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peterthewolf 02-03-2011 12:13 PM

Wrong dimensions - Help!!
Hi all-

We used the great fornobravo plans to build a 40 inch oven with firebrick. Our stonemason, who has done such wonderful work for so many years for us, said that he'd use the plans and had a lot of experience to create a masterpiece - especially because back in his village when younger and since, he said he'd built a whole bunch of hornos.

We referred to the plans and he got going. I was around a lot; building is not my skill set, so I acted as his laborer with cement and such. I checked in with him periodically, twice asking if it was really ok that his dome was significantly vertically steeper and higher (like a high beehive) than the plans showed in the pictures. I mean, he made it peaked inside at 30 inches high inside the dome. Forward of that we didn't just do a door; he built a tunnel 15 inches high, 20 inches wide, and about 20 inches long from oven floor to door, and a hinged fabricated steel door 1/4 inch thick that swings outward, hinge attached directly to the front face. For the distance from the dome floor to the door through the tunnel, there's an arch that is 18 inches high and at it's peak is an 8 inch hole that goes up to the chimney. Addition: later in the process for some reason I do not know, he did not attach the door hinges directly to the face but instead had me go back to the fabricator to weld a rod through them such that it would swing on a small friction point at the base, down inside a pipe to be included in the outside door cooling shelf area. I do not know why, but when it came time that he installed the door, he then had to build a brick structure on the top end into which to put the pipe that protruded upward from the door instead of attaching it to the wall as planned. It is a bit crooked in brick-laying, sticks out on one side of the oven front, and we have since had to hire a metal fabricator to come out and face it and it's opposite side in symmetry to disappear that.

Bottom line is that it seems to take 4 hours and 1/5 cord of split oak (by my IR gun) to read 500 at the dome peak (not white, yellow, red or any other color but slightly soot black up there) while the floor pushes 250 to 300 max. I was able to get 500 on the floor once, but that was 5 hours firing and a TON of wood. *Help*!! Can someone help about this??

Ok, 3 things to look at which have so far been suggested by people we've had look at it:

1. Lower the dome inside say, 8 inches? Options we've thought of include bolting a round or octagonal ceramics kiln refractory shelf up there, stuff some hi-temp kiln insulation blanket material (or pump some castable refractory) up behind it. Not a good idea, as with heat and flexion we would be breathing exiting fibers. Another was castible refractory material to fill the void behind it. Similar concerns; we do not know what might escape to be breathed while tending the oven. Still looking or material ideas there. That (would) reflect heat down mostly, not absorb too much. It's surface would be flat, not domed, but then that's not really that much interruption of the dome up there. Heat (would) come up to it for reflection from the heat absorbing walls, so that might work? Other ideas on that?

2. Curing:
I had not yet found this forum, so I didn't know to do slow firings gradually. He advised us to wait 2 weeks, then light up a big curing fire... Good news is that I did, and there was only a little cracking on the top of the dome; not worried about that. But man was there ever moisture blowing out of that brick in the front face while that thing was going. Steaming, even weeping water. Uh-oh I thought, this oven could blow or crack, but it was fine. *Then* I found these forums, and now 2 weeks later, learned to slow fire. Just to be sure, I have done the successive firings as I have found suggested reading in other areas. But again, it takes quite the fire even to get 200 floor when it's 500 above. FYI, at current proportions, the base of the smoke layer over incoming air through the door is about 12 inches high off the floor, which over a reasonable fire of say 3 pieces of split wood, the smoke layer sits about 5 inches above it.

*If* we close the door (it has 1/4 inch air leakage all four edges when closed) but leave the chimney open, there may be slightly more heat, but only 30 degrees or so. If we put a piece of sheet metal on top of the chimney to completely close it and let the door do *all* air/exhaust exchange, the smoke stays at same levels but looks richer of course coming out the top of the door, but again it still doesn't seem to affect the temp really.

So. (I) - not an engineer's mind - think that we've just got a lot of heat rising too far up, and bouncing around the high dome peak, and then exiting with smoke where it should have instead have heat-saturated a lower dome that would then be blasting it's head straight down into the floor.

Can someone please help on engineering insight here?

Thanks so much!

RTflorida 02-03-2011 01:31 PM

Re: Wrong dimensions - Help!!
you have not mentioned insulation. Is there insulation on the dome AND under the hearth? All of the re-engineering in the world will not help without proper insulation.


GianniFocaccia 02-03-2011 04:06 PM

Re: Wrong dimensions - Help!!
How about a pic?

texassourdough 02-03-2011 05:11 PM

Re: Wrong dimensions - Help!!
For a start you are almost certainly still wet. The dome issue is not wonderful but it is not why you are not heating. Tell us about your insulation (or lack thereof). The dome has almost nothing to do with your having a cold hearth. How thick is your oven, how much refractory do you have.

