclusion

Background

For Engr305 Appropriate Technology at Humboldt State University we will be constructing an earthen at oven at Bayside Park Farm in Arcata, CA. An earthen oven is an ancient device and is constructed primarily of clay, sand, and straw. Once a fire is burned inside the oven and removed the oven's walls can cook food via radiant heat. Due to the normally wet climate we have in Humboldt County we will need to be aware of protecting the oven through some sort of structure. We will also need to be aware of constructing the oven on a level surface. People that will be utilizing the oven will be community members who use the educational farm. Approximately 50 families currently buy produce from the community supported garden (CSA), the Bayside Park Farm. Ultimately the earthen oven will be used as a tool for the community in being able to bring people together. Bayside Park Farm would like the earthen oven to be able to bake a pizza. As well, the earthen oven could be used as an educational tool to show community members about appropriate technology and to possibly inspire them to incorporate appropriate technology into their own homes.

Problem statement

The objective of this project is to create a high quality earthen oven that can be used by the members of the Bayside Park Farm to create food for farm events. The farm fundamentally would simply like someplace to cook food. The earthen oven will be created by sustainable materials, will have an educational aspect of appropriate technologies, and will be capable of using Bayside Park Farm foods to create pizzas and breads.

Criteria

After speaking with a director of the Bayside Park Farm in Arcata these are the constraints that we felt were most important in carrying out this project.

Criteria Weight Constraints
Durability 10 longevity under seasonal weather patterns, especially Humboldt's normally wet weather
Educational Ability 7 Include a plaque or description of how the earthen oven was made
Functionality 10 Needs to be able to fit two large pizzas (14" each) at a time
Appropriate Materials 8 Nothing that would negatively impact the organic farming done at Bayside Park Farm
Cost 3 Not excessive
Maintainability 5 Easy maintainability for the farmers of Bayside Park Farm
Aesthetics 2 Make sure that the oven is not hideous

Literature Review

This is a review of the available literature pertinent to the earthen oven being constructed at Bayside Park Farms.

Masonry Oven basics

A masonry oven is an ancient device used to cook foods through the use of radiant heat. There are different types of masonry ovens across different cultures and the oven has evolved over time as well. The masonry oven dates back at least to the Roman Empire.[1] As well, there are different materials used for constructing masonry ovens, among these being cob (a mixture of clay and straw), brick (which is fire baked dirt), or modern electric or gas fired ovens. [2] A masonry oven cooks better than modern convection ovens, and is favored by cooking aficionados for baking artisanal breads and pizzas. Whereas a convection oven cooks via heating the air in the oven and blowing it around, a masonry oven cooks by radiant heat from the oven's hot walls, conduction from the oven's hot floor, and convection from "hot, steamy air." [3] Due to these characteristics a masonry oven does not suffer from warm/cold spots like a modern convection oven does. The oven is heated evenly throughout.


What is an Earthen Oven?

An earthen oven is a device that is used to cook food through radiant heat in a clay or brick structure. Over the centuries different styles of earthen oven have emerged. These include the Tandoor clay oven from India, [4] the beehive oven popular in Colonial America, [5] and the wood-fired brick oven perfected in ancient Rome as well as the Scotch Oven developed in the late 18th century. [6] The oven that we are constructing for Bayside Park Farm is a cob oven. Cob ovens come in a variety of shapes and sizes and are open to a good deal of artistic license. However,although there is much variety in the physical appearance of such ovens there are some technical specifications which are standard for cob oven construction. Among these specifications are that "door height should be 63%" of the height of the oven dome.[3]

Appropriate Technology?

A cob oven (versus a conventional modern oven) is the preferred choice for construction. This is because such an oven is appropriate, or exhibits the application of appropriate technology. Modern civilization faces environmental threats from various fronts. Over-exploitation of our natural resources and trenchant economic systems have degraded the environment and left countless billions in poverty. Sustainable technologies spurn the use of materials or systems (when available) that are not ecologically friendly, that do not take into account cultural variations, and that do not empower the people from where the technology is being implemented. Many, particularly those that ascribe to Ecological Modernization Theory, hold that continued economic development will be able to pull us out of the quagmire we have gotten ourselves into. Enter appropriate technology, which is being implemented around the world "to alleviate poverty and mitigate the destruction caused by excesses of consumer culture."[7]Although appropriate technologies use more basic construction materials (such as cob) than modern technological innovations, this does not necessarily mean that appropriate technologies are simple technologies not worthy of further research and implementation. [8]

