Intro: Insulated Clay Pizza & Bread Oven
How tobuild a high-temperature pizza- and bread-baking oven from easily-obtainable materials and avoid the use of expensive refractory bricks. And to get it to heat as quickly as possible, get maximum benefit from your fuel, and use basic tools. And to make damn nice pizza!
Bill of materials:
100 or so Recycled bricks
250kg Pottery Clay - I got mine from a local pottery equipment supplier, who delivered to my doorstep.
4 x100 liter bags of Vermiculite - also known as Perlite - obtainable from a building supplier. £20 per bag
2 x 25kg bags of concrete mix
2 x 25kg cement mix
4 x 25kg bags of mortar mix
10 x 65mm-thick reinforced-concrete Lintels
6m of 1x4 wood for the foundation formers
Hard-core builder's rubble for the foundation
1 recycled Chimney or similar (eBay is can help you)
Thin wood slats salvaged from a wooden venetian blind (from IKEA!), or long twigs, for the oven dome former
Some odds & sods...
Plaster mixing impeller
Angle grinder with masonry disc
Air compressor & pneumatic chisel
Step 1: How Does It Work?
The Working Principle
This oven works on the convection principle: A fire burns in the back of the oven. It is fed by the cooler air that flows in along the lower half of the cavity. The hot air and smoke rises to the top of the dome, and is then drawn out along the top half of the cavity through the chimney. The hot air in the dome causes heat to be radiated back to the ground and any pizza that may find itself in its way....
That takes care of the top of the pizza. The bottom layer in the oven is a material of high thermal mass (thick terracotta tiles in our case) that is heated up by the radiation from the dome and also by the conduction of heat from the fire. A porous clay-based material like terracotta is ideal as it temporarily absorbs moisture from the pizza base, which helps to give you a crispy pizza base. We intend to make proper pizzas here, not the thick, soggy things that those multinationals dish up, after all.
The fire is started in front part of the oven, and when it is ablaze, it is pushed into the back. The fire may appear to momentarily die because of the lack of oxygen, but the convection process starts to work within seconds and it grows again. The secret of keeping the fire going it to regularly feed it small amounts of wood, preferably using long tongs so that you can place the wood on top of the burning embers. You can test the convection process by dropping a few drops of water on the bottom layer of the oven: the resulting vapour should move into the oven towards the fire, not the other way around as you would normally expect.
There is no hard and fast rule of determining when the oven is hot enough. I just know it is ready for use when it feels much hotter than my domestic oven set to maximum temperature. I also measure the exhaust temperature on the chimney and when it exceeds 300 DegC (because that is how my temperature measuring device will go), then I know that the internal heat is well above that we are ready to go.
When the oven is ready, the cooking surface needs to be prepared by sweeping the any ashes either into the fire or out of the entrance using a natural bristle broom dipped in water. Then use a wet floor mop to pick up the remaining ashes off the cooking surface. You can also wrap a wet rag around the broom for the same effect.
You are now ready to bake your pizza.
Step 2: The Oven Design
Some design thoughts
A bread-baking oven typically has a thick internal layer made from a solid, heat-retaining material (i.e. a high thermal mass ) that will hold the heat for 3 hours or so. The idea is that you make a fire in it, let it heat up, and then scrape the embers out and put your dough in and close the door and let the bread bake. A pizza oven, on the other hand, has a fire burning in it - or has at least glowing embers in it - while the pizza bakes. It does not need a thermal capacitor like a bread-baking oven, but it needs to run much hotter. Both types of approaches require a good, heat-resistant insulator. This design serves the needs of both the pizza baker and the bread baker, by having an inner layer of material with a high thermal mass, and a surrounding heat-resistant insulation layer. The better the insulation, the less heat can escape, so you will get optimal energy from your fuel.
The outside of the oven is made of three layers and each layer serves a distinct function:
1. Inner layer: Clay mixed with sand. This is a refractory material, i.e. it can withstand high heat. It is also brittle and needs to be protect by the outter layers. This layer has a high heat capacity and therefore helps to even spread heat and to radiate heat back into the centre of the oven. It is about 3cm thick.
2. Middle, insulation layer. This layer must be heat proof and also act as an insulator. The choice of materials was a vermiculite (a.k.a. "Perlite") bonded with clay. It is about 10cm thick. This layer also provides the first bit of structural strength to the design.
3. Outter shell. Since every bit of insulation helps, this layer is also made from vermiculite, however, it is bonded with cement, which makes the shell rock hard. Since it does not get too hot on this layer, it is acceptable to use cement, which would flake and loose strength under high heat.
