I keep forgetting to post a photo of the new roof. There she blows!
Just a bit of shameless self-promotion here. Delicious magazine have just updates their website and have made a shiny new version of the article published last year about my oven (and how to build one!).
Although it contains most of the information you’d need to build a pizza oven, the devil is in the detail, most of which can be found in my eBook (funny that!!!) 🙂
Here’s the link:
In my book I outline the steps one should follow to build a traditional pizza oven. As a result of following these instructions, lots of people all over the world have now built their own ovens. One question I still get asked quite often is how much x do I need (where x can be clay, sand, bricks etc.). To be fair, I didn’t include lots of quantity details in the original book (new edition coming soon) because, to be totally honest, I didn’t record this information during my first build.
Anyway, at a recent build I made sure I took note of everything we used, and so here are those all important quantities and a full list of other equipment required.
NOTE: 1 bag of builders sand (approx. 15kg) fills a 15 litre bucket.
Normally I prescribe a 2:1 ratio of sand to clay but this is a rule of thumb. You need to aim for a mixed material which holds firm, is not too soft (or wet) and not too dry. If it is too wet (or if there is too much clay) it will slump around the base of the layer you are building.
SAND – 18 bags (approx. 270 kg)
- 10 for the dome former
- 4 for the first (oven) layer
- 2 for the brick arch, backfill and chimney
- 2 for the brick arch former
- 7 for the final layer (you will use the sand excavated from the dome and arch formers for this final layer which means you’ll have approx. five bags leftover at the end)
CLAY – 12.5 buckets (approx. 190 kg)
- 3 for the first (oven) layer
- 2 for the brick arch, backfill and chimney
- 3 for the insulation layer
- 4.5 for the final layer
OTHER MATERIALS AND EQUPIMENT
- 2 large bags of wood shavings
- 36 London bricks (11 for the arch, 25 for the oven floor)
- Rubble / hardcore (for the plinth fill)
- Large wooden “beams”, sleepers, logs or bricks for the plinth (this depends on how you decide to build it)
- Cement if you are building plinth out of brick
- Right-angled brackets and screws if constructing plinth from wood
- Glass bottles (optional)
- Old newspapers
- Plastic rubble sacks
- A bucket or two
- A drill with plaster mixer (optional)
- A knife
*Drop Test – Form a tennis ball sized clay:sand ball in your hands. Drop the ball onto the ground from shoulder height. If the ball explodes, the mixture is too dry, if it “splats” it is too wet. Ideally the ball should just hold together.
A very quick post about cracks. Lots of people contact me in a panic about cracks appearing in the oven layer during the build. Let’s start of by saying that it is very likely that you will get some cracking – in fact this is quite normal. It only becomes a problem if the cracks become big and penetrate right through to the inside of the oven layer. Normally I’d just say, try patching the cracks with extra material and carry on but if they are significant then it might be worth starting again! It’s your call.
Why do we get cracks though? Cracks appear when the sand:clay mixture dries out and contracts. If you mistakenly use clay only to build your oven layer you will see significant cracks appear. You should not do that! However, you can get major cracks, even if you use the correct mixture of clay and sand. This happens if the oven dries out too quickly. The trick is to allow the oven to dry slowly, naturally if possible. If the sun is blazing you can use an old trick that builders use when building walls to help slow the drying process down – cover it with a damp sack, or even a tarp. If you do light a fire inside to help with the drying process – keep it small and gentle.
Thanks to Mungo Finlayson, an oven builder from Scotland, who shared these photos with me. It seems that some rare Scottish sunshine dried his oven layer too quickly. I suggested Mungo should try and fill the cracks before having to resort to starting again.
When thinking about gathering building materials for building one of these ovens, one of the most important considerations is the mixture you need to construct the inner and outer oven layers. I suppose the fact that the oven is called a clay oven is rather confusing. These ovens are are not constructed using clay alone. The mixture we use is a combination of clay and sand -in fact we use more sand than clay in the mixture, somewhere in the ratio of 2 parts sand to 1 part clay. Maybe we should call them sand ovens?
So you might be thinking, is using clay alone really a problem? Well the answer is absolutely yes! Using 100% clay results in huge cracks forming in the most crucial layer of the oven (the internal layer) as the oven dries out. This often leads to catastrophic collapse of the oven which, I’m sure you’ll agree, is not a good thing. The confusion is particularly compounded when buying clay from clay dealers. More often than not, if you ask them you want clay to build a pizza oven, they will recommend buying clay which they describe as highly grogged, which basically means that it contains some pre-fired ceramic particles. Here’s an example:
This grogged clay is great for reducing thermal shock and shrinkage in pottery but has little affect when used to build these ovens. The double whammy is that grogged clay is normally more expensive than other types.
DON’T BOTHER BUYING GROGGED CLAY. Let me say that again, DON’T BOTHER BUYING GROGGED CLAY, you’ll just be throwing money away.
So are clay dealers deliberately trying to pull the wool over our eyes by trying to sell us this stuff? Well, let’s give them the benefit of the doubt and say no. Most people who regularly buy clay from dealers want it to make pots, sculptures etc, which will normally be fired in a kiln after it has been shaped. The firing process (at temperatures well in excess of 1000 degrees centigrade) drives of all the water and fuses the silicate particles together which creates the ceramic materials we see in all forms pottery.
These garden ovens NEVER reach the super-high temperatures created in kilns which means that, unless you intend on putting your oven inside a kiln to fire it (you can buy ovens like this) your oven will not become ceramic. I assume most clay dealers don’t know much about building this type of traditional, outdoor oven and so, inadvertently recommend grogged clay as the material to buy.
The simplest way to think about the mixture we use to build these ovens is to compare it to those chocolate and rice crispy (or cornflake) cakes that kids make. In that example, hot molten chocolate is mixed with the course grained, but normally loose, cereal which, upon cooling, binds the particles together to form a structurally coherent, and significantly stronger, material. The clay:sand mixture is exactly the same. The sand is equivalent to the cereal particles and the clay, the chocolate. All the clay does is bind together the sand grains. It is the sand that creates the strength in the oven, as well as giving it it’s excellent thermal properties. Sand is silica, which is what glass is made from and glass, as we know, has excellent thermal conductivity and radiant properties. It also has an extremely low coefficient of thermal expansion which is why it prevents our ovens from cracking when fired to high temperatures.
So the next time you need to purchase clay to build an oven buy the cheapest stuff available. It really doesn’t matter what type of clay it is. For the purposes of building outdoor ovens of this type, clay is clay is clay.