Earthen Oven Peripheral structures

Building a Base

The purpose of the base of the oven is to raise it off of the ground, to protect the oven from ground moisture, and to keep the entire structure in the same location when the ground gets wet. [3] Recommended heights for the base are between 24"-36", so that the oven is easily accessible without unnecessary crouching. The foundation will need to be built slightly into the ground. Due to the oven's large size at 36" the base will be approximately 60" across (scaled up from dimensions given for a 22.5" oven). Materials to build the base can be varied and can be almost anything. However, it is required that the base contain at least 6" of a "drain rock" such as gravel to prevent water from wicking up into the oven. As well, the top of the base needs to include an insulation layer.[3]This is to prevent anything in the base from getting dangerously hot as well as to best keep the heat inside the oven instead of dissipating into the base. This insulation layer can be made of multiple materials, such as glass bottles or sawdust mixed with clay. Once the layer of glass bottles has been placed on the base it is covered with a layer of sand. The hearth bricks sit atop the sand layer. Thus begins the construction of the oven itself.

Local Climate Concerns

Clay will erode over time due to rain. To protect against the elements a cob oven should be re-plastered annually or there should be some sort of roof above the oven itself.[9] Arcata, located along Humboldt Bay in Humboldt County, CA, receives on average nearly 50 inches of rain a year. [10] Although Arcata is currently in a state of "extreme drought" along with most of the rest of the state of California[11], a roof over the cob oven will be constructed.

Building a Roof

At one point Bayside Park Farm possessed a cob oven on their property. The oven was not intact for long though. Cob is known to withstand the power of Mother Nature very well. Its spongy character allows it to bear long periods of rain [12], but it has failed in the past to stand the many rainy days Arcata, Ca has to offer. For this reason, durability is a top criterion for our client. If water can be soaked up directly into the cob, or is gradually soaked near cracks, the oven could possible collapse. [13] The best way we think to allow our oven to hold out against the weather would be to add a roof. The best technique to use would be the “boots and cap” strategy.[12] This strategy requires us to make the roof eaves long enough to protect the walls. If we decide to take the simple route, there is a small A-frame roof[3] that is made of metal roofing sheet and bungeed down to the base of the oven that we can use. There is also the option to build a larger four-post roof. There are some poles available to us at Bayside Park Farm for roof construction. Our decision on which type of roof to build will be based on how well we think the oven will be protected from the rain. Another small step we can take to lessen the damage from rain is to give the oven an earth plaster. Earthen plaster is naturally mold resistant [14]. In addition using manure as our fiber for the earthen plaster will help with weather-proofing.[15]

Building with Cob

Cob, or earth, is a versatile building material that has been utilized in construction purposes for many thousands of years. Mud houses have been constructed in England since at least the 1200s and may have evolved from wattle and daub construction techniques. In the 20th century cob fell out of favor as a construction material in the Western world, although there was a resurgence in popularity in cob construction in the 1990s in both England and Oregon. The use of cob in Oregon was modeled after historic cob use in England, which has proven resilient to a wet and cold climate. [16] In particular, this resilience to wet weather makes a cob structure ideal for construction in Northern California. As a building material cob is completely sustainable. A cob structure does not exacerbate the problem of deforestation, it does not contain toxic chemicals, doesn't require power tools to construct, and does not contribute to climate change.[17]

Maintaining the Inside Cob Oven

Being able to easily maintain the oven is a top priority for our client. Every season Bayside Park Farm has a different set of hands helping tend to the land. Jaime, the current head farmer wants the oven to last for many seasons. Some tools that can keep the earthen oven maintained are a rake, a scuffle and a brass bristle brush.[3] All of these tools aim and maintaining the inside of the oven. The rake removes large chunks of coal and wood. A scuffle is a swivel attached to a stick on one end and rags wrapped around it on the other end. The rags can be dampened and used to wipe down the inside of the cob oven. The brass bristle brush is used to scrape food stuck on the bottom of the oven after cooking.

Maintaining the Outside Cob Oven

As for the outside of the oven, wear and tear will cause some cracks which are normal to see. Cracking is only a problem when the cracks don’t close after the oven is cooled. [3] Cracks can cause the oven to loose heat or to collapse, but it is a problem than can be fixed with a little bit of new cob and a few hours of labor. Problems that arise when repairing cracks in cob are due to 1. new cob shrinkage and 2. joining the new cob with the old cob. [18]To avoid the effects of shrinking cob, use a drier mix. If the crack is small you can just fill it in. When filling in larger cracks, you’ll need to cut a horizontal chunk of cob out, about half way through the original cob then fill that chunk in with new cob. [19]

Timeline

We have a semester to complete this earthen oven project. This timeline breaks up the construction of the oven over the course of the semester.

