HBCSL cobb oven
This cob oven was built in spring of 2011 at the up and coming Manila Hostel just west of Arcata, CA. The Humboldt Bay Center for Sustainable Living and DANCO are jointly restoring a house to serve as an alternative hostel that is renovated using sustainable and energy efficiency principles. We built a wood fired cob oven out of recycled and locally harvested materials that will eventually be integrated with a fully functioning outdoor kitchen. We intend this oven to serve as a learning and experiential opportunity for travelers and to of course be used to cook delicious breads, pizzas, cakes and other baked goods.
Earthen homes are an old building material and can be found in various forms around the world. Cob was widely used in Europe in the 16th through the 19th centuries and many of the structures continue to stand and are still in use today.
Cob is a simple mixture of clay, sand, straw and water and is mixed wet enough to shape and dry enough to layer up without the use of forms or skeletal structures. Generally it is mixed together by foot but can also be mixed by machine. The material is ideal for molding into any shape, lending itself to curves and sculpturing. Cob is incredibly durable and with a solid roof possesses a resilient ability to withstand fluctuations of weather.
Sand particles are the building blocks when creating a cob structure. The sand you choose should be very coarse for the thermal mass layer. The rougher the sand the more sturdy the structure . The kind of sand used in successful cob structures can be purchased at a concrete supply place. Ask for builders sand, for it is coarse and fairly inexpensive (depending on the quantity of purchase).
Straw is the plant structure between the root and the grain head after the leaves have been removed  . The internal structure of a piece of straw is tubular, tough, and efficient  . When choosing straw opt for fresh straw over old. Why? Fresh straw is stronger! Be sure to stay away from hay it has a faster decomposition rate and can sprout.
Plain water works best for cob mixes. Advantages: If you are in a rainy environment, pull out some buckets and capture your project water, we did! The Cobb builder's handbook notes that a bit of lime putty mixed with the water will strengthen the cob . Disadvantages: Too much water mixed with your other ingredients will make a soupy mess. If this happens take a break and come back to it after your mixture has dried a bit. Not all cob recipes are one-size-fits-all. Be sure to make test batches with your chosen materials to insure proper ratios!!
The thick walls provide excellent thermal mass and insulation to hold heat and therefore provide an ideal building material for an outdoor oven. A large wood fire is built inside the domed oven and burned to coals. The coals are pushed aside or removed and the oven remains hot for hours. You can cook anything in it that a "normal" oven can cook.
Cob Building Materials
|Straw (not hay)|
|Rubble / hardcore|
|Normal building bricks for oven base|
|Glass bottles (optional)|
|Hydrated S lime|
|Cat-tails (a type of plant, not from an animal!)|
|Wood for burning in the oven|
|Matches or lighter|
Cob building tools
|A few shovels|
|A screen for sifting clay|
|A screen for sifting sand|
|5 gallon plastic buckets|
|2 Big Tarps|
|Twine or string|
|Face mask and goggles|
|Old clothes to get dirty & boots|
|Basic carpentry tools: saw, hammer, drill, fasteners, screws, nails etc (not appropriate for every design)|
Mixing the proper ratios of material is important for stability and strength as well as ease of construction. Controlling the drying process of each layer is crucial to the integrity of the final product. It is important to let each applied layer dry enough before putting on an additional layer so that sagging doesn't occur, but also not letting it dry out so much that the next layer wont adhere to the previous. Clay is made up of microscopic particles that act as a binder. Advantage: As clay dries it becomes stronger due to its increase in bond . This makes for a wonderful building material in dry climates. Disadvantage: A cob structure in moist environments may need a shelter built to slow environmental deterioration. 
The cost to build a cob oven will vary depending on the source of the materials used. We have free access to red bricks, clay, rubble, and urbanite (broken slabs of concrete). Our clay was harvested on the grounds at CCAT []. Urbanite and rubble were taken, with permission from DANCO Construction, from behind the Arcata recycling center located in Samoa,CA. These two items are typical construction refuse and can usually be obtained for free. Straw can be found at any farm feed store. Our foundation bricks were items group members had free access to. Firebricks are available at most hardware stores. They are not to be confused with red bricks, or ceramic store bricks. All together our cobb oven materials ran us less than $150. Below you will find an organized chart of proposed project expenses.
