Making a pizza oven from scratch

Pizza...  mmm...

Recently in our house, a general consensus was expressed that everyone was a bit bored of our Christmas dinner options - either roast dinner or baked salmon for the last two decades, probs!

So this year we had a different christmas dinner concept than usual:  build-your-own-pizza!

The idea of doing pizza was surprisingly agreed without any dissent - but then the details...

Not just any old pizza - home-made, sour-dough pizza, stone-baked in a real wood-fired pizza oven

Great idea. Only one snag, we didn't have a pizza oven.

This was in mid-November and as a result the following challenge was laid down:

"Dad - can you build us a pizza oven?!"

Obviously the answer was "yes!". I could work out how later.

Well, here it is...

This was quite a build. I'm pretty pleased with this - it involved quite a lot of sussing out how wood fired ovens work, what you can build them out of and so on. 

Also, you can't just make an oven on the floor, so I also needed to create a pedestal base for it too. This is the full thing when finished.

This base acts as a support, as well as creating a sheltered woodstore to accumulate wood. The side and back walls have aerated courses, with holes in between bricks to allow good air flow which dries the wood out, for clean burning.

One handy thing about an enclosed oven, is that you can use softwoods as well as hardwoods.

And, as there is always wood waste to be found in skips after building works etc, in due course I'll fill this up with free wood. 

Design

To get a good design mainly involved doing a fair bit of (re)searching on YouTube and Instructables to see how other people had managed it. 

I found a load of really useful info, with some different approaches.
Roughly the stuff I found also fell into disticnt categories of online content providers...

• hobbyists - generally very generous and willing to share their fuckups - Instructables and YouTube both had plenty of good ones.
• marketorial - videos purporting to show how it is done, but really mainly focussed on selling stuff, brand building and occasionally people just bigging themselves up.
• traditional makers - the range of content showing how historically pizza ovens were constructed was pretty paltry - people in the two categories of content providers above asserted that things like: " traditionally, ovens were made like this... blah de blah", but there was little evidence to suggest they had ever researched it properly. Most looked like they had read some other hobbyists site and assumed that was true   

Anyway, the general consensus was:

• a parabolic oven shape is good to focus heat from the top of the oven down onto the cooking area
• though actually, semi-spherical and barrel shapes are traditionally used and work fine to the point that in most cases it doesn't matter, apart from small gains in efficiency
• the material needs to be heat-proof (duh!), but things are actually heat-proof is not that obvious (e.g. basic concrete is not that good if the oven temperature goes above a certain level)
• a lot of people saying: our ovens are bloody great/I know all about this (probably), etc
• you should use fire bricks, specialist refractory concrete and/or clay that mineralises and get more stable on heat
• You need good insulation between internal oven space and the outside
• You also need good insulation between oven floor and supporting platform
• you can choose to layer pre-cooked bricks to build up the dome, or go for a single monocoque concrete layer

So after comparing and contrasting a load of how-tos, I chose to 

• build a "close-as-I-can-be-arsed" parabolic dome
• to do this with a single monocoque moulded-concrete layer, rather than use bricks
• to reinforce concrete with steel rods and grid as required
• to use a wooden multi-part mould as the former for the main dome
• to use acryclic sheet and tape for the tricky chimney-throat area
• to use ceramic wool for the insulation
• to use Ciment Fondu for the internal concrete exposed to direct oven heat
• To use Pearlite as aggregate in the Ciment Fondu in the oven walls to give insulation
• To create an aereated wood-stack base to build the oven on, so scavenged/foraged wood can be seasoned naturally   

Construction diagram

To make up for a bit of a lack of decent photos, here is a quick overview diagram...

Actually missing in this diagram is the steel arch support that sits inside the brick arch. This gives tensile strength to the arch and alo acts as a support for the chimney (see below).

Construction

1 - First, build a base

Obviously, for a heavily-built construction one needs a decent foundation. And thus, I created a support slab of concrete for this. I used facing bricks at the front for a nice look and boards at the back to contain the slab without wasting any bricks that wouldn't be seen.

This was a standard 4:2:1 mix of 4x aggregate(25mm gravel): 2x sharp sand: 1x basic portland cement.

It's about 6 inches thick (150mm). I didn't bother using any steel reinforcement as the only real load on it would be compression from the weight of the rest of the build. There is no significant tensile load on it and it also has solid earth under it, with broken bricks as a base layer on the soil.  

