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.
The Clay Oven 
Hi, I’ve built a pizza oven pretty much following the method. As it’s drying out the clay seems to be shrinking back. I’ve fired it twice building temp slowly to dry out then properly today to attempt cooking. We’ve cook some great pizza but notice the smoke coming from the side of the arch between the dome and the arch. Should I just fill this again? It’s also coming away round the chimney to the arch as well but I guess this would fill?
If so should I use just clay to fill or mix it with sand.?
Any advise would b greatly appreciated.
Thanks in advance
Michael
Sorry for the delay, Michael. Yes, just fill the cracks. Always keep a bag of clay:sand mixture at hand for minor repairs.
hi, if i may add on the cracking issue. I worked out that the location of the oven is (was, in my case) an issue. during the natural drying process, 3/4 of the oven was exposed to the sun and the rest in the shade. although the oven was protected with old bed clothes, the quarter not receiving sun, did not dry out at the same rate and started cracking. Not bad enough to have to re-start but enough to distort its shape (first layer only I should add) patched up and recovered keeping the “sunny side” damper to allow the same rate of drying.
Hello Simon. I followed your guide to building an oven. The only clay I could find at the right price and in a dry format was bentonite, which is a local clay where I live in Colorado USA. Lots of cracks which I tried patching during the drying process, but what is more of a concern, is the inside of the oven surface is very sandy, and doesn’t look stable at all. Any suggestions?
Did you use a mixture of clay:sand?
Hello,
I’m looking forward to building such a lovely oven in our garden this year but now I’ve read that clay ovens aren’t lasting very long if they are exposed to rain/sun and because of the constant expanding and shrinking. What do your mean?
Thanks,
ines
Hey Ines
These ovens are made from natural materials, no concrete, no cement. If you left them uncovered in the rain for a long time, they would wash away like a snowman! – 😦 So they need a little bit of tender love and care. However, if you keep them covered from the worst of the weather, and patch up any major cracks every now and then, your oven will provide you with delicious food for a very long time. My oven is 7 years old this year. Yes, if you look back over the posts on this blog you will see it has come close to destruction on several occasions but that’s because I’m a bad oven owner. Remarkably she is still standing and works perfectly after all that abuse. There are also things you can do to help alleviate the major problems, like building a roof and covering up with a tarp in winter.
I’d say definitely build an oven – it’ll be on of the best things you’ll ever do!
Best of luck
Simon
Hi,
Firstly thanks for your fantastic blog , that has enabled us to construct our clay oven. It was a real family affair involving the kids, grandparents, cousins etc lots of messy fun had by all!! It’s now finished and looking good , but there are cracks appearing as it dries . I know you said not to panic and i’m trying very hard not to. I’m just wondering is it best to let it dry completely Before filling the cracks and when do you advise using the oven for the first time, it’s been drying for 4 days now and hardish but not changed to lighter colour yet.? The kids are being impatient and just want to know when they can eat pizza 🙂 any advice welcome.
Thanks again.
Hi Jacquie, thanks for the comments. It’s always nice to hear that family’s have had fun building an oven – thats what its all about! I’d start filling the cracks now. You can fire it anytime really but you will get more cracking if it is still damp when you do (you’ll get cracks anyway as the oven heats and expands but it’ll look worse if it’s damp). It’s up to you. Most of the cracks will be confined to the outer layer and around the chimney area and are extremely unlikely to result in serious failure. If the weather is fine, you could start a gentle fire inside to help dry it out (keep it going gently for several hours). Keep a bag of clay:sand at hand, fire her up, don’t fret about the cracks, then fill them in again afterwards. Finally don’t forget to cover the oven when not in use to protect it from the rain.
Send me some photos if you’d like to see them on the blog. Have fun!
Simon