Cracks in my dome! Need some tips

[sirvoice]

Hi all,

I know this is mostly about brick builds but i hope you can help me with some questions in my cob dome.

We have a fairly big crack tracing from the chimney, into the arch and also back into the internal dome a bit. As you can see it's big ish and as it's not fully dried yet (and we have not even started firing!) will probably get a fair bit bigger.

Should i try and repair this? Would heat LAKNO mortar work? Reckon this is a structural issue or will be fine?

Thanks so much.

[david s]

Repairing cracks in dry clay is invariably not 100% successful. It’s like a potter attempting to join a new handle onto a dried clay mug. The dryer the clay the less likely the bond. Any new wet material does not like to bond to the dry sides of the crack and as clay shrinks, the new material will pull away from the gap it is pushed into. You can try encouraging water into the crack before filling it, but it will quickly be absorbed into the dry clay that surrounds it. Some vinegar applied to the inside of the crack will assist bonding. The material, usually clay, needs to be forced in hard into the crack, thentry to keep the area damp with some wet hessian or similar for 24 hrs. You could try filling the crack with another product that will have less shrinkage, but usually the same material is a better solution. Good luck.

[sirvoice]

Thanks David! Do you think the crack will be a structural issue?

[david s]

No, not really, a dome is a self supporting structure. It looks like you have worked the cob mixture directly around the flue pipe. Because of the shrinkage of the clay against it there is considerable stress on the cob mixture in that are. Additionally on heating the very conductive stainless pipe will expand further before the cob that surrounds it gets hot. This results in even more stress and often leads to a crack. It is a good idea to allow for this expansion by wrapping some cardboard around the pipe when working around it. Remove the pipe before allowing the cob to dry, then replacing it minus cardboard, so it’s a slightly loose fit. This then allows for some expansion. Maybe do this for your next oven.