Vermiculite, Gym Ball Dome, Wood Fired Pizza Oven

Intro: Vermiculite, Gym Ball Dome, Wood Fired Pizza Oven

I built this pizza oven for my family instead of buying a similar one for at least £700 ($900). The ones you can buy still require a base to be made or bought and you have to assemble the oven as well. I was able to create something unique and bespoke to fit the small space that I had available.

I hope that you decide to make one for yourself. You will enjoy cooking (and eating) tasty stone baked pizzas that cook in seconds as much as you will enjoy the satisfaction of having made the oven yourself from scratch.

I spent under £320 in total on materials. The only parts I already had available for free were the 14 house bricks and scrap wood to make the jigs and some hardcore for the base.

I hope you enjoy making your own wood fired pizza oven! Go create!

STEP 1: The Base / Foundation

For an oven this size, clear an area larger than your intended base. For my base of 0.9m, I cleared an area of 1.2m square.

I used concrete paving slabs for the base instead of pouring a slab of concrete. They are very thick, strong, solid slabs - perfect for the base.

First, compact the ground beneath where you want to build your oven then add approximately a 15cm layer of hardcore material to build up a solid sub surface. The gaps between the hardcore should then be filled in with a few bags of 'scalpings'. Then go ahead and compact everything down as solidly as possible with a hand compactor or something similar and heavy.

Using string and pegs, mark out a square area equal to the final size of your four slabs and then fully bed the slabs in with a 3:1 mortar mix - 3 parts sharp sand, 1 part Portland cement to a thickness of about 15cm. Don't be tempted to economise on materials by 'dot and dabbing' as you will create a weak base with air pockets. Use a spirit level often to make sure the base is nice and level as you go.

STEP 2: The Vermiculite Gym Ball Dome

To make the dome you will need to construct a simple jig to support the gym ball. The jig is basically a crude table with a 700mm circle cut out of the middle. I made mine out of scrap wood that I had in the garage. Once you have made your jig similar to the one pictured, you can then inflate the gym ball in-situ inside the 700mm hole of the jig so that an upper hemisphere is jammed into the jig securely. Cover the top half of the gym ball and the lower edge with food wrap to stop the vermiculite cement mortar from sticking permanently.

To make the vermiculite cement mortar (or vermacrete), use a plastic plant pot or small bucket to measure out 6 parts vermiculite and 1 part cement. Begin to add water slowly. You don't want a watery slurry, but mouldable clumps - it's quite difficult to describe actually... Err on the slightly drier side.

Some people say that they add pins or poly strands to the cement mix but I added neither and my oven has had no cracks so far after one and a half years of usage, one cold winter and about 25-30 firings.

Start loading up the sides of the gym ball with the first layer of vermiculite cement mixture with your hands, clump by clump. With the first layer complete, add a further layer until the dome is about 20-30mm thick. There is no need to let the first layer dry before you begin the second layer.

Over the next week keep your dome slightly damp by spraying it lightly with water DAILY and covering it with plastic (I used bin bags). Don't soak it - the goal here is to slow down the curing process by not letting it dry completely.

After one week, uncover the dome and allow it to fully cure and dry out. Do this at room temperature. Don't be tempted to warm it up with a heater or put it in the sun. You want it to dry out slowly. This took my dome about one month to cure fully.

When I made my dome, I also made some test lumps of vermiculite cement mix so that I could see how the curing and drying process was going.

STEP 3: Building Up the Base Walls

Lightweight breeze blocks (as they are called in the UK) were used for the base walls of structure because they are easy to shape, cut to size and handle. I used a slightly weaker mortar mix of 4:1 for the brick laying. Four parts soft building sand, one part Portland cement.

Continue to lay blocks until you reach your intended height. I have no bricklaying experience and it wasn't to difficult - just use a spirit level all the time to check that it is all straight.

You are welcome to copy my design & layout for the block laying and I have included a detailed & to scale technical drawing. (See attached: Pizza Oven.pdf.)

I sculpted curved arches out of this versatile material to make a nice final arch shape. Six more breeze blocks were placed on the top horizontally to form a roof to the base.

Finally, cap the structure off with a further four concrete pavers - checking for level again as you go!

I used a total of 32 breeze blocks for the walls and eight paving slabs for the top and bottom.

STEP 4: Oven Floor Insulation Layer-up

The next layer is the calcium silicate insulation board. Release your dried vermiculite dome from the jig by deflating the gym ball and passing a knife around the bottom edge of the dome. You may need some help lifting the dome as it's a bit scary... dropping it now would waste a lot of time and effort!

