how big are cracks that cause a problem? and what to do about them?

[hulkiebear]

I have fired a couple of small fires in my completed dome, and last night I fired it up with a decent size one, not too big though, only with construction type timber.

Now, before this fire, I had some small hair line cracks, but during this fire, a couple of much larger cracks appeared, maybe 1-2 mm wide. They closed up slightly on cooling, but are still there. Obviously I expected some movement with the expansion with the heat, and whilst I don't see the cracks as effecting the structure(yet) I didn't get the temp of the oven up anywhere near the level it needs to be at. the dome had ben drying for well over 2 weeks, as I was building the chimney front and on holiday.

So, how worried should I be? and what can I do about it? I'm thinking, chase out some of the mortar and repoint the effected areas. or just leave it and hope for the best. My main fear is for them to become sturctural or to leak smoke, any advice?

I will try to post some pictures later!

[hulkiebear]

I went down the route of chasing out the joints, and repointing with a home brew mixture of 2-2-1-1 of FB dust, sand, lime and cement. This seemed to make a ver nice mixture, that was very workable. I kept it wet(well, the rain we have had in the UK for the last week did!) and then today, after a couple of days of drying out, I fired the oven again, only a small fire, but the crack did not reappear, and when the fire was partiularly smokey(due to the fact it was a very small fire and the wood was a little damp) a few smoke leaks at the back of the oven showed themselves, which is good as I can deal with them before I insulate and render!

It's good to be back on track and nice to seem the flames licking the ceiling of the oven, can't wait to get the bad boy upto temp!

[Neil2]

"maybe 1-2 mm wide"

Not unusual at all. Don't worry about them.

[Wookie]

It is easy to say that cracks will almost always appear, at some stage. If the dome has the thermal mass and insulation according to the FornoBravo guidelines you should have no trouble with the integrity of the dome. Cracks can add a bit of character. I agree with the previous sentiments, don't worry about them and spend your time enjoying the food. Forget about the oven looking faultless. Not many of us are masons, bricklayers or even trades people - just ordinary folk having some fun! Enjoy