The concrete slab was levelled by sliding with a baulk of timber in a see-sawing motion. I also used this to bash the concrete mix so the bubbles got released and the top layer was a pool of cementy slurry which gives a good smooth, even surface when set.

This was standard portland cement. This was fine as this layer would not get exposed to any heat. This takes a good two or three days before you want to build anything else on it, so it can stabilise and set properly (full strength is not actually for about 28 days!)

2. Build a platform support

Once the slab was set sufficient to build upon (usually OK after 2-3 days. I left it one week), I built a pedestal base to mount the oven on.

Before building it, I laid out the bricks to check the best way to bond them. NOTE - the holes in the back are deliberate to allow air to circulate inside the wood store. This is the best way to season wood - allowing air to blow through and carry off moisture. 

In this shot, I have laid the first three courses with mortar. The higher courses are not yet laid. This was just me setting them out to check where best to position the bricks.

You'll note the walls straddle the concrete base and the side bricks of the base container wall. This was to help bond these side bricks into the overall structure, and stop them tending to come loose.

The mortar for this was a standard 4:1 mix of 4 parts builder's sand with 1 part standard portland cement. To make it easy to lay, I used a good amount of washing up liquid as a plasticiser. This helps it all stick together and makes the mortar easy to work with.

3. Create a great big support slab

One I had a good pedestal of bricks, I constructed a big old support slab to sit on top of the base walls, which would form the main support for what would be a pretty heavy oven.

The first thing was to build a mould (aka a "former") to contain the slab. This was created from vertical supporting brace frames both inside and external to the brick walls, using 50mm x 50mm timber.

I screwed horizontal base plates of plywood on to these frames, to form the mould base. 

On the sides of these, I then screwed on outer containing walls, at right angles to these base pieces, to complete the mould.

As you can see, this wooden mould was made from any old wood I could find. The horizontal floor pieces generally needed to be thicker to bear the heavy weight of the concrete. The vertical outer containing walls were lighter, as these do not need to bear direct gravitational weight. They are just there to stop the sloppy concrete pouring off the edges and to create a nice clean square edge.

 

The gaps between wood and brick were padded with old newspaper, to prevent the concrete leaking out. The minimum mould depth for the slab is a little over 2 inches thick (55-60mm) although it varies across the whole piece.

Once the mould was finished I would fill it with concrete.

And given that the main oven would be pretty heavy, I used 8mm and 10mm steel reinforcement rods to add tensile strength to the slab. Essentially these mean the slab won't snap under heavy load...

Sexy!!

Once the mould was sound, I mixed a concrete using high-strength/fast-setting cement to fill it all in.

This is also a 4:2:1 mix of aggregate (25mm gravel): sharp sand: high strength cement (Blue circle rapid setting cement)

I made this quite wet, so the top layer would flow well and bubbles would rise out easily. This was helped by using a length of timber across the frame, to level it out. By tapping the wood, you can also agitate the bubbles so they pop out of the concrete. This is the same as I had done in the base. 

Here's a close-up concrete porn shot...

Incidentally, the tree in the background is a conker (horse chestnut) that is about 28 years old. It is semi-bonsai-ed by containing the roots in a pot (buried in the soil).

4. Design your dome

My design choice was to make the inside of the dome a parabola. The maths makes sense, but they are a bit fiddly to draw.

I didn't take very good pictures of how I created the parabola, but it uses a set square and nail to draw a constant line between focus and reference line.

I got this from this great YouTube video - respect to this person
https://www.youtube.com/watch?v=lVbfLlSLrZA

This involves marking out a series of point that are equidistant from the nail (left of pic) that represents the focus, to the wooden bar that represents the directrix (fixed reference line).

Sounds hard. It isn't. Watch the video :)

It is basically using the same method that small children used to use to make pin and string drawings in the 1970s, before fun was invented.

I drew the parabola on a white hardboard sheet (actually the backing from an IKEA cupboard).

I then cut this out with a Stanley knife (folding craft knife) and used it as a template to cut plywood parabolic buttresses, to support the mould...

I only had small sheets of plywood that I'd scrounged out of a skip. With some forethought, I found I could cut two of these from each sheet of plywood...

One solid and one with a gap in the middle. Like so...

Out with the handy jigsaw...

