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Tandoor Oven construction II


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Constructing an oil drum tandoor is no easy task , but Gwen Nathan went about this meticulous task and in his own words below the construction of a Tandoor oven from the drawing board to cooking some really delicious chicken.

This tandoor oven makse use of an Oil drum for construction.

I really love cooking and cooking outdoors over wood or charcoal adds the ingredient missing from the electric cooker in the house. I've worked out that replacing take away meals with food cooked at home - even on an exotic outdoors device - should pay for any wildly extravagant construction costs. And I've got a few weeks off work to engage in some wildly extragavant construction.

We already have a small kettle barbeque. I don't have much confidence in a device if nobody can decide how to spell its name. (Barbeque? Barbecue? BBQ?) I want something more exciting, more authentic, more unusual.

I've done some internet research and the options seem to be:

A wood-fired brick pizza oven in the garden. This is perfect. But after measuring out the dimensions in charcoal on our patio, I've realised that a half-ton of fire bricks makes it difficult to get out of the house and makes for very little light into the kitchen.I need something smaller and more portable for my outdoor culinary adventures.

a smoker. There are some great smoker projects out there, the Big Baby Double-Barreled Barbecue (spelling again?) caught my eye - perfect for that whole hog if you have the welding skills to balance one oil drum on top of another with connecting pipes. I don't have the welding skills, and I don't eat hog. Ruled out.

a tandoor. I love a large helping of tandoori king prawn with a nan. It looks about the right size for the garden and surely you can tandoori-fy thousands of other dishes...?

A thorough search of the internet turns up only two home-made tandoor projects.

Piers Thompson built a tandoor in his garden using bricks, vermiculite insulation, concrete and a commercial clay tandoor liner. Some guy called Bob has also tried it out and raves about how well it cooks. It looks good. I wonder how much a commercial clay tandoor liner costs? Ebay tandoors are about £400-£600 so I guess a clay liner is a fair chunk of that.

Paul Wright has instructions on his website for creating a tandoor out of an oil drum by filling it with a broken glass/concrete mix and then lining with fire cement. 2kg of fire cement costs over £5 at B&Q, making this look like a cheap project that turns expensive. I'm not sure if cement is good to eat. I think they mix it up with the wrong kind of lime (mineral, not fruit)?

Hmmm...looks like time to go back to the drawing board.

As promised, I've been back to the drawing board.

I've done some deep research into fire-cement. Wonderful product names - heat-proof screed, fire cement, fire clay, fire brick, castable refractory and my favourite, mouldable fire brick.

There is a whole wonderful world of refractory materials out there. Most are used for large industrial projects like building smelters, so there aren't many suppliers for small scale installations.

There seem to be two kinds of people who use these in a domestic setting - potters who want to build their own kilns, and people who want to make their own steel items such as swords (hold on - I need a new kitchen knife!)

I've found the tandooriq which looks extremely cool, but is over my budget. I've spoken to Clay Ovens - the suppliers of clay liners used by Piers. They were incredibly helpful and friendly, but I feel a need to carve my own path.

The design in the drawing above is based on a combination of a diy pottery kiln and a tandoor. It should be a bit lighter and efficient than the concrete in an oil drum.

"My arms hurt. My legs hurt. My head hurts. I think I'm going to cry...with relief.

I just rolled an oil drum about a mile through the streets of London. Our local car place returns oil drums back to the depot so I walked up to the next garage who gave me one free.

I wish I'd taken the car.

Ladies and gentleman, boys and girls. Do not try to roll an oil drum more than a few hundred metres. They are heavy."

Build day 1. Objective: Clean the oil drum and cut the top off.

The guy in the garage said it would be easy to remove the top of the oil drum - "just chisel it off".

Made reasonable progress on this, although it would have been easier to hit the cold chisel and not my hand if I had a nice big club hammer. The top came off in one piece leaving a sharp jagged each which I hammered down.

After this I hammered the oil drum back into shape where it had been dented, and then cleaned it out with (kitchen) degreaser to remove the remains of the oil. A small hole in the top of the drum has let some water in creating a rust stain. Far from ideal, but the steel is thick enough to last several years and I'm not going back to get another oil drum.

