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.