Cob oven when should i start to take the sand out?

[Tboney]

Hi
I just wanted to get some advice on when i should start taking the sand that i used for my mold out of the dome? I completed building the dome 1 week ago, Should i leave it longer or start to take the sand out now? I do not want to rush it and cause problems down the line. Any advice will be greatly appreciated.

Thanks

[david s]

If your mix is just sand and clay then it does not need any damp curing, but it will also shrink a bit as it dries. Because the sand mould is in the way this could lead to some big shrinkage cracks, so the sand should be removed as soon as the clay is firm.

[Tboney]

Thank you. I took the sand out yesterday and the dome is firming up but can still be impressed when u put pressure on it. When should I start to build small fires in it?

[david s]

That all depends on your weather conditions, but as a rough guide one week per inch of thickness.
It may look dry on the surface but if the centre of the walls still contains moisture and it suddenly turns to steam you will split the clay in the centre quite violently as the steam expands. This takes place at approx 200-300C so take it really slow.

[Tboney]

Total thickness is about 7 inches I'm in the UK so its damp and cool at the moment few dry sunny days mixed in. So 1inch per week do you mean I should wait about 7 weeks before I start to put small fires in?

[david s]

You can hasten drying with fire, but try not to let any flame impingement on to the dome. When you think it’s dry throw some plastic sheeting over it to see if moisture is still condensing on the underside.

[Tboney]

I started small fire well it started out small but it flared up and it took a little time to dampen it. A crack quite deep is running along the top of the interior of the dome. I think it was way to damp inside. I have put a very small camping stove in jus at a very warm temperature until I can put a proper fire inside. Should I repair the crack now I'm worried about it, or should I wait for the oven to dry out completely?

[david s]

Filling the crack with more cob mix will not be successful if it’s dry, as a wet mix will not adhere to the dry crack. Fill it ASAP.
You will have to wet down the crack with a paintbrush and water then force the sand/clay mix hard into the crack. Leave it for a min of a week before any more fire.

[Tboney]

Shall i use cob mix or outdoor oven cement? Thank you for all your advice. Once i have filled it shall i continue to use.the small camping stove on very gentle heat or leave that out also.

[david s]

Shall i use cob mix or outdoor oven cement? Thank you for all your advice. Once i have filled it shall i continue to use.the small camping stove on very gentle heat or leave that out also.

I’d be using the same mix used for the dome rather than oven cement. I’’d be leaving it alone for a week before starting the stove treatment. Charcoal or barbecue fuel works well too. Just try to avoid direct flame impingement.

[Tboney]

[Tboney]

Pictures above show the cracks in my oven I tried to repair them but after repairing and leaving them for a week the cracks reappeared. Is the oven going to be a waste of time I'm really worried that I have wasted my time. Advice appreciated.

[Tboney]

The outside has no cracks.