Ahh...nice clean oil drum

I also created a wire former to make the shape of the clay on. In the photo you can see the cuts used to let the top taper in to take a pot shape. Not sure yet if this is going to be the best approach to moulding the clay.

Tools used today

protective goggles

ear defenders

protective gloves

washing-up/latex gloves

1 steel oil drum (capacity 208 litres)

25mm cold chisel

1lb hammer

2 pints degreaser and/or detergent

Galvanised chicken wire (13mm weave)

Tin snips

Thin galvenised wire

Flat-nosed pliers

Building Supplies

After finding inspiration in their website, and a quick chat on the phone, we went to Bath Potter's Supplies (BPS) today to get the clay and insulation. BPS were incredibly helpful and supportive in working through the options with me.

After discussing the requirements - shape and size of the pot, that it would be heated repeatedly to about 300-400°C and left outdoors in frosty conditions, BPS recommended Potclay's original raku clay.

Original Raku (1154)

Superb thermal shock resistance. Low shrinkage. Designed for Raku process but also suitable for slabbing etc., and as a stoneware body.

Buff to off-white body. Especially good for slabbing and handbuilding. Suitable for for large constructions and tiles needing good warp resistance.

Firing range 900-1300°C

I also bought 5m of 13mm*600mm superwool 607Max insulation. Superwool 607Max is rated for continuous use at 1260°C and is "body soluble" - the human body can break down any fibres that get into the lungs.

It remains to be seen if this is enough insulation. At our working temperatures, the insulation should be worth about 0.07 w/m•k, but I'm not finding it easy to work out what that means in charcoal burning terms.

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Build day 2. Objective: Cut the air vent, attach castors, fill base with concrete.

Right. Wasted loads of time this morning cutting the hole for the airvent with a junior hacksaw blade. I tried filing down the rough edges and that was going too slow as well.

I am now the proud owner of an angle grinder. Suprisingly cheap from the local DIY superstore and its made very quick work of finishing the air vent. Final dimensions are about 10cm x 20cm which should be big enough to sweep the ashes and to let me put my arm in for any interior construction work.

The castors were pretty straightforward to attach using some large locking nuts and bolts. Be careful to ensure the castors can rotate all the way round and that the castor brakes don't foul the edge of the oil drum.

My Dad popped round with 25kg each of dry mortar and aggregate and we mixed it up with some water to fill the base. The mixture came to about 3-4cm short of the bottom of the air vent to allow for the insulation and clay. My Dad's top tip is to add a dash of detergent to the water - he thinks it helps the concrete take the form of the barrel.

Tools used today

protective goggles

ear defenders

protective gloves

washing-up/latex gloves

Drill + variety of HSS bits

Junior hacksaw + lots of blades

Angle grinder + 115mm grinding plate

1 half-round File

Emery sponge

3 x 80mm swivel nylon casters with brake

12 bolts & stainless steel nylon lock nuts (we used M8)

set of spanners and sockets

25kg General Purpose Mortar

25kg 10mm Gravel

water

A dash of detergent

trowel and/or shovel

sheet of wood + newspaper for mixing (or polythene sheet)

scrap wood (for tamping)

Build day 3. Objective: Fit door to air vent.

A short build day today as I spent the morning spent messing around with routers, ADSL and wireless lans. We do now have every computer in the house connected up, and the laptop is wireless enabled for garden internet surfing. Woo!

The concrete didn't look quite set this morning - we mixed it as dry as possible to ensure there was plenty of grip on the nuts holding the castors on. We added another pint or two of water to make sure it sets properly.

I cut the door for the air vent from the lid of the oil drum. Its a little larger than the air vent hole to allow for a flusher fit. Smoothed it off with the angle grinder and file and then fitted with the hinge. The hinge isn't designed for this kind of joint, so I've mounted the door on the outside of the hinge.

I've got a great book called Tandoor - The Great Indian Barbecue. The technique shown in the book for building a tandoor is the same as suggested by the pottery shop - slab rolling. So I think tomorrow I will have a go at a smaller scale effort as shown here

Day 4. Objective: Lay insulation. Start building the clay pot.

Not much progress this morning.

Once things dried off I put the insulation in the drum. There is a double layer in the bottom and a single layer on the sides. Hopefully the 13mm of insulation will keep enough of the heat in - I won't know until it's fired up for the first time. The insulation cuts very easily with a knife or scissors. I used a ventilation mask and latex gloves to do this.

