Crack!

[CvC]

Hi there,

after almost 5 months of building my 47"-oven, I started my first curing fire on Sunday.

The dome was completed one week earlier and I thought this was enough time for the mortar (I used home brew) to develop some strength.

The first fire was no problem, about seven hours at low temperature.

Also the second fire was o.k., something around 8 hours at just a little higher Temerature than the first one.

The third fire again, was just slightly bigger than the one before and lasted for about seven hours. After this fire, I recognized some hairline cracks on the dome, but didn?t worry too much about it.

After the fourth fire, I noticed a monster of a crack on the dome, it started 20 cm from the highest point of the dome and went down all the way to the first course of bricks. It was so wide (around 6 mm i.e. about a quarter of an inch), that I could look into the oven at several places.

The bricks are all still in place, but I?m really concerned about the structure of my dome.

O.k., maybe the last fire was still too hot, but what can I do about the crack?

[Aegis]

Re: Crack!

After the fourth fire, I noticed a monster of a crack on the dome, it started 20 cm from the highest point of the dome and went down all the way to the first course of bricks. It was so wide (around 6 mm i.e. about a quarter of an inch), that I could look into the oven at several places.
The bricks are all still in place, but I?m really concerned about the structure of my dome.
O.k., maybe the last fire was still too hot, but what can I do about the crack?

Hi CVC, Don't sweat the crack, I have a crack that may rival yours and it has been almost a year without further damage. My oven works fantastically, it is stable, and no further problems have resulted. I did a "repair" of the crack, by grinding out a v notch on the outside and is the widest part filled in with a thinly mixed homebrew and firebrick shims. The rest was done with just homebrew piped in with a corner cut off a plastic bag, and then pushed in the best I could with a trowel.
This was more for keeping the heat in the oven than any structural support. The dome shape is almost self supporting and needs little help from mortar.
With that said, try to figure out why/what made that crack. I had made my walls around the hearth, or the hearth is inside the wall rather than the wall sitting o top of the hearth. I left an 1/8 inch gap between the hearth and wall, but failed to remove some firebrick chips, debris, from that expansion joint. Therefore when I heated the oven the hearth expanded and pushed on the walls until a crack gave some pressure relief. Please post pics of your problem and give some more details as everyone can learn and help. Here are a couple of pics of my foolishness!

[david s]

Re: Crack!

When you drive the water out of the oven the top of the dome dries out way faster than the bottom because heat rises and with wood it is difficult not to get direct flame impingement on the top of the dome. Once the bricks are dry they climb in temperature even more, resulting in a huge difference in temperature between the top and bottom of the dome. Temperature difference means expansion difference and t his is what does the damage. To make matters worse if you do your curing fires before the dome is insulated the outer surface of the bricks will be cool while the inside surface is getting hot-also not good, more temp and expansion difference. If insulated the bricks will heat more evenly, even though the insulation may be getting moist and not as effective. I've found firing for 24 Hrs and not exceeding 250 C works to eliminate enough of the water so the oven doesn't crack. But every oven is different so it's hard to make rules.