Building the Cob and Tadelakt Tandoor Oven

As many will know, tandoor ovens are traditional clay ovens that have been used for millennia in much of the world. They are cylindrical in shape, often tapering and/or curving inward at the top to promote maximal heating. Food is cooked on metal skewers which dangle over the open wood fire built at the bottom of the cylinder. A hole at the bottom acts as an air intake, allowing the tandoor to behave like a giant chimney. The heat of the tandoor is absorbed and radiated by the clay,  creating an even cooking heat and searing in juices while the hot metal skewers cook food from the inside out. Juices and fats drip on to the fire to create a uniquely delicious smokey taste.

After building our cob pizza oven and getting so much use out of it, we wanted to try using cob to build a tandoor. However, after much internet searching we were unable to find any information on how to do this. As such, we have pioneered our own adaption of one and it has worked out wonderfully. Here is how we built it. Hopefully some of you are inspired to build a cob tandoor of your own.

Or maybe you already have one and will be inspired to share your own experiences.

To begin with we needed a plan. We sketched out many different designs and ultimately settled on the one shown below. Our tandoor would be conical in shape, tapering and curving in at the top. It would have a rock foundation and a dense fire brick bottom insulated from below with wood ash and glass bottles. In BC’s lower mainland, dense fire bricks can be bought from Clayburn Industries in Abbotsford. Well worth the expense! The walls would be built in 2 layers using two different mixes of cob. The dimensions were chosen for aesthetic reasons and based on what we thought would make for a comfortable working size and shape.  You can make your oven any number of ways. This is just the system that we came up with for ours.

 

 

A site for the oven was chosen. Ours is right next to our pizza oven in the back yard so we can have combined tandoor and pizza nights. A trench was dug, roughly 8 inches wide and 12 inches deep in a round of ~3’ in outer diameter. It was then packed with chips and dust and tamped down. This foundation will hopefully slow the rate at which our oven sinks. You will need your trench to be below the frost line in your area.

 

 

Large rocks were arranged in a circle to form the base. They just sit on the tamped trench and were arranged to create a tight fit that looked nice. Gaps between the rocks can be filled with smaller rocks if needed. The inner diameter of the rock circle is probably about 30”. The inside of the circle was filled with a few inches of wood ash from the pizza oven and some beer bottles, all of which was then covered in a layer of dry sand.

 

 

 

DENSE fire bricks were arranged and leveled on top of the sand to form the bottom of the oven. These were tamped down to level in all horizontal planes with the handle of a hammer to create a smooth, edge-free oven bottom. This is where the fire will be built. To maximize the bottom area of the oven within the constraints of the rock circle we had to cut some of the bricks. We did this using an angle grinder fitted with a hard stone cutting wheel. We decided to use dense, rather than porous, bricks because we thought they might retain more heat and allow the oven to remain hotter longer once the fire died out.

 

 

We wanted to use a sand form around which to build the tandoor, much like how a cob pizza oven is built. However, we were unable to create one in the shape and size that we wanted because it kept collapsing. Presumably this is possible, but we didn’t know how to do it. To get around this we made a card-stock form that would contain the sand in the shape we wanted.

We used an online cone calculator to work out the flat shape of the inner straight walled part of the cone of the oven. Our cone had a base 24” in diameter, a top of 20” in diameter, and a height of 20”. Using the online pattern we fashioned a cone out of heavy card stock from the art shop and centered it on the brick base.

 

 

We initially tried to fill the cone with damp packed sand but the form tragically exploded, tearing the cardstock to shreds. Turns out the card stock was not strong enough to withstand the pressure and moisture of all the sand.

We made a second cone and this time only filled it about 6” full of moist packed sand before we started cobbing. The inner layer of cob is fire cob. Our mix consisted of 1 part clay rich soil (Burkeville Ready-Mix left over from building the pizza oven) to 4 parts coarse sand. This was combined with water by stomping and rolling it on a tarp to make a coarse, crumbly mix that could be packed with force in to strong balls. Before packing it just looks like damp sand on the tarp. This mix has no straw or animal hair in it. It is in direct contact with the fire and has nothing flammable in it. We then began cobbing, working around the base of the oven to the height of the sand before adding another 6” or so of packed sand and then continuing to work up with the cob. As with the pizza oven, this cob was very gritty and required a lot of force of get a good pack. In this way we worked our way to the top of the cone form.

 

 

When our cob was about 6” from the top of the cone, we filled the cone to the top with packed sand and then continued to build the sand form up over the top of the cone to create a rounded dome with a radius of roughly 10”.

 

 

 

The sand dome was covered in wet newspaper to prevent the cob from sticking to it and the cob was built up and over to completely close over the top of the cone.

 

 

Our cob was a little too wet and because of this we had problems with it slumping down the side of the oven. Once the top was completely cobbed over it looked like a saggy blob but we shaved off the sides with an old wood saw to get a clean even shape that we were happy with.