There is a small tear in the top - the insulation is more fragile than I expected.

The weather dried up long enough for me to get the base in.

I've been practicing slab-rolling but as you can see the bottom isn't smooth. I'll post some pictures of the slab-rolling process tomorrow.

The layer of clay is about 2cm thick, and has used nearly a whole bag of clay (12.5kg). I'm going to have to use thinner layers for the walls as otherwise I'm going to run out before I get to the top. (I've done a little spreadsheet to calculate the volume of the base and the walls and divided that by how much clay is in each bag).

I've tried to smooth the base using slip, a mixture of clay and water with the texture of yoghurt. It's messy to make.

The slip has smoothed the base out but I think its gone a bit too watery. I'll check again in the morning.

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Day 5. Objective: First level tandoor wall.

I got a lovely email from Paul Wright last night. He's very interested to hear about my cooking experiences with this tandoor. He ominously says "be careful the heat generated inside the tandoor is amazing". Must remember to get some nice long welding gloves.

The base didn't fully dry overnight, so I've wiped up the watery mixture with a rag. There is enough firm clay in the base that I'm happy to proceed.

My slab rolling technique is working much better today. I bought some 12mm x 12mm wood to use as a spacer when rolling. I'm rolling out on a sheet of canvas which stops the clay sticking, and using a normal household rolling pin. I'm then cutting them to size. More photos on this tomorrow.

The slabs are about 20cm wide and of varying length. The best ones are about 30 to 35cm long at the moment. My wife has done a great job of cutting all these nice and straight.

Part of the trick with these slabs seems to be to let them dry for an hour after they've been cut out. They seem a lot easier to handle then.

Lightly score the edges and add some slip to the sides and bottom...

... and jiggle them into position inside the tandoor which has also been lightly scored.

More clay slabs and some soggy clay forced into any remaining gaps and it looks like this.

After I left it to dry for about an hour ("leather dry") I cut the hole for the air vent. Then I reinforced the hole with a piece of cardboard and stuffed clay around the cardboard to create a reinforced arch.

Looks pretty solid from the inside. I've created a little ramp so that the base leads smoothly up to the edge of the hole. This should make it easier to sweep ashes out.

I've also glued some small magnets onto the door to act as a catch. Hopefully the araldite will be able to take the heat.

I've just had another look in Ranjit Rai's tandoor book. He describes the slab build technique for tandoors and says

"Slabs about 12-15cm wide and 50-60cm long and 2-3cm thick are made.

I've only got enough clay to build the tandoor 1cm thick. I hope the insulation and oil drum walls will provide the strength and thermal properties to get away with this. There's no way I'm going back to Bath and buying another 120kg of clay.

Time for a rest. There's lots of grovelling on the floor to roll out the clay and then bending deep inside the tandoor. I should add that I rolled all the clay this morning and realised it was too thick so I had to do it all again. Oh well, tomorrow should be easier

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Day 6. Objective: Second level wall.

As promised some step-by-step pictures of making slabs.

First, knead the clay together so it is all mixed together and there are no air bubbles. You can see I'm working on a piece of canvas to stop the clay sticking to the ground.

Then smooth out the clay by hand into roughly the right shape. In the photo you can see the two wooden batons used to get the clay a consistent thickness.

Folding the canvas over the top seems to stop the clay sticking to the rolling pin.

Roll the clay out flat with the rolling pin, using the wooden batons to get a consistent thickness.

Mmm...flat clay.

Mark up the dimensions

A rectangular slab of clay. It will stay on the canvas for about an hour to firm up before going in the tandoor.

The results of today's work. The air vent arch is finished off and the cardboard removed.

A second level of the wall in the tandoor. My wife tells me the first level of clay shrunk in the night as it dried out, so there is a chance the two levels will match up a little better by tomorrow morning.

Today's quandry is building the next levels. They need to come towards the centre so that the opening at the top is about half the diameter of the base. We've given up on the wire mesh idea as it will be supporting from the wrong direction and I'm worried that if it gets very hot it might cause the tandoor to collapse.