Completion Date Task
2/2/14 Criteria for oven, choose a specific location
2/3/14-2/12/14 Chose a design, made a list of materials
2/19/14-3/7/14 Begin collecting materials, making cob bricks
3/8/14 Build the base of the oven
3/15/14 Finish the base and the insulation with the layer of cob
3/28/14 Get the firebricks in place and cob around bricks/outline oven form
4/2/14 Build the first layer of the cob oven with ENGR 305 slaves
4/12/14 Put on the second layer of cob
4/19/14 Fix any cracks that we see on the inside or outside
4/26/14 Design the outside of the oven
5/3/14 Make the A-frame roof and build the first fire in the oven


Budget

Budget: This is the budget for what was spent on this project. This is an appropriate technology class, and so we took the appropriate approach and found all of our materials either on Craigslist or from donations. We paid almost nothing to construct this oven.

Qty Material Needed Source Cost Total cost
2 Wheelbarrows Bayside Park Farm Already available $0.00 $0.00
A lot Dirt Bayside Park Farm Already available $0.00 $0.00
21 Firebricks Piersons Hardware Donated $111.88 $0.00
12 regular bricks Arcata Salvage $0.35 $4.20
3 5 gallon buckets Bayside Park Farm Donated $7.50 $0.00
1 9”x 14”x 2” slab of wood Pierson's Donated $6.98 $0.00
1 Lots of beach sand Free from the beach free $0.00
1 Cubic Yard of Urbanite Peter from Craigslist Donated $30.00 $0.00
12 cubic yards Sand that is less than 1/8" thick Pierson's Donated $16.97/2 cubic feet ($50.91 total) $0.00
30 gallons Decorative rocks Frank from Craiglist Donated $50.00 $0.00
10 gallons Clay HSU Art Department Donated $5.64 $0.00
20 Empty Beer Bottles Brendan (not from Craigslist) Donated $0.05 CRV ($1.00 total) $0.00
2 Bales Straw Bayside Park Farm Donated $5.00/bale ($10.00 total) $0.00

Construction

Constructing our earthen oven at Bayside Park Farm was much more difficult and time consuming than we had originally hoped the project would be. Hopefully if you are interested in building an earthen oven you will be able to learn how to do so from the following construction instructions as well to learn from out mistakes so to not repeat them. Ultimately there are a few important lessons we learned. We learned that we weren't able to glean very important information from our test bricks, although finding the right composition of cobb is very important. Our failure the first time was to not have enough sand content in the cobb, causing cracking of the whole structure. We ultimately had to construct our thermal mass layer four times before finally coming to a mixture that worked for us. While test bricks were somewhat helpful, all of the test bricks became very hard upon drying. We did not realize that when applying the mixture to the oven these mixtures would cause severe cracking. Finally, we also felt that it was important to use a parabola shaped piece of cardboard to shape our sand mold. These insights are elaborated below.

Below is a list of how to build our earthen oven.

How to Build your Own Earthen Oven in 13 easy steps!
ImageStep
Backpack frame bike trailer Step 1 : Find a site for your earthen oven.
Aleiha's parabolic solar cooker Step 2 : Dig a slight foundation.
Aleiha's parabolic solar cooker Step 3 : Build the base.
Aleiha's parabolic solar cooker Step 4 : Build the insulation layer inside of the base.
Laying the fire bricks Step 5 : Lay the fire bricks.
Aleiha's parabolic solar cooker Step 6 : Construct the sand dome.
Constructing the thermal mass layer Step 9 : Build the thermal mass layer. We made our layer about 3" thick. The thicker this layer is the longer it will take the oven to heat up, but also the longer the oven will stay warm. In making this layer, lay your orange sized balls of thermal mass around the base of your sand dome. Never push into the sand dome but push down when making this layer.
[[File: |180x180px|Aleiha's parabolic solar cooker ]] Step 10 : Lay on the insulation layer.
[[File: |180x180px|Aleiha's parabolic solar cooker ]] Step 11: Place on the finishing layer.
[[File: |180x180px|Aleiha's parabolic solar cooker ]] Step 12 : Dig out the interior sand dome, wait for the oven to dry.
[[File: |180x180px|Aleiha's parabolic solar cooker ]] Step 13 : Bake pizzas!
How to make your cobb/thermal mass!
ImageStep
Laying out the sand bricks to dry Step 1 : Do a shake test/sand brick test for clay content.
Stomp around to make cobb/thermal mass Step 2 : Place your ingredients on a tarp. Jump around on it and fold it burrito style so that you get a homogeneous mixture. Pack it in to orange sized balls and place on your oven for thermal mass. Make grapefruit sized balls for cobb.
Orange sized balls ready to go for application Step 7 : Orange sized balls of thermal mass ready to go for application on the oven.
The finished cobb layer Step 8 : Here is what the finished cobb layer should look like. The mixture for cobb is exactly the same as the thermal mass layer except with added straw/other tensile material (most commonly straw). This layer will not be necessary for every oven. For our oven our base was not built large enough to support the eventual thermal mass layer and finishing layer, so the cobb layer was built purely for structural support of these future layers.
What to Avoid!
ImageStep
Caption Step 1 : The crack from the first attempt at constructing the thermal mass layer. Our problem was continuously a lack of enough sand.
Footer
More of What to Avoid!
ImageStep
Caption Step 1 : If you see cracks in your thermal mass layer, do it again. This is the first crack.
Footer


Heat transfer within an earthen ovenCaption
EfieldSolarCell.jpg Fig 1: Demonstrates heat transfer within an earthen oven. From Kiko Denzer's "Build Your Own Earth Oven."