4. Weather-proofing layer: A layer of external cement will keep the worst moisture out when it rains, but allow the clay to continue to dry out as it ages. One could also tile the outside for decorative effect.
The inner height of the entrance must be 63.9% of the inner height of the dome at its highest point. Who and how this was calculated remains a mystery to me.
A chimney that can provide a good draft is required to help draw the hot air and smoke from the oven, so as a rule of thumb, the chimney should be at least the height of the inner dome height.
Note to Academics, Engineers and other wonderful mathematically-inspired types:
I made no formal calculations, even though I did once do courses in thermodynamics and structural engineering at university. Regrettably, beer was more interesting at that time. After one year and regular use, the structural design held up pretty well.
It is also unlikely that this oven will ever be hot enough to fire low-temperature clay pottery - I have tried.
Step 3: Cast the Base
Make a wooden frame the size of the base of the oven. The size of this will mostly be determined by the length, width and number the concrete lintels that you will across the base. This one was 1m wide and 1.6m deep.
Collect as much builders' rubble (a.k.a. "hard-core") as you need and dig a hole deep enough to hold the wooden frame and swallow all the rubble up. Ensure it is level and knock any protruding bits down and tamp it all down.
Mix 2 x 25kg bags of concrete mixture (this consists of cement, sand and gravel aggregate) with water and pour it over the hard-core until level with the wooden frame. Roughly spread the concrete and then smooth it with a plastering float.
You can start building on the base as soon as the base is set.
Step 4: Build the Wood Storage Level
I decided to keep the pizza oven at a low level so that one can look into the oven when seated. As a result, the wood storage level was about a 1 1/2 foot high. Arches on both the storage bin and on the oven look good and keep things simple, so make a semi-circular former out of 3/4 inch plywood for use on both the wood storage and oven entrance. In the end the former is also use to shape the doors for the wood storage bin and oven, using a router. The nail-and-string-pencil trick works well for drawing the semi-circle, BTW.
It is not too late to change your mind under duress if you should rather use your foundation to build an outhouse instead. Otherwise, if you have managed to silence the missus, carry on building the pizza oven!
Build up the brick courses using recycled bricks and standard mortar mix from a DIY store. Test the spacing of the bricks around the arch before committing to it. And use plenty of mortar for this bit - the excess can be scooped up and used for the level courses. Keep checking for vertical and horizontal plumbness with a spirit level. It is only a pizza oven and mistakes are not in any way affect the eventual function, but you might as well make it look professional...
Because my pizza oven is built into a slight mound, I created a crude damp-proofing barrier from some left-over packaging plastic and then backfilled with dirt and then trimmed the excess plastic off.
Lastly, top out the last course with mortar to create a level plane for laying the lintels on. Let the mortar set before continuing.
Step 5: Lay the Lintels
This is the least complicated step, but is the most most exhausting, because the lintels way about 20kg each and need to be moved around to fit as perfectly as possible on the platform. I only managed to get 110mm-thick lintels, but 65mm-thick lintels would have been just as good, lighter to shift into place, and cheaper!
Step 6: Take a Break
You should now relax and have a BBQ - British style, with venison burgers ("Bambi-burgers"), Lincolnshire sausages, spare ribs with dubious red food colouring in the marinade and good ol' plain Porkers.
Step 7: Create the Entrance Arch and Baking Surface
First build a dam in which the lower insulation will be held. I used decorative bricks. Then build the second arch - but with a difference: It needs a keystone with a large hole in it, on which to rest the chimney.
The keystone in the end was made made up of two pieces of sandstone (salvaged from a rockery in my garden), and I used a pneumatic chisel to form a slot in one of them in order to form the hole. There was a lot of fitting and even more mortar to make it look respectable. I also decorated the front-facing keystone with an craved year-date, using my pneumatic chisel and sharped the lines with a craft knife. It turns out that sandstone can be carved quite nicely with a stout craft knife, but don't count on using the blade for anything else afterwards!
Once the arch and keystone are set, mount the chimney. I used 4"-diameter clay sewerage pipes to get a good column of air flowing, and will put a large decorative Victorian chimney on there later on.
The bottom insulation layer is made from a mixture of cement, water and vermiculite. Start by making a bucket-load of cement slurry and and then gently fold in the vermiculite. A plaster mixing impeller is really handy here for making the slurry, and a lot of slurry needs to be made, so go ahead and invest in this cheap tool. (It is also a great tool for mixing vast amounts of pancake batter for the village fair). With the vermiculite mixture ready, spread it across the dammed-up surface and use a plasterer's float to smooth it.