Maybe we can build a cardboard template and form the clay on that. And once the clay is dry it will be strong enough to support iself. Or maybe it will all collapse in...?

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Day 7 + 8. Objective: Levels 3 and 4 of the wall.

Yesterday's task was to create the third level of the wall, starting the inwards contour of the pot.

Suprisingly, this all seemed fairly easy - particularly because my wife provided a very able pair of hands holding the clay slabs in place as I worked my way round.

I used the handle of the knife as a measuring stick to ensure I was getting a consistent distance from the edge of the oil drum to the clay. I then cut each piece to fit as each slab needed to be wider at the bottom than at the top. I needed help to hold the pieces in place as I trimmed them, but once each slab had another piece of clay on each side they all seemed pretty stable.

After drying the third level overnight, the fourth level also went in without too many troubles using the same approach (and helpful wifely assistance).

Cutting each piece to shape seems to provide a lot of strength to the pot. It feels quite stable and strong.

After the photos I worked my way round again to fill the gaps and make the clay nice and smooth between each slab. I need to decide how much more time to spend smoothing off the clay before we start firing.

To help get the angles right, my wife suggested we create a template with a cutout the same as we want the inner diameter of the top of the pot. The template also helps make sure the pot is being built up dead centre in the oil drum.

So I'm really happy. Not far to go - we are about 5-7cm from where the top needs to be and the pot feels relatively strong and sound, and is looking like the right shape! The bottom layers are feeling quite dry now. I think it will take 2-3 days to fully dry out, depending on the weather.

We have been looking at suppliers of wood or charcoal as it is going to take quite a bit to fire the pot. Some of the local supermarket supplies of charcoal have been of low quality - lots of sparks and all the coals burn out very quickly. Our local coal merchants might be able to do charcoal...

We also haven't had much luck with skewers. I might try popping to Southall or Wembley over the next few days.

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Day 9. Objective: Finish clay construction.

We got to the top!

In this photo you can see me using the cardboard template to start shaping the rim. The final slabs of clay were 10cm wide and protruded by about 5cm giving nearly enough clay to form a rim.

And the finished pot!

I've done a little diagram below to show roughly what shape a rim should be. I didn't quite get this to begin with, so I had to do a lot of moulding backwards and forwards until my wife bought a sense of perspective. The rim is just a reinforcement of the wall to give it some extra strength, as shown by the grey shaded piece. The rim I constructed added a flat piece on top, but the important element is the shaded section which sits on top of the clay walls.

On closer inspection, the tandoor book shows pictures of the rim being built exactly like the diagram shows.

I'm very pleased with the quality of the work, even if the inside of the pot won't be winning any construction awards. I've still got the last piece of pottery I did before this - a hideous, rubbish, mishapen "cup" I made when I was about 13. This is so much better.

The rim of the pot sat about 1cm above the top of the oil drum when I had finished. Overnight it seems to have settled to the same height. I've also added two coach bolts to hang the tandoor skewers on while the food is resting.

To celebrate this success, I went with my wife and parents to our local curry restaurant. We all ordered tandoori food and probably spent as much time looking at the size of the skewer holes and the texture of the marinade as eating. The restaurant staff were absolutely amazed that I was building a tandoor oven, and gave me some useful contacts for catering equipment and a free tandoor skewer! They also invited me back tomorrow (or next week) for a marinade and tandoor lesson and they seem genuinely excited by this project!

The next big stage is firing. The pot will have been air drying for about 36 hours since we last did any work on it. We need to "season" the inside surface with jaggery^ and then gently build up the heat over several hours. The jaggery is solidified sugar cane sap and tastes like a cross between fudge and molasses. I hope I've got enough because I keep nibbling at it.

I've got two big bags of charcoal but no thermometer capable of going up to anything like 1000°C. Charcoal was original used before the invention of coke to smelt iron from ore, which needs 1530°C, so getting the temperatures we need is theoretically possible. I'm planning to use a chart of glowing metal colours which suggests that the firing temperature will have the (inside of the) pot glowing between dark orange and white hot.

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Day 10 part 1. Objective: Line tandoor with jaggery.

Started work today by cutting up the jaggery. That is a 1kg block you can see in the picture. The texture is a bit like fudge - it cuts up pretty easily.

The jaggery has a lot of water in it so it melts easily.

