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:

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.



Another course and some sand:clay volumes

Happy May 2014 Cohort

Happy May 2014 Cohort

Yesterday I ran another clay oven course at the Petersfield Sustainability Centre for 18 lovely people. It was a glorious sunny day again and everyone seemed to have a brilliant time. I certainly did.

During the day I asked one of the crew to make a note of the volumes of clay and sand we used (I’m not great at recording this stuff normally so thanks so much for doing this). Here’s what we used.

NOTE: 1 bag of builders sand fills a 15 litre bucket.

Sand Former – 10 bags of sand

First (oven) layer – 4 bags of sand, 3 buckets of clay (note that is a ratio of 2:1.5)

Brick arch, backfill and chimney – 2 bags of sand, 2 buckets of clay (we used a clay rich mix which made the material easier to manipulate). Also used 2 bags of sand to use as the former around which the arch was built. 11 bricks were used to make the arch.

Insulation Layer – 3 buckets of clay, 2 large bags of woodshavings.

The thing worth noting here is the ratio of sand to clay. Normally I prescribe a ratio of 2:1 but this is a rule of thumb. You need to aim for a material which holds firm, is not too soft (or wet) and not too dry such that it can’t be moulded very easily. If it is too wet (or if there is too much clay) it will slump around the base of the layer you are building. Clearly the plasticity of the material will depend upon the moisture content of the clay and the sand you are using. The drop test will help. I’d say always err on the dry side, you can always add a little water (or more clay) if you need to.



Clay Oven The French Way

I really want to share this little video that Gill Connors sent to me which is a time lapse video of their oven build at their home in France. As you can see it is pretty much identical to my oven but with a few neat variations. The chimney is offset to the side slightly which they have been told is to help with stability. Although I’m not 100% convinced that this is necessary it does look rather cool. The other modification is that they have used a lime render (mixture of 2 parts sand to 1 part St Astier NHL2 lime). This looks great and is, apparently, weatherproof and breathable. It looks brilliant.

So there you have it a French clay oven. Voila!

Clay Oven 2.0

In a recent post I demonstrated what happens when a traditional clay oven is neglected and abandoned to the vagaries of a British winter. After waiting for weeks for the weather to pick-up a little, I finally managed to get out into the garden this week and got on with the long-awaited repairs. I thought you’d like to see what I have been doing.

First off, here’s a sad reminder of what the oven looked like beforehand.

Collapsed Pizza Oven

You can see that the brick arch and chimney had collapsed. You will also notice the house brick which I had placed in the hole which had burnt into the front beam of the plinth. This was always meant to be temporary but I had never got around to fixing it.


The first thing I did was remove  the arch bricks, the chimney remnants and then strip-off the outer-layer of the oven. What you can see below is the inner layer, around which is wrapped  the insulation layer.


I then removed the brick from the charred hole in the plinth beam and fitted a new, fire-proof tile in its place (I used a stone floor tile).


For ages I have wanted to make a wooden former around which to build a new brick arch. As you can see below, I made this out of a few pieces of wood offcuts which I marked-up and then cut with a jigsaw. The arch pattern will allow me to build a much neater arch but also make it much easier for me to make a nice, tight fitting oven door.


Next I fitted the arch pattern/former in place at the front of the oven. This required me to slightly re-shape the front of the oven layer which I did easily with a knife.


At this stage I realised that it might be tricky to remove the arch pattern once the bricks had been built around it, so I added a couple of brackets to the front which could be used as handles. I then built the brick arch using clay:sand mixture as mortar, making sure that the key stone, in the top-centre of the arch was positioned to take the weight of the arch either side of it. This is important, if you don’t do this the arch will collapse under it’s own weight.


You can see below how I then began to backfill the gap between the new arch and the oven using clay:sand mixture.


Another modification I have been wanting to make for ages is to add a ready-made chimney. I managed to get hold of a clay pipe fitting, the front of which I rested on the top brick of the arch and the back which I rested on the solid, oven-layer. I cut a couple of house bricks and fitted them snugly, either side of the chimney, in order to give it extra support and then packed the gaps out with clay:sand mixture. I think it looks rather splendid!


Here’s a close-up of the chimney.


Finally, I rebuilt the outer layer with good old clay:sand mixture and, voila!


Clay oven version 2.0 – done! What do you think?