|Quantity||Material||Source||Cost ($)||Total ($)|
|50 lb bag||River sand/fine sifted||Pierson, Eureka||12.00||12.00|
|50 lb sack||Hydrated Lime||Ace Hardware||10.00||10.00|
|1||Straw bale||Three G's Hay and Grain||7.00||7.00|
|18||fire bricks||Thomas Home Center, McKinleyville||3.50||63.00|
|1 bag||Mortar Clay||CCAT||donation||0.00|
|30-40 slabs||urbanite||Arcata Recycling center in Samoa, CA||donation||0.00|
|2 bags||black copper and yellow oxide||Phoenix Clay Supply, Arcata, CA||10.00||10.00|
This section includes all possible criteria & constraints thought up by the group members for this project as well as the site manager, Sean Armstrong.
|Criteria||Constraints||Our weight (0-10)|
|Maintainability||Must be able to use properly without repairs for five years.||8|
|Safety||The proper materials must be used during construction that allow the oven to function safely.||10|
|Effectiveness||Must be able to distribute and contain heat properly.||10|
|Ease of Use||Must be able to figure out with instructions and or supervision.||8|
|Longevity/Sustainability||Must be able to withstand in Humboldt County's climate and continue to function properly.||8|
|Accessibility||Must be in close enough range to the hostel for easy access.||6|
|Cost||must be less than or equal to our budget of 500 dollars.||10|
|Aesthetics||Must be inviting without compromising the functionality of oven||9|
How to Construct
Materials: urbanite (reclaimed concrete, sand, rubble, bricks, mortar clay, recycled glass bottles
Foundation: Leveled ground and laid 3 inches of small rock.
To construct our base we went to the local recycling center and found chunks of urbanite. We made a circular base with an outer circumference of 43-46 inches using large pieces of urbanite (Fig.1-2) and left a jagged empty core that we filled with a mixture of sand and rubble of small to medium size rocks (Fig.3) also found for free at the recycling center. We built up the base like this, with urbanite and rubble, till about 32 inches tall and leveled off as flat as possible (Fig.4).
At this point we added a circumference of two layers of reclaimed bricks (5" total), laying them in mortar clay and fine sand. To fill this space we first made an insulation layer that consisted of sawdust from the local mill, sifted clay, and enough water to make a thick clay slip (Fig.5). We put an inch of this material on top of the urbanite and rubble layer and buried recycled glass bottles one inch or so apart (laying flat) and filled in around the bottle with just the ridges of the bottles showing (Fig.6).
On top of this layer we immediately mixed a dense layer of a 1:1 ratio of sand and sifted clay (Fig.7). Our clay already contained a lot of sand and often the ratio for this ratio is 2 sand to 1 clay. We let this layer dry for 4 days.
After it had dried somewhat we laid fire bricks on top of this dense layer (Fig.8-10).
We drew a circle with a circumference of 27 inches to encompass the fire bricks and constructed a moist sand dome that was 16 inches tall at the center; sand should be moist enough to pack together and hold form (Fig.11-12). After a uniform structure is reached the form is covered in damp newspaper to enable a clear division with the first layer of cob when digging out the inner sand dome to leave a hollow space (Fig.13).
Thermal Mass Layer
For this layer we mixed batches of river sand and clay at a 2:1 ratio and water until each particle of sand was thoroughly covered with clay (Fig.14-16). We knew the mix was ready after performing a "drop test" several times. For this test we took a fist full of the material and molded it into a packed ball, held it up at shoulder height and dropped it to the ground below. If it went flat and "pancaked" we knew there was too much water and so continued to mix, adding more sand sparingly; we were looking for the ball to hold together, smooshing out only halfway or so. Once we reached the consistency we were looking for we made balls of all the material and started building our first layer.
This layer should be a uniform 4 inches thick. Starting at the bottom and building around the base, pushing down instead of in, we built up, completely encompassing the sand mound inside (Fig.17-18). After we finished we took a two-by-four and rocked it back and fourth all over the mound to ensure cohesion and uniformity, using the same "tamping" technique to compact the sand dome; we then pockmarked the entire mound to provide a rough surface for the next layer to stick to.
We let this layer dry for a couple of days.
Aka "woolly mammoth layer." This layer consists of straw, clay and water. We made a soupy clay slip by de-solving sifted clay in water and dunked fistfuls of straw in (Fig.19). Making sure the straw was thoroughly coated we then lightly packed it on top of the previous thermal mass layer, again starting at the base, working around and up, and building a 2 inch thick layer. With this layer it is important that the straw not get too compressed; it is its ability to hold structure and air that allows it to perform as the insulate layer. After the layer firmed a bit we cut a hole for the door and excavated the sand from the inside (Fig. 20).