Note that in the last photo covered the vermiculite surface with a mixture of clay and sharp sand - this was a mistake! It shrank horribly and cracked to the point of being completely useless. The best baking surface turned out to be large terracotta tiles. More about this a few steps later.
Step 8: Make the Former
This oven is build using the so-called inverted basket approach. Instead of using traditional basket making materials, the former is made from the left-over slats of IKEA window blinds. You will need about 30 of these slats. Instead of slats, you can use any other bendy sticks: hazel, willow, palm, etc.
Start with a base circle 20cm less in diameter than the eventual outer width of the oven. Attach vertical slats to the circle and form a dome. Then form the oven entrance using a similar technique. This is probably the easiest step so should not require too much explaining, but see the animated picture for more details if you get stuck.
Mount the oven former on the base, stand back an visualize the eventual beauty of what you are about to make!
Step 9: Wallpaper the Dome
Well, sort of. Mix up some wallpaper paste. Cheat like I did and use an impeller on a drill. Paint the paste on the dome former and lay the first layer of newspaper down. Paint over the paper and add a few more layers of newspaper in this way. You should have enough layers of paper on there to be strong enough to stop the sand/clay mixture from falling though the gaps in the oven former.
Use at least 4 layers of newspaper and let the newspaper dry before continuing to the next step.
Step 10: The Thermal Mass Layer
The inner layer of the pizza oven needs to be made from a material that is heat resistant and can hold a lot heat (i.e. has a high thermal mass / heat capacity) and also be able to conduct heat, in order to evenly distribute and radiate the heat around the oven and to avoid hot spots. In other words, it is a bad idea to make the inside of the oven from a heat insulating material. However, we also do not want to escape from the surface of the oven either, so this layer needs to be protected by a thermal insulation layer of clay mixed with vermiculite.
Make the Thermal Mass layer mix
Again, mix sharp sand with potters' clay in a 80% to 20% ratio. You will need about 4 large buckets worth of mix in total.
Apply Thermal Mass layer
Cover the papered-over shell with a 1-inch thick layer of your mix. Ensure that the layer is of even thickness. Roughly score the surface so that the next layer will adhere better.
Let it sun dry for a few days. Crack may appear, as they did on mine. Simply fill the cracks in with more sand/clay mix, and top the layer where the thickness is less than 1 inch, and then let it dry again.
It is important that this layer is a sound as possible, as this is the pizza-facing surface and you don't want bits to fall into your food.
Step 11: The Insulation Layer
The insulation layer needs to be heat-resistant, needs to be made from a refractory material, and has to be mouldable over the existing shape of the dome. We make this from a runny mixture of clay and vermiculite (a.k.a. Perlite).
Make a clay slurry with a 12.5kg slab of clay and 5 liters of water, using a spade to chop it up into smaller chunks. Add a spadeful of sharp sand to it to help with the chopping up to prevent it from annoyingly sticking to the spade. Leave the clay for a while to absorb more water. Then use the impeller on a drill to turn it into glorious gloop. It should be the consistency of double-thick cream.
Gently fold in 50 liters of vermiculite (half of a 100-liter bag) into the slurry in such a way that the vermiculite is fully coated by the slurry but does not get compressed - this is a bit like folding beaten egg-white into a soufflé mixture, actually. You will need to repeat this 5 or 6 times to make enough insulation mixture, or if you have a large enough mixing container and are feeling brave, do it all in one go.
Place 5 cm high mounds on the outside of the dome to act as a thickness guide. Now use your hands and pack the clay and vermiculite mixture over the dome. The mixture is surprisingly mouldable and elastic. If it is too stiff, wet it with some water. It will all eventually evaporate in any case. However, be careful not to press it down too hard, or you will squeeze all the air out of the vermiculite which will cause to to loose its insulation property. Come compression is unavoidable.
Finish off the dome by smoothing it with your hands or a plasterer's float. Using your hands is more fun! When using a float, remember to only ever move in an upwards direction, just like when you plaster a real wall. All in all, you should now have a total wall thickness - including the clay layer underneath - of about 8cm.
