Jen & Ryan have been intrigued by cob construction for years. An oven would be the perfect introductory project!
Cob building is a project for warm summer days and evenings. August 2012 had perfect weather for cob construction.
As many will know cob is a material made from clay, sand and straw... all of which we had on hand. The proportions of each vary to best suit the structure being built. For the oven various mixtures were tested to get the best results. The test pucks now act as decorations and paper weights throughout our house!
If you follow these instructions step by step at the end you too will have your own Earth Oven.
What will you need?
- yard space
- good weather
- clay type earth
- sharp edged sand
- straw, dog hair, sawdust
- assorted empty bottles
- water
- weed whacker
- card board cylinder forms
- premixed cement
- gravel
- 3/4" ply
- chicken wire
- staple gun
- level
- news paper
- piece of 2x4 or equivalent
- dowl for poking holes
- fire bricks
- insulating bricks
- buckets
- wheel barrow
- shovel
- tarp
- wood scraps, metal scraps
- lime plaster
- tint for the plaster
- rubber gloves
- old clothes
- matches and wood for your fire
- time and energy plus patience!
Step by Step Oven Construction
(1)The tube molds for the concrete foundation. Fill with concrete. Let set and peel off the card board.
(2) Once the oven’s site was decided on, a 3’6” round was marked out and a ring of earth was dug out to below the frost line (~1’) and packed with chips and dust. 11, 8” cement cylinders were poured and arranged to provide the foundation for the oven base.
(3) The cob mix consisted of 1 part clay rich soil which was collected from Bukeville, Richmond (aka Burkeville Readimix) and 1 part sharp sand. Straw and dog fur (we have plenty of that on hand) were added liberally to add grip and tensile strength to the mix.
(4) The cob was mixed on a tarp by stamping. Boots were worn as the Burkeville Readimix was from an urban site and had all varieties of rusty sharps and glass mixed throughout it, even after aggressive sifting. This proved to be a great workout with sore legs the following day!
To step back a bit... the dirt and sand were mixed with straw. The straw had been put in a 30 litre garbage pail and chopped in suitable lengths with a weed whacker. Once this was done, all was mixed on the tarp and water added. The mixture was stomped. The the edges of the tarp were folded inward (kind of like kneading dough) and the mass was stomped again. This process and the addition of water was repeated until a well mixed and workable consistency was achieved.
(5) It rains a lot in Vancouver and the ground is often wet. Having a base acts as a moisture barrier plus it brings the oven to a comfortable working height. Use your imagination and make a base that pleases you. The base of this oven was built using a glass bottle/cob mosaic on top of cement posts. Bases are traditionally solid but we wanted ours to be open to serve as a heated dog house when the oven was fired.... or for those poor souls without a dog it can be used for wood storage:)
Thus so far... the ground is cleared, a circular trench is dug, gravel is put in the trench and tamped down, the cardboard forms are filled with cement creating 11 cement posts which are carefully arranged around the gravel trench with an opening left for access into the base. Once these are placed cob is build up on the cement posts. Bottles of various colours and shapes were laid amongst the cob. This gives a decorative stained glass effect to the project. It was necessary to consume a lot of different beers and beverages to get the required bottles!
(6) The wall grew quite quickly and was checked periodically with a level to ensure it was level in both the horizontal and vertical planes. The gap in the wall will eventually become the arched entranceway to the base but for now it is being left alone so that the wall proper can lock up and provide support to the soon-to-be arch. The walls were built by taking handfulls of cob, forming them into loaves and splatting them down on the previous level. Shaping and sculpting was all done my hand. Yes, you will get very dirty... but it all washes off!
(7) When the base got to the height that felt right a platform was required for the oven itself. A 4’ diameter perfect round was cut from ¾” plywood. Reclaimed chicken wire from the last summer’s fence repair was stapled liberally to the underside of the ply round and around the edge to provide a mechanism for the ply round to lock down onto the cob wall.
(8) Chicken wire was also stapled to the upper edge of the ply round. The arch is slowly taking shape below.
(9) A cob wall was built up around the edge of the ply round to provide a well in the center to enclose the insulating layer below the oven.
(10) Once the cob edge wall was finished a layer of dry sand was spread into the centre well. We have now created a cob "doughnut" with decorative bottle placement and a dry sand layer above the plywood base. The "blue beer" had great bottle colours but no one liked the beer :( This acts as insulation from the oven above.
(11) Empty beer bottles were then placed over the sand layer and were then buried in more dry sand. This provides an insulating layer that the fire bricks sit on and prevents excessive heat loss from the oven through the base.
(12) The centre of the plywood round was carefully marked out and dense fire bricks were arranged on the surface of the sand ... starting from the centre and working outward toward the periphery. Each brick was accurately levelled and pressed tightly against each of its neighbours to provide a perfectly smooth, even surface with no edges on which a pizza peel could get caught while sliding it in and out of the oven. This is the cooking surface of the oven so it must be perfectly level and smooth.