Until I had 8 pairs of these...

With some gaffer tape, I then made a base former from these buttresses. That's what I call a parabolic dome...

This looks like a great mould, but actually it is not practical because it is an internal mould and once concreted over, you'd struggle to remove it. Later on, I had to deconstruct it and build into removable modular sections, that would keep the overall parabolic dome shape, but that could be removed through the narrow mouth of the oven.

5. Lay out the oven floor

The floor of the oven was built up from firebricks. The plan section was based on the mould in the previous step. Basically, I just laid a load of bricks out then drew round the mould with charcoal and cut the bricks to match.

They are arranged in a herring bone pattern to allow the pizza shovel to slide in easily.

This was done in the shed. I numbered them before moving, so I could tell where they fitted together when I put them in situ later.

Here they are transferred to the slab. You can't see it here, but they were laid on top of a 25 mm layer of ceramic fibre, with steel mesh reinforcement on top of it, with Ciment Fondu concrete to bed it down. I used a mould round the outside to contain the shape (not shown) and the mesh was left long, so it could be linked up to the dome later, for reinforcement strength.

Ciment Fondu is a specialist cement that can withstand very high temperatures and is used for making furnace walls and also for hard-wearing concrete art sculptures. It dries fully in 48 hours. Obviously it way more expensive than normal cement, about £45 a bag.

Another view...

6. Build your dome

This was by far the least certain part of this build. Pizza ovens are usually built using special fire bricks, built up into a hemisphere (easiest way to do) or parabolic shape (better heat distribution, but not easy to build using bricks).

My design was to build a single self-supporting concrete monocoque shell over the mould. Using a mould is by far the easiest way to get a specific shape. In my case I want a (near-enough) parabola. Would it have mattered if I'd just used a half-sphere? Honestly, probably not. Let's face it, if you have a 400C-500C oven, it's gonna cook stuff OK, no matter the shape, more or less, I reckon.

BUT, being a bit of a fan of parabolas, I made a parabolic dome. Hey, whatever floats your boat.

Here, you can see the new modular mould. The top half is essentially the cap of the wooden buttress frame shown above, cut off from the rest of the buttresses. The cap was also now split into two halves. Each is a set of buttress pieces with hardboard panels to fill in the gaps between them. The panels were screwed onto the buttresses, then reinforced using good old gaffer tape.These are resting on more hardboard panels, all of which are taped together using more gaffer tape. 

The original dome shape was recreated using about 6 building blocks that, when put together reformed the orginal shape. Crude, but crude works. The reason for using tape to hold the pieces together, is that the tape could be cut between blocks and then each removed in turn later. 

You can also see the oven entrance bricks laid temporarily in place with the steel arch former. This hole is the size that each piece needed to fit through when being removed. 

I'd also added a lip to the front of the dome to prevent the concrete slopping off the mould. This shot shows the first structural layer of the dome concrete. The wood poking out of the dome mouth is supporting the mould, but in a way that can be removed later, when set.

Also clearly visible is some of the 15mm square steel mesh, used to add tensile strength to the concrete.

Some of this mesh is from the base and some was added as I built up the dome concrete layer.

The most important thing to note here, is how the concrete itself was mixed. This is a 1:4 mix of Ciment Fondu (the super strong, heat-resistant cement mentioned earlier) to Pearlite. 

Pearlite is a type of volcanic glass mineral that occurs naturally. It looks like fluffy white beads, much like polystyrene beads, but being glass it can tolerate very hight temperatures. It is an extremely good insulator as it is full of holes that trap air.

The concrete is sort of equivalent to making chocolate rice krispie cakes. The Ciment Fondu is the chocalate glue and the Pearlitre is the lightweight padding. Obvs, don't try eating it. That would be silly.

Despite being quite lumpy, it is quite easy to trowel on and spread out.

This first layer is about 35-45 mm thick. This is not really thick enough, even with steel reinforcement, so I added more. 

The next stage did several things: 

1. add another layer of Ciment Fondu/Pearlite over the first one to get the thickness up to a good solid 60-70mm
2. build the brick arch for the oven mouth, using a custom steel former I made from an old steel shelf (about 2mm thick)
3. fix the chimney in place at the rear side of the head of the mouth 

You can't see it here, but the central span of the steel arch has a semi-circular hole in the rear-facing face. This is to allow a flue to be formed at the mouth of the chimney.

