Cob Oven Construction
The Creation of a Pizza Monster!
By Steve Nicolini
A cob oven is becoming a key element on any property whose
residents are practicing permaculture.
The reason for this is that earthen ovens perform many functions, including:
- Give reason to throw a pizza party (a.k.a. build
community)
- Provide inspiration for further natural building
projects
Building the oven itself is quite easy. The difficult part is deciding what to use
for the foundation. As we all know,
foundations are
essential for any building.
We had just completed our root cellar and had a bunch of extra small to medium sized
stones lying in a pile. We also had
extra chicken wire from the earthbag cellar project.
I know that a light bulb just turned on in your head. Put the stones in the chicken wire and tamp
them down! That is indeed what we
did. We were able to make a three and a half foot
diameter cylinder of stones that was about four feet tall. There was still a little more than a wine
bottle’s length of chicken wire extending beyond the stones.
The next step of our cob oven construction was to make an
insulated base. What we did was lay down
about a three inch thick layer of straw, then mix up some slip straw type stuff
with perlite. After that, we started
inserting empty wine bottles upside down and backfilling between them with
perlite. When the circle was packed full
of about 80 wine bottles, we put another thin slip layer on top and leveled
it.
Fire bricks were placed atop that last layer in a way that
maximized oven floor space. We also made
sure all the bricks were on the
same
vertical plane (easier to slide the big pizza spatula in and out!).
The “oven” part of a cob oven is just a void. The strongest voids on this earth are
domes. The interior of our cob oven is a
dome. We used sand to make a form for
the dome. A string tied to a pencil made
a great compass for drawing a perfect circle from the exact center of the brick
floor. The height of our form was about
70% of the diameter of the form.
Strips of wetted newspaper were placed around the form as a
separation layer between the form and the actual oven. This step was followed by the mixing and
application of a
thermal layer
(which is a mix of coarse sand, sifted 5/8” gravel, and clay). This layer was about 3 inches thick and
applied directly around the form. The
second layer has a lot more coarse sand than gravel and also has chopped straw
in the mix. This is the closest to
regular cob that we got. This second
layer was added in “cobs,” which look like loaves of mud.
The plastering step is when the pizza monster was
created. We plastered the cob oven this
summer.
To make an earthen plaster one needs a few
different materials and tools:
- Clay
(we have a clay bank on site)
- Sand
(fine beach sand is good for plastering)
- Finely
chopped straw (weed whacked loose straw in a 55 gallon drum for about 10
minutes)
- Ungulate
manure (optional)
- Mixing
tools (hands, hoes, shovels)
- Container
to hold plaster (bucket, wheelbarrow, etc.)
- Throwing
device (trowels, hands)
The process
is just like making cob, except you need to sift the straw through the 1/4 inch
sifter. The clay and sand should be
sifted through the 1/8 inch screen. Once
these steps are done, add water to the sand, clay, and straw and mix it up best
you can. You know it is ready when you throw a wad of plaster at the oven and
it sticks. That method is called harling.
We used
trowels to get as smooth a finish as possible to the cob oven. With the extra plaster, Isabel (a student)
began to sculpt some eyes; then some artistic rays beaming out from them. It sort
of looked like an Owl. It kind of looked like Jaba the Hut. It somewhat
resembled a nightmarish koi. It
looked exactly like a PIZZA MONSTER!
Our poor
pizza monster had her mouth filled with sand for a whole week while the oven
set. We helped her out on that seventh
day by removing the sand and starting a small fire inside her mouth. She breathed fire like a dragon while the
newspaper burned off the thermal layer. She
is now ready to start pumping out the pizzas.
