Starting new 36" build

[deejayoh]

Re: Starting new 36" build

Update on the curing -
I lit up 6 chafing fuel containers on Weds, and 9 last night. The oven temps were up around 270 at the peak and 170 at the floor with 9 containers going. Outside of the dome was up to about 115. Tonight I'll light up 12. I am hoping it will get up over 300 degrees. inside, and maybe 150 outside the dome. As of yet, I haven't really saturated the dome with too much heat so I can't assess how much cracks could be an issue. Saturday will be the first fire - but nothing too big. I will be trying to keep it under 500 degrees.

In retrospect, the chafing fuel approach has a couple issues. The fuel itself is pretty expensive, and it doesn't get as hot as I thought it might. I've kept a plywood form over the door to try to keep the heat in, without that the temps don't go up as high.

[deejayoh]

Craaaaacks

Well, I got a little carried away with the curing tonight it seems. Friday, I lit a small fire and got it up around 450. Last night, a bigger fire and I had the oven up around 550 at the peak of the dome. My plan tonight was somewhere between 600 and 700. I had the fire going in there, and it was only about 450. So I threw a couple more pieces of wood on the fire and WHOOSH. Temperature at the dome spiked to over 800 in a couple of minutes.

Before I knew it, I had a crack running from the top of the dome to the entry arch. It started quite small but widened out within a few minutes to what you see in the picture. There are also some smaller horizontal cracks that showed up between the 3rd and 4th courses that are less concerning.

Biggest worry I have with this is that the crack goes straight through my arch. How concerned should I be?

[clg763]

Re: Starting new 36" build

Most people have cracks in their domes and arches, don't worry, they give an oven character .

[Amac]

Re: Starting new 36" build

Dennis
I am impressed by the strength of that mortar mix - seems to have split one brick rather that follow the joints.
I got a lot of hairlines mostly following the horizontal courses higher up the dome around and once or twice following a zigzag downwards but alwasy along joints. One followed out to the entry arch. It was the only one I tried repairing (but only because I could see it and it was irritating) by angle grinding the mortar out and replacing it. I expected it to come back at the next firing but it never did.

I think it won't cause you any problems and will act as an expansion joint. I wonder should all the ovens be designed with deliberate straight line or not mortared joints rather like you would do with a large concrete slab. Then you could point it out as a "design feature"

[SCChris]

Re: Starting new 36" build

As long as you don’t have loose bricks, leave it alone, at least for now. If you’re concerned about the arch and you don’t have buttressing you might feel better by doing a bit of buttressing.

It’s my feeling that getting a bit of semi-permanent insulation over the dome will help to keep temperature differences to a minimum and so the thermal stresses might also be kept to a minimum. If you find that nothing loosens up in the final curing fires, button her up and cook. If you feel like doing a bit of patching, pull the insulation back, patch, insulate and button her up..

Enjoy the oven, you’re right there..

Chris

[Tscarborough]

Re: Starting new 36" build

One of the advantages of a barrel vault is that the expansion joints can be built in. The front and back are built with slip joints, and thus no cracking. I did get some cracking on the concrete cladding, but they are shrinkage not thermal.

[deejayoh]

Re: Starting new 36" build

Thanks guys -
I am not going to freak out about it. The dome just seemed to want to stretch out. But it all seems solid. No loose bricks. I may patch it with some of the black goo furnace cement if it widens out at all. But when I looked at it this morning and it had shrunk back a bit.

I also need to poke my head into the oven now that it is cool to see if it showed up on the inside.

Assuming all that goes ok, I probably fire a few more times this week and then put on the insulation next weekend. I'll probably pick up some steel studs for framing as well.

[Tscarborough]

Re: Starting new 36" build

As a rule, you don't want to patch cracks on the inside at all unless they are big enough to allow flame to escape the dome.

[deejayoh]

Re: Starting new 36" build

Finally got the chance to inspec and the crack is visible inside the oven too - which is worrisome. But never fear, I am going to fire it up again tonight and see what happens. First pies with a sourdough culture dough!

thanks Tscarborough. I was thinking about a patch on the outside, not the inside - but I think I'll only go there if I see any widening of the crack with future fires

Chris - I probably should put the insulation on, but it's been such a crappy, rainy June that whenever it has been nice I've wanted to focus on curing!

Next report tomorrow after some cooking!

[deejayoh]

Re: Starting new 36" build

While I'm at it - here's my first draft of an enclosure design.

I am planning for a 12" overhang in the front, and 6" on the sides and back. Does anyone have any thoughts on the scale of that?

Also shooting for the lowest point of the roof trim to be at least 6' 3", as I don't want to be knocking my head on that. But that causes some issues with the height of my chimney at the top. Current view has an ~18% slope on the roof, in order to give the 2' chimney room to clear the roof. I think even at that, I may need to go back and raise the chimney up by a few inches to give me a little more clearance for the box. It needs to be flashed and finished - which will be tough if it is only 4-5 inches higher than the roofline.

[clg763]

Re: Starting new 36" build

Why are you only going with a 2' chimney instead of the recommended 3' minimum for your oven? I would think you should be trying to find ways to make your chimney higher rather than keep it short,

[deejayoh]

Re: Starting new 36" build

hmmm. Where are you seeing that 3' is recommended? I checked this a lot and used an 8" vent - which is I think where people have issues, but AFAIK 2' is ok for a 39" oven. 4 fires in, the oven draws fine. All the FB kits of this size come with the same 2' duravent pipe.

What I read is that chimney height is kind of a design choice, and that you should worry more about the diameter - but maybe I missed something.

[Amac]

Re: Starting new 36" build

Finally got the chance to inspec and the crack is visible inside the oven too - which is worrisome

You'll feel better after you listen to the sage of montreal.

".there is a crack in everything
that's how the light gets in.."

The song should be the oven builders "Anthem"
Anthem, Leonard Cohen, Sharon Robinson & The Webb Sisters - YouTube

[deejayoh]

Re: Starting new 36" build

Cranked the oven up again last night and got it to ~700 degrees. I had a pretty good fire going in there and no new cracks. It was clear on the side that I pushed the fire over to, but not at the top of the dome. I could see that the existing crack widened out a bit when the oven was hot. By this morning it had shrunk back.

I cooked a couple of pizzas, which came out just ok. Crisp on the bottom, but not with the char on the crust that I want. I need to get the oven hotter still - which I am going to try doing on Saturday if the weather holds. I am having some friends and family over and am going to crank it up, try to get the dome totally clear. I figure it's ready now for a real burn.

I would get some insulation on too, but I am still waiting for the stuff to seal my heat break. Turns out the place that I ordered from is shut down for vacation for 2 weeks and not sending out any orders. Took my money, but didn't tell me that part. Grrr. NWIronworks.com is the name of the place if you want to know where NOT to order.

[SCChris]

Re: Starting new 36" build

I'm sorry to hear about the insulation vendor. :-(

Do you have other options that make sense?

Chris