Operation

Maintenance

Schedule

Instructions

Conclusion

    • Section to be completed with the eventual completion of the earthen oven. **

Testing Results

Discussion

Lessons Learned

Next Steps

Troubleshooting

Problem Suggestion
Example issue Example solution or suggestion
Does not turn on Make sure it is plugged in
Another issue Et cetera

Team

These are the people who were responsible for constructing the earthen oven in the spring 2014 semester for ENGR 305.

References

  1. "Cooking Fireplaces and Bake Ovens". Buckley Rumford Fireplaces. http://www.rumford.com/oven/articleoven.html. Retrieved 2014-02-08.
  2. "Rules of the VPN Association". Verace Pizza Napoletana Association. 1998. http://anticapizzeria.net/vpn/get%20certified%20new.html. Retrieved 2008-09-28.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Denzer, Kiko, Field, Hannah (2007). Build Your Own Earth Oven. Blodgett: Hand Print Press. ISBN 978-0-9679846-7-4.
  4. "All About Tandoor Ovens: What they are and How they Work". Apartment Therapy. 2009. http://www.thekitchn.com/all-about-tandoor-ovens-what-t-90121. Retrieved 2014-02-08.
  5. "Early American Fireplaces and Cooking". Colonial America:The Simple Li. 2009. http://colonial-american-life.blogspot.com/2009/08/early-american-fireplaces-and-cooking.html. Retrieved 2014-02-08.
  6. "A Brief History of the Pizza Oven". Forno Bravo. http://www.fornobravo.com/pizza-oven-resources/pizza_oven_history.html. Retrieved 2014-02-08.
  7. Pearce, Joshua, (2012). The case for open source Appropriate Technology. Environment, Development, and Sustainability. pp. 425-431. doi:10.1007/s10668-012-9337-9.
  8. Kammen, Daniel, Dove, Michael (1997). The Virtues of Mundane Science. Environment.
  9. "Build a Clay (Cob) Oven in Your Yard". 2013. http://buildnaturally.blogspot.com/2013/06/build-clay-cob-oven-in-your-yard.html. Retrieved 2014-02-08.
  10. "Arcata, CA Weather Averages". Weatherbase. 2014. http://www.weatherbase.com/weather/weather.php3?s=64537&refer=&units=us. Retrieved 2014-02-08.
  11. "Another drought designation for Humboldt". Redwood Times. 2014. http://www.redwoodtimes.com/news/ci_25010252/another-drought-designation-humboldt-farm-service-agency-passes. Retrieved 2014-02-08.
  12. 12.0 12.1 The Cob Cottage Company (1996). Earth Building and the Cob Revival: a Reader. Oregon: The Cob Company. pp. 2-3.
  13. "Engineering Properties of Cob as a Building Material". Journal of Applied Sciences. 2006. http://www.scialert.net/abstract/?doi=jas.2006.1882.1885. Retrieved 2014-02-06.
  14. Freed, Eric Corey (2008). Green Building and Remodeling for Dummies. New Jersey: Wiley Publishing, Inc.. ISBN 978-0-470-17559-0.
  15. Denzer, Kiko (2007). Dig Your Hands in the Dirt! A Manual for Making Art out of Earth. Blodgett: Hand Print Press. ISBN 978-0-9679846-6-7.
  16. "The History of Cob". Michael Smith. http://www.networkearth.org/naturalbuilding/history.html. Retrieved 2014-02-08.
  17. "Cob Cottage Company". DeaTech Research Inc.. http://www.cobcottage.com/whatis. Retrieved 2014-02-08.
  18. "Maintenance and Repair". Devon Historic Buildings Trust. 1993. http://www.devonearthbuilding.com/leaflets/the_cob_buildings_of_devon_2.pdf. Retrieved 2014-02-06.
  19. "Maintenance and Repair". Devon Historic Buildings Trust. 1993. http://www.devonearthbuilding.com/leaflets/the_cob_buildings_of_devon_2.pdf. Retrieved 2014-02-06.
  • W., Allan (2011). Build Your Own Wood-Fired Oven: From the Earth, Brick, or New Materials, 1st Edition, Rosenberg Publishing

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