The chimney has a diagonal cut across about 2/3 of its rear-facing lowere end. This completes the flue . Once the arch was built, the chimney mouth is very slightly higher than the mouth of the oven, so hot fumes will naturally flow out of the chimney, not out of the mouth of the oven.

Also, not easily seen, is the moulding piece, between the first layer of the dome and the steel plate. This is a piece of acrylic sheet moulded by softening with a blowtorch to fill in the gap between concrete, chimney pipe and steel arch.  The acrylic is about 4mm thick and again is held in place using removable tape - in this case masking tape.

    

Here is a close-up.

This was quite fiddly, but once all the moulding was secured, I could build up the side walls and then fill in the gaps with more Ciment Fondu. The total thickness of this inner dome varies, but it is at least 50mm thick over the main dome and in most places a bit more. It is even thicker in the areas where brick joins dome/chimey and also at the base.

This shot was taken after I'd built the arch and added about half the extra layer of the top of the dome. The steel mesh is showing on the near side, where that layer had not yet been fully applied.

Here is a close-up of the arch/chimney/dome intersection.

Note the bricklaying mortar is 4:1 builder's sand to Ciment Fondu

The dome is 4:1 Pearlite to Ciment Fondu.

This shot from the front shows the wooden internal support that props up the acrylic part of the mould, before the concrete had set. It was removed later. The acrylic is actually screwed onto a plank inside the oven with further planks to bear the weight.

Another close-up of the joint.

This shot shows the oven mouth two days later, when the mould and its supports have been removed.

I also applied another layer of the insulating concrete to form a cleaner more even shape.

7. Add some insulation to your dome

The top of the oven needed further insulation. Although Pearlite is a good insulator, it is set in concrete. So, although it is a much better insulator than standard concrete, it still needed more. For this I added a thick layer of ceramic wool. This is a ceramic fibre spun out to form a blanket. This is an extremely good insulator. I believe it is about 5 times as good as the Pearlite concrete, so this is like having an extra 250mm thick layer in the concrete dome.

Here, you can see the ceramic wool blanket over the internal dome. It has a tight mesh dome of chicken wire mesh streched over over it. This holds it in place and also acts as a steel reinforcement layer for the top layer of mortar that I laid over the ceramic wool.

As with the internal dome, this outer dome was applied in several layers. Each one was about 20mm thick and as I built each one, I smoothed the mortar render to a finer and finer dome shape.

This mortar did not need to be Ciment Fondu as it doesn't get unduly hot, because it is over the insulation layer. 

For the mortar, I used a 4:1 mix of builder's sand to high-strength cement.  

Below is the final coat. 

The smooth finish was achieved by using a standard float towel and a fairly wet mortar, so the surface could be puddled a bit, by patting as I trowelled, and then using the wet surface to smooth out a pleasing, even dome shape.

The plastic sheet you can see is a rain cover. The high strength cement is fast-drying, so it only needed to be protected from rain for about 48 hours.
  

Conclusions

This is not a build to take on lightly, but an especially satisfying one. It is quite crude, but it works.

The oven itself can get as hot as 500C+ on the roof and 300C+ on the oven floor.

It also is quite efficient. a fire based on a pile of waste wood of about 16" across will heat it up to Pizza heat and then because of the excellent insulation, it keeps it heat really well. 

I have since baked bread and potatoes in the the residual heat, which can stay hot for several hours.

If you are attempting this, watch a few videos of how other people have done it, then  choose a method. 

I like to use reclaimed materials, because it is good to give things new life, and also because it is much cheaper .

These things I bought:

• Ciment Fondu (2x 25kg bags) for the heat-facing walls of the dome
• Builder's sand (6-8x 25 kg bags) for rendering and bricklaying
• Sharp sand for concrete (2 x 25kg bags)
• Aggegate for concrete (8 x 25kg bags)
• Pearlite (about 100 litres)
• Ceramic wool (about 2 metres)
• Gaffer tape
• Wire mesh used to reinforce the concrete

These things I reclaimed from skips:

• All the red bricks (mostly Victorian and some miscelaneous 20th century bricks)
• The fire bricks
• The chimney pipes (offcuts from a wood burner installation)
• Acrylic sheet for moulding
• Plywood and timber for moulding

Next...

I have also made tools for using the oven. That is another post