Again, let it dry in the sun for a few days. Cover with a plastic sheet if there is a chance of rain - it will still dry out nicely under there. This will not crack as badly - if at all - as the underlying clay layer when it dries, because the vermiculite is made of multiple sheets of a mica-like substance that allow for some internal slippage. But some cracks will appear, so back-fill them with some left-over clay,
Step 12: First Firing
So, you have waited for the oven to sun dry as much as possible, right? Because if you have not, you risk damaging your hard work if you proceed on to this next step:
Like all things made from refractory materials, they need to be gently fired with a small fire at first and then fired with increasing size fires over time until the material has stabilized. Since we know very little about the materials that we created with sand, clay and vermiculite, it is a safer to err on the side of caution and to "run the oven in" over many small firings before putting it to proper use.
The first firing will consume the internal wooden former, so stack a small fire pile of tinder inside against the former and start it with a blowtorch. In my case, the oven former was built from venetian blind slats, which quickly ignited. I had a roaring, very hot fire in there for about 2 minutes, and then it was over. Feel around the outside of the oven and look for any hot spots. Note them for fixing later with some more vermiculite/clay mixture, but ideally (and surprisingly!), it should be perfectly cool on the outside of the oven.
Let the oven cool down for a day and sweep the ashes out. You can see the original baking surface shrinking and curling up in the picture - we fix this in the next step.
Step 13: Pizza Baking Surface
The baking surface is made from 3/4" thick terracotta tiles. This are heat resistant and easy to work with. They are also slightly absorbent, which creates a crispier pizza base. Typically, these tiles need to be replaced over time, so it is a good idea to only lay them once the dome is fully built.
Be like the wicked witch from Hänsel&Gretel and put your head in the oven to take measurements. Cut the terracotta tiles using a fret saw (the blade does not last long but the cutting is quick) and shape it with a masonary file. Lay them on a bed of clay slurry for a snug fit. I used a tea-light while working in the darkness of the oven.
Step 14: Outer Mortar Shell
The outer shell is a layer of standard builders mortar (cement & sand) on a matrix of chicken mesh. It is about 1/2" thick. If you don't use chicken mesh reinforcement, then you will quickly get small cracks in the outer shell, as in the picture.
For some decoration, you could tile the shell with ceramic mosaic tiles.
Step 15: Finish Chimney
I found a victorian chimney on eBay, which I fitted over the existing sewer-pipe stack with some more mortar. I filled the gap with between the two pipes with left-over vermiculite, so that the outer pipe never gets too hot. A standard chimney cap and a turned wooden finial completes the chimney.
Step 16: Make Pizza Tools
You will need make or buy the following tools for a happy pizza making experience:
- A tool for inserting and removing the pizza into the oven. I made a pizza lifter from oak. I also use it to scoop cold ashes from the back of the oven.
- A brush for brushing the ashes on the baking surface aside. This is a natural fibre brush mounted on an oak batton.
- A smaller scoop for dealing with errand pizzas and pushing burning embers to the back of the oven
Also make a door to fit snugly over the oven entrance. Use the original arch former for a shape and make some insulation on the fire-side with multiple layers of alu-foil. I decorated the door with some engraved teutonic witt: "The Hot Herta" (An old-fashioned german woman's name, and Hert is a german word for oven), and "This Way To Hell".
Step 17: Make Pizza
Make a wooden fire in the middle of the oven and let if burn until you have mostly embers. Experience and experimentation will tell you how much, and what wood to use.
Push the embers to the back of the oven. If the embers appear to die, get a convection current going by blowing along the baking surface of the oven - the embers should flame up again. The temperature of the exhaust air should be in excess of 300 deg C. Occasionally add some more wood to the back of the oven to keep up the heat.
Perpare the baking surface
Dip the brush tool water and brush the remaining ashes to the back. Wrap a wet rag around the brush tool for a final polish of the baking surface.
The baking surface should be very hot now and ready to bake a pizza!
A truly thin-crust pizza should not be in the oven for more than two minutes.
Unless you have mastered handling floppy pizza bases with the pizza lifter, should practise and experiment with bought pizzas first. For better control, make one pizza at a time, regardless of the number of hungry people slathering for a slice. Put the pizza in the middle of the oven and turn the pizza around once for an even bake.
Cook other stuff
Using an insulated door to the oven, this becomes a proper oven that uses the residual heat from the embers and inner layers.
- The best Christmas Turkey is one that I slow-cooked overnight in the pizza oven, and browned off on the BBQ. All in the middle of winter!
- You can make amazing slow-cooked stews - ox-tail stew is my favourite.
- Confit de canard - the slower and longer the cooking, the better.
- Chicken and roast veg
- Bake bread - varied success...
Grand Prize in the