HOW TO BUILD A COB (ADOBE) OVEN
Using broken concrete (urbanite) and Portland premix, we created a cylindrical oven base and then filled the base with debris like more broken concrete and any soil we removed from the area.
We hydrated Portland premix (Portland and sand in a dry ready to use mix) with water to mortar the broken concrete pieces into place. Wine bottles were added for decoration into the base of the oven. After peeling the pine poles using a draw knife, we dug into the ground 18 inches and sunk the poles down using a post hole digger and then tamp packed small rocks and earth around the poles to get all the air pockets out. These poles were sunk in order to form a foundation for the backs of the straw bale benches and to hold a roof over the cob oven to protect it from the elements.
We formed the base of the straw bale benches using broken concrete to form a separation between the bales and the ground. Once the bales were set in place, we sharpened some Yaupon stakes and pounded them through the bales and into the ground holding them from moving around. Using more Yaupon, we screwed the long branches to the pine posts creating a frame for a back rest and then added vertical branches pounded down into the bales.
As we built up the interior of the oven base with earth and bricks, we gauged the depth with an inverted beer bottle. Once we had a good, comfortable height then we filled the cylinder with as many upside down empty beer bottles as we could fit. This method gives us a great insulation barrier under the fire brick. Then, we filled around the bottles with sand until full. The theory here is using the bottles to create a dead air space will prevent the heat inside the oven from leeching down into the base because it is mass and will draw the heat downwards and away from the oven.
We took clay and sifted it through a screen to get out big pieces, glass or any other debris in order to make cob. Cob: The materials used are called a select mix, and it is soil and clay mixture in natural ratios that make great cob! It comes straight from the ground like this. We added water to the clay on a tarp and stomped it in. Then, we took the ends of the tarp and “taco’d” (this is the name given to folding the mix over on itself) the clay mixture and stomped it some more. Then, we added straw from straw bales and stomped some more until we got a good sticky consistency. After the cob was ready, we formed cob sweet potatoes or “loaves” which is what the word cob means, and formed a cob line of people to pass the cob loaves and put them on the bricks of the oven.
You have to be quick and know where the next cob loaf is going or you could end up getting whacked with a flying loaf! David likes to place the cob with a nice slam to the top of the base which makes sure that the cob is forced down into any and all crevices ensuring good "tooth". We formed the top of the base and the bottom of the fire brick seat. After using all the cob we created, David realized the depth + the height of the cob was now more than the length of a bottle... no worries!! We used a bit more broken concrete pieces and soil to build it up to the proper height. We then laid out the fire brick to size. Five bricks across gave us a 20" oven which is a very nice size. David mixed the high temperature mortar after setting the fire brick into place. Then, the fire brick was mortared and ready for a sand mold.
Sand Mold:
After the fire brick is mortared in place, a sand mold is formed using only sand and water, much like a sand castle at the beach. Once the mold is formed, then ripped pieces of wet newspaper are used to create a separation layer, much like paper mache.
Fire Cob Layer:
The first layer over the sand mold is the fire cob layer. This is made with sand and clay with water only. David had some pure clay that he brought specifically for this layer. The ratio is about 70% sand and 30% clay. This creates a refractory layer that will get harder and harder with each firing of the oven, much like firing a piece of pottery in a kiln. This layer is about 2 inches thick.
Insulation Layer:
The insulation layer is formed next. It is a mixture of Perlite (expanded volcanic sand) and clay slip. We first mix a batch of clay slip (clay and water) to a consistency much like thick chocolate milk and then add the Perlite. We test it by packing the mix into a ball by hand and squeezing it. If it shatters, the mix is good. If it doesn't, we rework the mix until it does. The insulation properties of this mix allow for a thinner layer than one might expect. 1” of this insulation is acceptable.
So, the fire brick is mortared; the sand mold is formed; the fire cob layer is in place; the insulation layer is in place. At this point structural cob is packed around the exterior which forms the finished shape of the oven.
David found a galvanized rain gutter at the Houston Re-Use Warehouse and thought it would make a cool chimney for the oven. We added a “sloppy” batch of cob to the oven to make a rough earthen plaster and give the oven a more uniform shape. It was applied by smoothing it over the shape by hand and filling divots and misshaped areas of the oven. After the rough earthen plaster was put on, we cut brick ends using a grinder with a ceramic cutting blade to create an arched front to the oven entry. The next step is to put a lime finish plaster and do a brick cleanup.
Robert cut the front prep shelf for the oven out of a piece of composite scrap. We propped up the prep shelf with a Yaupon stick, and then David and Robert cleaned out all the bricks and sand from the inside of the cob oven. Then, David started a small fire inside the oven with some newspaper and wood shavings.
During the course of building the cob oven, we hand mixed clay, water and straw for the straw bales. Then, because hand mixing is so time consuming, we decided to use a concrete mixer to do the mixing and save time. We added clay and water to the mixer for the plastering of the bales. During this time, David added Yaupon wattles to the pine poles for a back rest for the straw bale benches. He screwed them in and now, the back rest is ready to receive cob as well.
The penetration layer had been added to the bales. This is the process of forcing the plaster up into the bales to give it a great hold on the individual straws (we forced the clay down into the bales by throwing it into the bales and smearing it by hand). After this mixture dries, it’s on to the thick cob for the seat surface. We made that cob by adding water to clay and stomping it in just as we had for the cob for the oven. Then, we added straw and stomped that in and turned the cob over and over in a tarp just as we did for the oven. We didn’t get around to cobbing the back rests because the weather did not cooperate. This oven is a work in progress, and we still have to finish the benches and make a roof to cover the cob oven.
Due to the size of this project, so far, we have spent three days and several hours building out the cob oven and the straw bale benches. I foresee another weekend or two to finish the job. Then, we can celebrate and make pizzas in the oven!