Hi all, I built a cob oven recently, and when I removed the sand from inside I found some fairly substantial cracks inside (around 1cm wide, running for at least 40-50 cm each), which luckily do not appear to be structural. I have not yet fired the oven, what I would like to know is whether I should patch the cracks before firing, or just leave them be? Any assistance given would be much appreciated!
Yes, patch them with a mix that is heavy on the sand. It could be that your cob mix was not sandy enough to begin with, or a combination of not enough sand and not enough fiber.
1cm from the get go is a big crack IMO. If the original mix was not optimal, you may find that the over continues to crack as it heats and cools. Patching it will help, but it may never be as durable as an oven made with a proper mix. Again, assuming it is an issue with the initial mix.