Happy Pizza Oven Builders

Last month’s Delicious Magazine article has already motivated some folks to have a go at building their own ovens. You can see a couple of readers attempts in this follow-up article. 

http://www.deliciousmagazine.co.uk/articles/meet-the-readers-who-built-a-pizza-oven-after-reading-our-how-to-guide

Fantastic work. Anyone else built an oven recently that they’d like to share with us all? Drop me an email or pop-by the Facebook page and tell us all about it (and send some pics obviously!).

I hope you have all had a fabulous summer. Role on next year!

Simon

Oven Building Course, September

I’m running another oven building course at the  Sustainability Centre, nr Petersfield if you fancy coming along.

This next course is on Saturday 13th September and costs £75.00/person for a whole day. This is a hands on (and hands/boots dirty) course. You will learn everything you need to know to build your own oven and experience for yourself making and eating delicious pizzas for lunch, cooked in the Centre’s own clay oven.

YOU ALSO GET A FREE COPY OF MY “HOW TO BUILD” BOOK.

Full details can be found here:
http://www.sustainability-centre.org/docs/course_id_179_course_pdf.pdf

To book a place or for further details please contact Sam on 01730 823166 or email her at courses@sustainability-centre.org

Places are limited and are selling out quickly so call soonish to guarantee a place.

Here’s some photos from our most recent course, earlier this year:

http://clayoven.wordpress.com/2014/05/18/another-course-and-some-volumes/

It’s a great day out (even though I do say so myself!)

Hope to see you there.

Simon

Cracks!

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.

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.