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!