Just add water!

Hey, oven lovers! This is a tiny little post to show you that the clay:sand mixture we use to build these super ovens can be recycled (or rejuvenated).

As you will see, if you read my previous post, I had to do some pretty major repairs to my oven recently. Anyone who has gone to the effort of building one of these ovens will tell you that the most tedious part of the whole process is puddling the clay:sand mixture. It is hard work and very boring so, while you are going to the bother of doing it, I recommend mixing a little more than you need and storing the extra in plastic sacks. You will use it for filling cracks and maybe, one day, to undertake more major repair work. You might also need to partially, or even wholly, remove one of the dry layers, particularly if you abuse your oven as much as I have mine. Well don’t throw that dry stuff away, and if your bagged material is dry, make sure you keep that too.

All you need to do is add some water to it, leave it for a while and, hey presto, it’s as good as new. Sweet!!

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Beware of the Clay!

Rice Crispy Cake

Don’t try to build with this mixture!

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:

http://www.bathpotters.co.uk/earthstone-sculpture-pizza-oven-clay-es180/p5636

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.

 

6. The Insulation Layer

Entrance dry? Chimney dry? Great! Let’s start building the next layer – the insulation layer.

What you’ll need:

  • Some clay
  • Some wood shavings
  • Some water
  • A bucket
  • A wheelbarrow
  • A spade
  • A power drill and plaster mixer (optional)
The insulation layer complete.

The insulation layer complete.

First thing you are going to do is make a clay slip which is simply clay mixed with water. The simplest way to do this is to put some water in a bucket and slowly add chunks of clay, squashing, squeezing and mixing with your hands as you go. Alternatively you could add water to half a bucket of clay and leave it to soak for a couple of days. You could then squish-up the clay quite easily.  You are aiming for a consistency similar to that of thin natural yogurt.  

I spent quote a long time making my slip until I discovered a short-cut method using a drill and plaster mixer. It works really well but is very messy! Chuck your clay and water in a large bucket (or large bin) and blitz it with the mixer.  Job done!

Next throw some wood shavings into a wheelbarrow. I bought a huge bag of wood shavings from a local pet shop and I still have three-quarters left (any takers?). Add some of the clay slip and mix well with a spade or get your hands dirty. The mixture should be wet enough to form “bricks” similar to those you made for the clay-sand layer.

Build up the insulation layer using exactly the same technique as before. Simple! Leave it to dry and then you can move on to the last step in the build – woo hoo!

3. The Clay-Sand Mixture and Puddling Technique

So now the fun begins!  Before I get on to detailed instructions of how to build the oven I want to provide some details on preparing the all important building material – the sand-clay mixture.

Ingredients

  • Builders sand
  • Clay
  • Water (optional)

Equipment

  • Shovel
  • Bucket
  • Tarpaulin
  • Thick plastic bags
  • Wheelbarrow
  • Wellington or other sturdy boots
  • Legs!

You can buy builders sand from any building suppliers, some garden centres and DIY stores (e.g. B&Q). Either buy it in individual plastic bags or get a job-lot delivered – you will use quite a lot.  

What type of clay should I use?

I have had quite a few questions about the type of clay to use.  As far as I know you can use any type of clay you can get your hands on.  I dug my clay from a local farmer’s field here in Hampshire (clay overlying Upper Cretaceous chalk if you are geologically minded – maybe Paleogene?).  The team at River Cottage get theirs from a pond on site, in Dorset (I think it is Blue Lias).  If you can’t find any clay locally you could always buy potters clay which would be wonderfully homogeneous – free from large particulate matter.  Which reminds me, try to get clay does not contain too many stones – they are liable to form the focus of cracks in your oven if left in the mixture.  

Quantities

The ratio of clay to sand is 1:2 (one part clay to two parts sand).  I used a bucket as a convenient measure and found that one bag of builders sand almost filled two buckets which was nice!  

I think it is wise to make-up just enough mixture (with a little bit extra) to complete one layer of your oven at a time.  Why?  Well if you make up a huge batch (enough to complete your oven) it could dry out before you get a chance to use it if you get delays between layers (if it rains for example).  So how much do you need for one layer?  This will vary depending on the size of your oven.  For mine, if we define one “batch” as two buckets of sand mixed with one bucket of clay, the first (oven) layer took three batches (6 sand to 3 clay).  This left enough over to fill small cracks after drying and to begin building the chimney.  The outer layer required more mixture because it is covered a larger surface area.  I used four and a half batches for this layer.

Mixing or Puddling

Clay chunks distributed onto sand ready for puddling.

Clay chunks distributed onto sand ready for puddling.

Mixing the clay and sand is by far the most tedious part of the whole build process.  What you are trying to achieve is a well-mixed material with no pockets of unmixed sand or clay and the only way to do this seems to be by using your feet.  The process is known as puddling and it goes like this:

 

  1. Spread a tarpaulin out on a firm surface (double it over in case you get holes in it).  
  2. Tip two buckets of sand in a pile onto the tarpaulin and spread out a little.  
  3. Next, fill another bucket with clay.
  4. Take a chunk of clay and break it into small pieces (thumb sized) and distribute them over the surface of the sand (like throwing broken-up pieces of mozzarella onto a pizza base!).  Take this opportunity to dispose of any stones or sticks you might find in the clay.
  5. Puddling Sand-Clay mixture with help from my dog Scout!

    Puddling Sand-Clay mixture with help from my dog Scout!

    Wearing your boots, start to mix the sand using your feet (puddling).  The best technique is to tread and twist.  I recommend putting some music on and get into the groove! Seriously, you will look pretty daft doing this but you need to twist – Chubby Checker sty-lee!  This is where a group of friends come in very handy.  Many feet make light work of puddling sand and clay.  It is great exercise though so keep reminding yourself how much good it is doing you when you start to get knackered and bored!

  6. Keep mixing until the clay lumps have disappeared then add more clay chunks and get back to puddling.
  7. Keep repeating this until the bucket of the clay is mixed into the sand.

I found it takes somewhere between 45 minutes to 1 hour to mix one batch.  One tip I discovered which speeds things up is make sure the sand is damp before you start mixing.  It definitely helps the clay mix in better (essentially you are coating sand grains with clay and water helps break down the clay bonds I assume?).  Add some water before hand if you need to but dont go mad with it!

The last thing you need to do is check that the mixture is of the correct consistency i.e. not to wet and not too dry.  You might be surprised at how sandy the mixture seems but this is how it is meant to be.  The team at River Cottage HQ had a great method for testing the consistency though which I will share with you.  

  1. Sand - clay ball after drop test.  It held together just right!

    Sand - clay ball after drop test. It held together just right!

    Grab a handful of mixture and form a ball about the size of a lime.  

  2. Hold your arm directly out in front of your body at shoulder height.  
  3. Drop the clay-sand ball onto hard ground in front of you.

The ball should hold together quite coherently.  If it splats flat the mixture is too wet and you should add more sand to dry it out a little.  If the ball breaks to pieces (explodes) it is too dry and you can add a little water.  Simple!

Once you are happy that your mixture is just right shovel it into a thick plastic bag to keep it moist while you mix the next batch.

If you are ready to start building your first layer I recommend shoveling the final batch into a wheelbarrow for ease of use.  Wheel it over to your plinth – it’s time to start building your oven!