After completion we allowed it to dry for about a week (Fig 21).
To seal the oven from Northern California's damp and boggy environmental conditions, we used a lime plaster for a final oven layer. We used a recipe from a natural plaster Appropedia page as an outline [] and then made the following substitutions:
12 cups mortar clay + 12 cups lime-putty + 20 cups fine sifted sand + 12 cups cat tails + 2 cups wheat-paste = 48 cups plaster / 16 = 3gal plaster total.
To give the plaster color we added a bit of goat blood (Fig.22). Unfortunately the blood didn't alter the color that much, but it did make for an educational experience(Fig.23). Lime is caustic so we applied the plaster by hand using safety gloves. Once the plaster was covering the oven we smoothed out all bumps and holes by lightly dragging a smooth piece of plastic over it. Yogurt lids work great because they are flexible and soft. We let the plaster sit over night and returned in the morning to find our cob oven speckled with red spots(Fig.24). Apparently tiny plasma pockets had exploded overnight resulting in a "chicken pock" look. We found this look unattractive(Fig.25). Our devised solution was to cover the oven in a lime wash tinted with various oxides.
The purpose of the lime wash was not to add a total covering to a structure, but to include a thin layer of protection and color that will help to enhance the underlying appearance of the structure. After the lime plaster set for 48 hours, we went over it with a lime wash. Our lime wash consisted of slacked lime, yellow oxide, black copper oxide, and water(Fig.26-28). The resulting consistency was that of whole milk. We applied it to the oven with potters sponges. After the lime wash set for about an hour we used our yogurts lids to smooth out the texture (Fig.29). As our oven dried we cleaned up the site and removed and lime putty that fell one the foundation (Fig.30). Over the next week we will re-visit the oven to stoke small fires to help with the drying process (Fig.31).
Here is a short clip of us working with our lime wash
The first thing that needs to be done is to let the oven dry out more and then return to actually cook the first pizza or bread within. This will allow us to ascertain its heat retention abilities and overall baking effectiveness as well as its idiosyncrasies and time that it takes to heat up. Directions for operation must be completed and posted to ensure ease of use. We also need to return at some point and put another layer of mortar around the two layers of bricks in the base; our application was sub par in places.
We envision this to become part of a larger outdoor kitchen that contains a cutting board and counter top and feel that this addition will make it more inviting to cook in. The roof covering the oven is not quite big enough and needs to have, at the very least, a gutter attached to provide 4 more inches of overhang.
Maintenance for the oven consists of building a heating fire inside the dome about once a month to ensure that it remains dry and well seasoned; this is especially important if the oven is not being cooked in on a regular basis.
Overall we are very pleased with the final product. However, there are some important lessons that we learned through our process. The biggest being that we continuously underestimated how long things would take. From gathering the urbanite to grating the clay to building the base and getting the first layer complete we always thought it would go faster than it did. Part of this was from lack of clear communication between group members and the tasks that needed attention while the other component was a miscalculation in amounts of materials needed. Always be prepared to need more than you think.
Another lesson was to do multiple test bricks or other consistency tests. We ended up gathering materials from hither and yon making the test bricks that we originally made a mute point. Luckily after we constructed the base insulate layer and saw it cracking as it slowly dried we realized that we needed to add more sand to clay and so upped our ratio from 1.5:1 sand to clay to 2:1 for the oven layer.
But still we had problems with this mixture when constructing the first insulative layer of the actual oven. We made the mixture, unbeknownst to us, too wet and as a result the whole thing began to slump at a surprising rate. Trying to get it to stay in place by pushing up the material only sped up the slumping process so we decided to call it quits for the day and give it a rest, coming back the next morning to try again. We ended up taking down this layer altogether and remixing it with more sand and clay. In this process we refined our "drop test", realizing that it could not pancake nearly as much as we had originally anticipated.
Overall, we learned that this project is one all of us want to recreate over and over again. We have all learned so much and want to continue sharing what we have experienced.
Megan Colosimo []
Jodi Hart []
Thanks to CCAT for countless buckets of clay, Mitra Abidi & Myles Danforth for inspiration and guidance, Eric Recchia & Aaron Gallo for lending a hand (and foot). Special thanks to Lonny Grafman & Sean Armstrong for invaluable advice, patience, and for the opportunity to partake in such a powerful project.
- Bee, B. (1997) The cob builders handbook: you can sculpt your own home. Groundworks, Murphy, Oregon. Cite error: Invalid
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