I smeared the jaggery on the inside of the tandoor using a wooden spoon and a spatula. It was very diificult to get a smooth surface as the jaggery became very sticky when the clay cooled it down. I was able to fix this later once the fire was going

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Day 10, part 2. Objective: Room temperature to 300C

Hour zero. This is the start of the charcoal fire. I read up about firing clay, and the first crucial stage is around 80°C-100°C to dry out the clay. I decided we would need to stay at that temperature for several hours.

This is the fire three hours later. I've built it up a little. The internal temperature was still about 100°c at this stage. Over the first four or five hours we had a lot of problems with rain. There was a lot of standing round outside in the rain hoping the umbrella didn't melt.

At six hours we started to build the fire up. You can see that there were a few hairline cracks forming in the clay. The base was really quite cracked up, but the cracks filled with ashes.

The higher temperature at six hours makes a big difference to the jaggery. It has hardened and started to peel off the clay. I think it has left a thin layer of sugar - the seasoning.

And at about eight hours the fire is substantially bigger...

And shortly after that we reach the maximum of my oven thermometer.

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Day 10, part 3. Take it to the max

At about eight hours some major cracks appeared near the top of the pot. There had also been a lot of steam coming from the insulation around the pot so I had taken all the accessible insulation out. It looks like the steam came from the pot drying out. The insulation was so effective, the steam was condensing on the inside of the oil drum.

The good news was that the coals were now very hot. I heated some metal rods and hammered them to form traditional shaped hooked tandoor skewers.

At nine hours the fire was really roaring. Very uncomfortable to get close. The clay wasn't glowing, so we must have been somewhere between 300°C and 700°C. It certainly felt and looked entirely different from 300°C.

To get it hotter I fired a paint stripping gun into the air vent.

At ten hours there is a raging inferno with about 4kg of charcoal in the tandoor. The oven thermometer is sitting outside the tandoor, inside the oil drum, and registering about 100°C.

We decided to bake a potato in the tandoor. It took about 10mins and was delicious, if a little burnt on the outside.

I'm writing the blog now at 14 hours. The interior of the tandoor is back down to a much more comfortable 300°C. I'll probably put a metal lid back on in about an hour or so and then see how its looking in the morning. We've burnt about 14kg of charcoal today.

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Review of the Tandoor firing.

Phew. The Tandoor was still warm the next morning. And still standing in one piece!

The clay definitely seems solid enough to cook in. You can see that the base is cracked. I'm pretty sure this is because the hot charcoal was sitting directly on the wet clay. It would have been better to use fire bricks for the base as they are already "cooked" and can take the direct heat from the charcoal at the start of the firing. It will still be fine for cooking and the ashes help fill the gaps.

There are some vertical hairline fractures in the pot. I should have massaged the pieces into place more diligently. I think the wet clay I used to pad out the gaps didn't dry properly and this caused the cracking. It should still be fine for cooking though.

The two large cracks on the outside of the pot in yesterday's photo seem to have stabilised. They are probably less than a quarter of the pot thickness. The large cracks look like a problem to do with the clay taking on the shape I wanted. I think maybe either there were air bubbles in the clay, it didn't dry enough or I should have made sure the clay was fully bashed into the curved shape. I think I might fill them with fire cement to ensure there is plenty of strength.

It's worth noting that the pot has shrunk quite a bit during firing. It maybe sits half an inch below the level of the oil drum now. Not a problem, but I'm glad I made it a bit too big in the wet clay.

I think a lot of these problems could have been avoided if I had used fire bricks in the base, worked the clay in the vertical joints more and dried the pot for a week or two before firing.

The final learning is that its not worth doing the coating with jaggery to go up to these very high temperatures. It produced a lot of acrid smoke, carbonised and quite a bit of it started burning. I think it would be better to fire the pot and then do a second cooler firing with the jaggery. At about 200°C the jaggery dries quickly and peels off the clay, leaving a thin sugary layer on the clay - this has to be the right process for lining the pot.

All that said, the pot is definitely good for cooking, and I'm confident its structure will now be stable up to the highest cooking temperatures. I need to work out how to cover the insulation in the top of the pot - fire cement is likely to be the right answer here too.

And of course, I now need to get cooking and for that I need some drier weather again

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