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.