(13) Once the bricks were leveled, 4 porous insulating (non heat retaining) fire bricks were placed at the front to provide an entrance shelf into the oven. The ornament that was sculpted on the rim was not only decorative but provided a support for the front brick shelf. The shelf was locked in on the sides by little blocks of cob.
(14) The largest possible circle was drawn in chalk on to the bricks. This circle represents in the inner area of the oven. The circle is centered to align with the center of the plywood base. The supporting blocks for the shelf are clearly visible in this photo.
(15) A layer of wet newspaper was placed over the bricks and moist sand was poured onto the paper. The paper’s function was to prevent sand from working its way between the bricks and unsettling them. Gradually more sand was added as a semi-sphere was sculpted to the size of the chalk circle. Underneath the arch continues to slowly take shape.
(16) The sand form continued to take shape. For this oven, the diameter of the sand dome, which represents the inner area of the oven, was 82cm, the height of the dome was therefore sculpted to 41cm.
(17) Once the shape of the dome was achieved it was smoothed out using wet hands and by rocking a 2x4 board over its surface. Since this is the inner shape of the oven it was important that its shape and texture be as perfect as possible.
(18) The dome was covered in wet newspaper in preparation for cobbing. The purpose of the paper is to prevent the cob from sticking to the sand ... if that happens the inner dome will have loose sand that will fall on the food and make a mess of things.
(19) The fire cobbing begins. This layer of cob is on the inside of the oven and will be exposed to the fire inside. As such, a different blend of cob was used, known as fire cob. It consists of ~5 parts sharp sand to 1 part Burkeville Readimix without any straw or fur. It was gritty and required a fair amount of force to get a good pack.
(20) The fire cob layer grows.
(21) Once the sand dome was completely covered in fire cob, the cob was worked smooth using a 2x4 board that was rocked over the surface repeatedly to get a nice finish. The fire cob was left to set for a couple of days and then the door was cut out. The height of the door was 63% the height of the inner sand dome. This ratio has been shown to provide the optimal airflow to promote a good burn without allowing excessive heat escape. We built a thick wooden door out of scrap wood that was cut to plug the door opening tightly to trap the oven heat in once it had been fired to temperature. Some will add a chimney to the dome. For our oven these dimensions, along with a fire door, have worked well and a chimney has not been necessary.
The tarp was put up to protect from direct sun, so that the cob dried slowly and evenly. It also protects against Vancouver's rain onslaught.
The arch continues to slowly take shape.
(22) An arch was also built around the doorway with firecob to protect the insulation layer of cob (coming up) from the flames when the oven was fired.
(23) Finally, when the firecob had firmed up sufficiently (~2 weeks) the sand was scraped out to hollow out the interior.
(25) The fire cob is drying.
(26) Dry!
(27) Our first attempt at pizza. The oven got incredibly hot! It broke our oven thermometer that had a max temp of 700F! The first pizza wasn’t pretty but it was awfully delicious.
(28) Our first attempt at bread in the oven. Delicious!
(29) Getting ready to put on the insulation layer. The oven got extremely hot with the fire cob layer but it lost it’s heat quickly once the flame was removed. The insulation layer is designed to help slow the rate of the heat loss. Our mix was composed of 1 part Burkeville Readimix to 1 part sharp sand. We added a liberal amount of straw, fur, and sawdust shavings from the woodshop to create air pockets within the mix. Unlike the firecob layer which was designed to be very dense, this insulation layer was designed to be very porous to trap and keep the heat. Once this was in place the oven would still be warm the morning following a burn.
(30) The insulation layer is complete. Holes were punched into it to provide a key for the plaster layer. Ryan is firing the oven to dry out the insulation layer. He is holding our first pizza peel made from a piece of scrap metal.
(31) Another attempt at pizza. This time the oven held its heat for a good long time. When the flames were removed it kept a temperature of 350F for over 5 hours and was still warm the following morning. The arch is finally nearing completion.
(33) Dogs, and human friends alike, love pizza nights!
(35) A firing door was made out of a piece of scrap steel. The handle was carved out of a piece of lumber left over from Ben’s cat run (that is a blog in it's own right). This door made for a hotter burn and helped direct the smoke upwards, rather than out towards people gathered around the oven. The arch is now complete. If made freeform the arch is best built slowly to prevent sagging... each layer is firm before the next is added.
(36) Tidying up the arch.
(38) Baking bread.
(39) The dome of the oven was plastered in scratch coat and a tinted finishing lime plaster coat was applied to the base. This colour will fade a lot over the next few days.
An Earth Oven is a great gathering point in the yard as well as being very functional. Hopefully some of you will be inspired to build your own.....
This oven was built in part based on the wealth of knowledge available in:
Kiko Denzer's 'Build Your Own Earth Oven' 2007
Adam Weismann's 'Using Natural Finishes' 2010
This oven was built in part based on the wealth of knowledge available in:
Kiko Denzer's 'Build Your Own Earth Oven' 2007
Adam Weismann's 'Using Natural Finishes' 2010
Jen & Ryan have now signed on for a hands on Cob Home building workshop in Oregon! That is to happen in August 2013. Ovens to houses in 12 months...it simply boggles the mind:)
WOW!!
