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.

 

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11 thoughts on “Beware of the Clay!

  1. Hi, we have now finished our oven using grogged clay which we ordered online. Have only just read this bit of the blog😬 Otherwise we followed your instructions. We kept the sand in until the final layer was on, we are drying it out very very slowly and filling in cracks as we go, the sun really brings them out so we have now got a tarp up and are going to let it settle for a good month before lighting any fires in it. There is SO much conflicting information out there, and when I found ‘Pizza oven clay’ I thought my prayers were answered. Do you reckon our oven is doomed?!? Any tips for its survival?!?

      • We used it straight out of the bag, ‘pizza oven clay’ from the clay cellar and was assured when I asked that we wouldn’t need to add anymore sand. It has got quite a lot of sand/’grog’ already in but definitely not 3:1 ratio, I just wanted to check it wouldn’t completely fail!!

  2. I have built my third attempt at a clay oven. The first was pure clay (we got excited, I nkow I know) and completely cracked while still on the dome. The next was a mix of clay and sand and it also collapsed. Finally i got my third mix i thought right, cleared it out and a few days later it also collapsed.
    Any thoughts as to the culprit? Walls to thin? Too much clay? Too much sand? heeeelp

    • HI Pamela. Sorry to hear this – it sounds like a nightmare. If you follow the instructions on my blog (or in my eBook) you should not have any issues at all. I have never had a collapse in many years of building and teaching others to build the ovens. Make sure you use the correct ratio of clay:sand, the correct thickness of layers (7-10cm) and leave the first layer to dry somewhat before removing sand.

      I’m not surprised that the clay only version cracked though!

  3. Hey Simon, I wanted to thank you for your blog. I built my oven 3 years ago and it was a very successful project. I have cooked in it often since then. I work at a pottery supply company in Canada and we have always had a lot of people coming in looking for clay for various oven projects. While doing some research I came across your blog and I liked how you demystified the process and made it less intimidating. I was inspired to make my own oven following your design and having gained some valuable experience, I have a lot more confidence when I am asked for advice at work. I agree that the first clay that would pop into the head of a person with pottery experience would be one that is heavily grogged. Grogged clays are nice for building large, sculptural forms. But, yes, an oven is a different sort of animal.
    The most valuable things I learned from your blog and from building my own oven are 1) Use whatever clay you can get your hands on. I used a variety of different clays that I could get cheap, both moist and dry, and they all fulfilled the function they needed to. Follow the general rules about how much sand and or wood shavings to add and you’ll be fine.
    And 2) Get over perfection. Your oven will develop some cracks. It will occasionally require some repairs or maybe some modifications. That’s okay, it’ll still work great.

    I talk to lots of people who are looking to build an oven and everyone has a different design. I think there are many approaches. For me this oven is simple, low tech and it works beautifully 🙂
    Thanks again!

  4. I used grogged clay but also stayed with Simon’s sand-clay ratio…in fact I think I went with even more sand than he recommended because it didn’t feel quite enough yet. I am now going on 4 years with no internal cracks. The external layer cracks fairly severely (few cm wide) but they reduce when cool. I dust with water after use to smooth them out. The cracks may not look pretty, but haven’t caused any sturctural problems yet.

  5. Oh dear!

    I wish we had come back to your blog a couple of weeks ago, rather than hunting it out now to find out why we have such massive cracks in the first layer of our clay oven. I was hoping it was the heatwave that had dried it out too quickly but it looks like we’ve fallen in to the pottery sellers grog-trap, totally innocent on their half I’m sure – at worst probably just naïvety on their part and ours.

    We’re going to have to start afresh and the puddling just can’t be avoided. I don’t think the other-half will be best pleased with the news. At least we’ve caught it now before we get too far down the road!

    Thanks for your excellent blog, so much excellent information here.

    • I feel your pain Sam! Never mind though, it won’t take that long and you’ll soon be making delicious food and wowing your friends. Don’t forget to cover when drying the first layer in hot weather.

      Best of luck

      Simon

  6. Pingback: Cracks! | The Clay Oven

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