Place the dome on top of the calcium silicate board and outline the shape onto the board with a pencil. Use a jigsaw to cut the ceramic insulation board shape but be careful, this material is quite brittle. You'll also need to wear a dust mask as the dust is apparently harmful and can irritate your lungs. Fix the board permanently on top of the four paving slabs with some fire cement applied with a tiling comb and then press it down firmly. Fill any breaks or cracks with fire cement. My board did crack as you can see in the picture - it's no big deal.

Fire bricks: Test arrange your 25mm firebricks on top of the silicate board. Once again, place the dome on top and trace the edge to make your cut line. Number the bricks if you are worried about mixing up the bricks during cutting. I cut my bricks (badly) with a hammer and bolster chisel but it worked. A cleaner finish would be achieved with an angle grinder if you have one.

Next, bond the fire bricks down with another thin layer of fire cement. You can skip this step because the whole insulation slab will be encapsulated with a final layer of vermiculite concrete layer later on.

STEP 5: The Brick Archway

Now it's time to cut your arch and bond the dome to the base.

First, build a semicircular jig from scrap plywood to support the brick arch as you build. You can get the size of the semicircle from the scale plan PDF attached to step 3. Trace the contour onto the dome and cut it out with a jigsaw or handsaw.

Bond the dome to the base with either left over fire cement or a quantity of vermiculite cement mortar using the same recipe as for the dome itself.

Allow to dry for a few days and then go ahead and light the first of many 'curing fires'. Make a fire out of a small quantity of sticks, nothing more. You are just trying to get the dome slightly warm.

You must protect your pizza oven from any rain between the curing fires- rain will soak in to the vermiculite and potentially cause damage at this stage in the build.

Trim your house bricks with a bolster and hammer or an angle grinder to match the contour of the dome as closely as possible. Bed the brick arch in using the arch jig for support and using a standard 4:1 mortar mix. Four parts building sand to one part Portland cement.

I added an extra brick on the top of the arch as a design feature / somewhere to put a glass of wine :)

STEP 6: Insulation Blanket Layer

The whole point in a pizza oven is that they reach insane temperatures. To achieve these high temps, you need to add lots of insulation: Vermiculite dome / ceramic blanket / more vermiculite / exterior render.

Unroll the ceramic fibre blanket and cut 'darts' so that the blanket can follow the curves shape of the dome. Cut out an approximate chimney hole roughly where the the flue will go. The hole you will cut through the dome will be a 130mm hole which is smaller than your stainless pipe. This is so that the pipe will sit in its position without falling down through.

Pin the blanket down by stretching chickenwire over the whole dome. I used a staplegun with long 30mm staples.

Light another gentle curing fire whilst you're at it - not too hot please but bigger than the last fire!

STEP 7: Final Layer of Vermiculite Insulation

Wrap your flue pipe with a layer of cardboard. This will act as a shim and protect the pipe as you add the next layer of vermiculite insulation.

Mix up batches of vermiculite cement and begin packing it over the blanket and chicken wire. It sticks quite well and is very satisfying. I did the whole lot in one thick layer.

Once dry (not cured yet) you can slip the cardboard covered flue template and you will find that you have created a nice socket for taking your flue down when the oven is not in use. When in use, seal the flue to the oven using a length of ceramic stove rope.

Light another curing fire... no high temps though yet.

Allow your dome to dry out - I felt like mine was sufficiently dried in five days as it was summer, but use your best judgement.

Light a curing fire as often as possible over the next two weeks, gradually increasing the temperature and size of the fire every time. Be patient - don't rush or skip this step or be tempted to light a big fire as the oven may crack if you go too hot too soon. Towards the end of this process you could go hot enough to cook a pizza.

STEP 8: Render the Exterior Surfaces

The render coat of plaster is made with the following 6:1:1 recipe. That's six parts plastering sand, one part Portland cement and one part hydrated lime. The lime is important because it gives the plaster some elasticity.

Splat the render on with a plastering trowel if you have one. I'm no expert plasterer and we're not looking for a perfect finish here! Aim for about a 10mm coat. Go ahead and plaster the front and sides of the base whilst you are at it.

Allow to dry for a day or two and then paint with exterior masonry paint - I used Sandtex.

STEP 9: Make a Door

Use the brick arch jig template you made earlier to get the rough size for the door. The door is an important part to be able to get your oven to its maximum temperature.

I made the mould out of yet more scrap wood and a bendy strip of fibreboard. I used copper pipe insulation cut in half to mould air inlet passages.