 

 

We wrapped our oven up in a tarp and left it to sit overnight. The next day the cob had locked up somewhat so we cut the openings. To get a level, centered top we placed a 5 gallon bucket over top of the oven and used a bubble level to set it perfectly. We used an old kitchen knife to trace around the edge of the bucket, marking out the line and then sliced off the top of the dome with horizontal cuts, following this line. We also cut the lower hole which acts as a fire feed and air intake. To do this we used a flower pot to trace a perfect circle at the base of the oven where we wanted our opening and again used the knife to cut the hole. Cutting cob destroys knives! Use one that no one in the house will miss.

 

We left the oven tarped for another couple of weeks before taking out the sand. Once the sand was out we peeled out the card-stock and left it another week or two before lighting a fire in it.

 

 

 

We wanted to try cooking breads on the curved inner top of the oven but found the inner fire cob to be too rough. To overcome this we plastered the inside with a fine fire cob plaster prior to the first firing. We took some pure lumps of clay, ran them through a fine sieve to get out the grit and then added 4-5 parts of sand sifted through fine window screen. This created a silky plaster that we applied with our hands to the inside of the oven after moistening it well with water. It resulted in a smooth, even interior finish. We used polished stones, the lids of plastic yogurt containers, and damp sponges to smooth the clay plaster into an even pearly surface. So far it has worked great though we still need more practice getting the bread off the oven without dropping it in to the fire!

PRIOR TO FIRING, BE SURE TO READ ABOUT CURING TANDOORS!
Having never used a tandoor prior to building this one, we were not aware of the process of curing the oven before the first firing. We have since read extensively on this process and intend to attempt curing our next tandoor after smoothing the inside and before first firing it. Presumably this will allow us to manage cooking the breads better.

When we first lit a fire we were astonished by how quickly and efficiently it heated up. We half expected it to collapse but it didn’t even crack. We used dry wood bucked in to fairly small pieces (4-5” in diameter, 12” long) to make the fire. It was cotton wood season when we first lit the oven and we could see the fluff in the air getting helplessly sucked in to the air intake. We couldn’t hold our hands even a foot over the opening without getting burned. The poor plum tree overhead had to change its branch trajectory after getting repeatedly scorched. Our tandoor worked amazingly well!

We started cooking on our tandoor as soon as we first fired it however shortly thereafter we decided to add a second layer of cob to try to improve its heat insulation. This cob was made using 1 part Burkeville-Ready-Mix, 1 part sand, and copious amounts of straw and wood shavings to make an airy mix of mostly straw glued together with just enough clay to stick it all together. The fire cob core was wetted with the hose and the insulation layer was applied. We left it to set for a couple of weeks, wrapped in a tarp before firing the oven again. Letting cob set slowly apparently helps prevents cracking and creates a stronger end result. We have since realized that this layer doesn’t insulate so much as add thermal mass to the oven that allows it to adsorb more heat and hence cool more slowly.

 

 

 

As a final touch to our oven we plastered it to both decorate it and help protect it from weathering in the rain. We first applied a coarse scratch coat which we made by combining 1 part NHL2 lime with 4 parts coarse sand. To this we added copious amounts of straw that we chopped in to short segments by putting it in a garbage can and then going at it with a weed whacker. The oven was dampened with water and the scratch coat was applied in an even but coarse layer. Wear GLOVES when working with lime!

 

 

The finishing coat was made from 1 part NHL2 lime and 2 parts sharp masonry sand which we sifted through fine window screen. To this we added a liberal amount of horse fur that had been previously washed, dried, and carded. We noticed that our trials of plaster that had horse hair in the mix cracked significantly less than equivalent plasters without horse hair. The hair did nothing to alter the final finish of the plaster, it completely disappeared from view. We tinted this layer with pigment which we purchased on line from EarthPigments.com. To get good coverage of our tandoor we used 1kg of tint for the darker areas, and roughly 500g of tint for the yellow band. Our colours are Envirinox Dark Brown, and Curry Yellow. We finished our plaster layer with a tadelakt technique which we describe fully in our fireplace blog.

 

 

 

 

We left the plaster for several weeks without firing it in an effort to prevent the plaster cracking with heating however even with this it still developed a fine crack the runs the height of the oven. It doesn’t seem to interfere with it’s function though so we have just left it.

Our tandoor oven has seen lots of use since it was first built in April 2013. We have even continued cooking on it through the cold winter months as it heats up quickly, cooks food fast, and provides a nice warmth to gather around while dinner is cooking. The tadelakt does a good job of tolerating the wet winter months and we riveted together an aluminum cone to keep the inside rain free. After the winter months the tadelakt looks a little tired but come spring we wash it with soap suds and it livens right back up again. We are very happy with how it has turned out and continue to have tonnes of fun cooking over it. Hopefully some of you are inspired to build one of your own.