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!

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2. The Plinth Foundation, Plinth and Brick Oven Floor

The Completed Plinth

The Completed Plinth

Your plinth has two main functions.  Firstly, it raises your oven to a height that makes it practical to use.  Imagine trying to slide pizzas in and out of the oven on a baker’s peel if the entrance is at ankle level. You would end up with a bad back and a charred mess in the oven!  Secondly the top of the plinth forms the all important brick base to your oven.

Materials for plinth construction

Your choice of materials is dependant on personal taste, availability and budget.  If you scour around the Internet you will find plinths made from natural stone, bricks, cob, wood and earth and wood alone (like mine).  No one solution is better than the other so take your pick.  I am going to show you how to build a simple but beautiful plinth made from wood.  I was lucky enough to get hold of some oak beam off-cuts from a local timber merchant.  They are very attractive and very heavy!

Plinth Foundation Construction

If you are going to use a construction material with substantial weight from which to build your plinth and are building on soft ground you will need to establish a foundation first.  This is pretty easy.  In my case I dug a 40-50 cm deep hole slightly wider and longer than my intended plinth dimensions (120 Cm x 120 Cm) and filled it with hardcore (rubble) to a level just below the top of the hole.  I then topped this off with a layer of builders sand and finally laid a paving top onto the sand.  Continuing with the spirit of recycling, I managed to get hold of some broken slabs which I laid as crazy paving to form the flat level of my plinth foundation.  You can of course use other foundation methods such as concrete.  If you are lucky enough to have a chosen a location with a solid floor on which to build your oven then you can skip this step.

Plinth Construction

So you have a nice flat and solid base it is time to start building the plinth.  This is where building with timber comes into its own because it is just so simple!  Before you start building though you need to think about dimensions and this will involve a little forward planning. 

Plinth Dimensions

Plinth Dimensions

The plinth needs to be wide enough (and as your oven is going to be circular I suggest you make the plinth square) to accommodate all three layers of your oven.  The most important measurement is the diameter of the first layer – the inside of your oven.  You need to decide what is practical for your needs.  If you only ever intend to bake pizzas in it then it could be quite small but if you want to use it to roast legs of lamb or pork joints you need to make the oven layer wide enough in diameter to accommodate a roasting tin.  The internal diameter of my oven layer is 80cm which therefore means the brick floor on the top of my plinth is also 80cm x 80cm (you wouldn’t want any part of the wooden walls to be inside the oven when you fire it!).  Working out from here, each of the 3 layers should be approximately 7cm thick, so the total thickness of the oven walls Will be around 21cm.  Add this dimension to each side of your brick floor width and you will end up with the correct dimensions for your plinth.  Phew!  So my plinth top dimensions are 120cm x 120cm

Once you have your dimensions sorted, construction of the plinth is very easy.  Simply cut the timbers to the correct length, not forgetting to leave enough space for the overlapping end of each side.  Then build the sides up as if building a wall – overlapping the lengths of wood to add strength to the structure.  You shouldn’t need to use any “cement” between layers if you are using heavy timbers like mine.  Try to make the top layer as level as possible as this will help when trying to level the brick oven floor.  I used four internal corner angle brackets on each layer in order to prevent the structure from moving out of shape.

Plinth part-filled with rubble showing internal structure and angle brackets.

Plinth part-filled with rubble showing internal structure and angle brackets.

Your next task is to fill the plinth “box” with rubble (hardcore) to a level of about 30-40 Cm’s below the top. 

After this stage I decided to add a layer of builders sand followed by a layer of glass bottles (whole with tops removed).  My reason for doing this is that the glass bottles should provide an extra layer of insulation and retain heat below the brick floor of the oven.  I have no idea if this actually works but I had loads of bottles lying around so thought I’d give it a whirl!

Finally add another layer of sand up to a depth below the surface of exactly one brick deep.

Brick Oven Floor Construction

This is the last stage in construction of your plinth.  The brick floor is obviously a critical component of your oven so I suggest you take your time with this and do it right. 

You’ll need some bricks, something to cut them with and some more builders sand.  I used bog-standard London bricks and a hammer and bolster to cut the bricks (although I’m not very good at this and broke quite a few!).  Use any bricks you can get your hands on but try and look for ones with a nice flat surface, without cracks and any that look too porous.  Bricks with hairline cracks will break when you come to cut them or later when they are fired in the oven.

Laying the brick oven floor.

Laying the brick oven floor.

All you need to do then is fill the top layer of your plinth with bricks.  I chose to use a herringbone pattern which holds together nicely without cement.  Remember to try and make the oven floor as level as possible. 

Once you have laid all of the bricks in place brush handfuls of building sand into any gaps to prevent any further movement of the bricks.

That’s it!  Your plinth is complete and ready for the next stage – building the oven!