Pack the mould with more vermiculite mix (same recipe) and allow to cure for a week or so. I incorporated an unused kitchen door handle into my oven door which gets a bit warm but does the job.

Whilst my door was drying I couldn't resist and fired the oven up using the arch mould as a door. As you can see from the picture the door didn't last long - the pizza was still excellent!

STEP 10: Aftercare & Tips

After use, once the oven has cooled down, remove the chimney and cover the oven with a garden patio table cover from the garden centre.

I will probably repaint the exterior every 2 years or when we fancy a chance of colour (it's yellow at the moment)

Start your fire at about the same time you make your dough (Jamie Oliver recipe by the way!)

Buy a £10 infrared thermometer from eBay!

Pizza cooks best with a flame (in other words, not just hot embers) the best way to achieve this is to have your fire going at the back and keep a few small logs on the opposite side "heating up'. When you are about to cook a pizza, throw the warmed log onto the embers and it will burst into flames!

Use coarse semolina to stop your base sticking to the pizza peel.

We tend to have parties where we ask guests to 'make their own' and then I cook it for them in the oven - it goes down very well.

I wish you the best of luck and enjoyment out of your new pizza oven. Go entertain your friends and family!

Comments

[kaapstad]

Would this work indoors with a routed to the ceiling chimney. Are there restrictions that would stop this?

[phammy57]

Hello, i would think it would spill too much smoke out through the front, especially on a windy day. I wouldn't recommend doing this sorry. Also there's a fire risk being technically an open fire in your house. Sorry I don't have better news.

[Microbe]

When we have guests for our pizza oven, they make a pizza, but it is to share. So people have a variety of tastes.

Also, when it comes to toppings, less is better, don't be tempted to overload your pizza.

[paddy2604]

How many meters of ceramic blanket did you need to cover the dome? You say a meter but my dome is roughly the same size as yours and I recon I'll need at least 2 meters. Hopefully mine will look as good as yours.

[KirklandV]

Great instructable! And one of the nicer looking one of these I’ve seen. How long does it take to get the oven up to 350C? Is it difficult to get it higher? What kind of heat retention does it have afterward? Does it cool down quickly or can you cook other stuff in it post-firing? Thanks!

[phammy57]

It takes about 1.5hrs to get really hot compared to just 20-30 mins on my dads metal one he bought which is also insulated. It does keep the head really well. Still just warm in the morning is you have your pizza party in the evening :) it can go a lot hotter but my temp probe maxes out at 350. I find it hot enough to cook a pizza in 1-2 mins.

[Klisa2]

How do you staple the blank down? And in to what do you staple the blanket to?
thanks sharing your plan, we can’t wait to use our pizza oven.

[phammy57]

Sorry for the late reply. For the benefit of others... Yes the felt is held down to the dome with chicken wire which you sort of tie/sew together with more wire as you go. It doesn't have to be perfect looking :)

[Stables89]

Hi. Iv just recently put my dome together. Did you find it was quite crumbly at first

[phammy57]

It is a very strange material to work with but the strength comes from drying and hardening over time. Yes very crumbly indeed, hard to get the first layer started! (sorry for the delay in reply)

[RobL35]

When mixing the vermicement do you use ordinary cement or fire proof cement.

[phammy57]

Hello, it's just standard cement. The fireproofing comes from the properties of the vermiculite alone. Ours is still going after many years and cold winters outside - we do protect it from rain with a cover.

[Stables89]

Hi. Iv just recently put my dome together. Did you find it was quite crumbly at first?

[gt_man002]

Hi Phammy57,

I'm just starting my Pizza Oven project using your post as my primary guide. In your description of the first layer over the fitness ball, you mention to make the cement/vermiculite layer approximately 20 centimeters thick. In the images, it doesn't appear anywhere near 20 cms. Maybe 5cms. Can you clarify the thickness?

Regards,
Glen

[phammy57]

Sorry for the delay in replying...
It should be mm so about 20mm - although rereading the instructions it was more like 30mm
I will correct the instructions and thanks for spotting the mistake!
Pascal

[gt_man002]

Ok - thanks for clarifying - that makes a lot more sense! Looking forward to getting started with the next step! Thanks again for writing up this instructable and posting it.

[phammy57]

Your jig looks perfect for the job! - good luck with it all!

[WWW_PLAWSKI_PL]

And here are some flatbreads from it https://youtu.be/LuLexbdrmis

[WWW_PLAWSKI_PL]

I have built one last year https://youtu.be/HIeOjXM4ST4

[wrksnfx]

WARNING VERMICULITE IS A FORM OF ASBESTOS, A KNOWN CANCER CAUSING AGENT!