And you want a door that SEALS with NO gaps so you can keep the steam and the heat in the oven.

peterthewolf 02-03-2011 06:13 PM

Re: Wrong dimensions - Help!!
Hey all-
Wow, thanks for getting on this so fast. Ok. First, he did follow the plans for insulation. He made a 2 inch reinforced concrete base. On that he put 4 inches of vermiculite with 1/5 portland cement (into a wood frame which after formed we removed leaving the slab). Then on that, about half an inch of mortar. Then on top of it, regular (hard yellow) firebrick floor, flat sides up without mortar between them.

For the dome, we had brick of course. They were done brick and mortar with 3/8 inch to slightly thicker mortar mix between bricks. As I recall, he broke a number of bricks in half so that when stacked it looked less "toothy". But of the mass, I think it worked out about the same thickness of brick that leaving them whole would have been. Mortar wherever applied was of 6 shovels sand, 2 shovels portland and 1 shovel clay - as was the reinforced slab mix). Then outside that, about 3 inches (minimum) of the same vermiculite. Over the vermiculite (for casing) he used Sonoma fieldstone. It's a pretty non porous volcanic rock. Each stone was 1/2 head size to head size. Grout between the rocks was the same mixture. Of course since the stones are semi-round to whatever else, there were big voids between each stone so lots of cement mix was put down into between each rock. Then it was finished outside with about one inch grout lines between stones on the outer decorative surface. So in total the dome walls are nearly a foot thick I'd guess, half of it being decorative outside the insulation. I think the insulation job was pretty good because on the night of the big pizza party at the peak of the big "firing" I went above to mess with the chimney flue. The dome was only warm to the touch, which really surprised me. That was after at least 4 hours of firing all that wood inside.

Texassouourdough - yes I've been looking at this door seal issue, thinking it might be simple enough to use hi-temp wood stove silicone caulk to bead out the brick into which the door swings shut. Now, baking, I hear you need to close the door. But we were inspired by a friend's oven built from these forno bravo plans, to build ours when we attended a pizza party there - all pizza cooking was done with the door open and that sucker was *hot* right at the door. With a tiny fire going, pizzas were flying out of that oven in about 90 seconds.

Photos - good idea. Not sure how. Go slow folks, I have other skills... Hopefully there are two attachments here, here goes...

peterthewolf 02-03-2011 06:14 PM

Re: Wrong dimensions - Help!!, who can tell me how to post photos?:rolleyes:

Faith In Virginia 02-04-2011 04:58 AM

Re: Wrong dimensions - Help!!
One of the things noticed is you dome height to entry height. 30" dome and 15" entry is 50% the magic number for WFO's is 63%. So you may not be getting enough air to get the fire you need.

My oven has lots of thermal mass 7" all the way around so 4 to 5 hours is not unusual but I always start with a slow small fire to start.

I'm also wondering about the shape of your dome and how the air flows.

texassourdough 02-04-2011 05:09 AM

Re: Wrong dimensions - Help!!
There is no question that having the dome be too tall creates a dead spot and impairs heating but it sounds to me like there is something else going on. You are almost certainly very wet. Vermiculite takes a long time to dry out. One big fire, and a bunch of little ones aren't nearly enough in my experience. Doesn't sound like you are mass challenged. Get a small fire going and keep it for days - slowly building it up. You have to push the heat in to get the water out.

Good luck!

dmun 02-04-2011 07:12 AM

Re: Wrong dimensions - Help!!
Smells like wet insulation to me. I think that has to be addressed before we start considering any fine points of dome proportion. Vermiculite concrete holds a LOT of water and that all has to be driven out. It also has the ability to re-absorb water so it has to be kept REALLY dry.

Photo uploading

peterthewolf 02-04-2011 11:47 AM

Re: Wrong dimensions - Help!!
dmun&Texas - thanks. Wet makes sense, especially because we had it pretty soupy going into the slab. Less so for the dome layer. So firing: Last 2 nights I lit a fire before dinner, kept it going a few hours, capped the chimney and shut the door for a long smolder and went to bed. It was 150-170 this morning with small coals, just enough to throw some little sticks on to get going, so I started it back up and will fire all day until bed, same deal tonight. A few days? I'll keep you posted. Would I be correct to see more heat generally as water leaves? As for drying and keeping dry, other than cement seams between those rocks, it is *rocks* outside. Perhaps this means I'll actually be slower than say, stucco or something because these are not porous rocks. I'll pay attention to that and fire longer. Do y'all light up a small one during rainy seasons to maintain a level of relative dryness?

Faith- The dome is quite vertical, and pretty pointy at the top. I'll post photos separately. As for bringing down the dome, what do you folks think about pushing a round ceramic kiln shelf up into the point of the dome and bolting it in place with insulation behind it? Does anyone have a better idea? *Certainly* heat is wanting to go up, up , up, but it does get lost up there, and then wanders out the door within the smoke layer. I can tell this because it's twice as hot on the surface of the top of the arch that contains the chimney where smoke is exiting the dome, as it is on the oven wall for two feet up. I realize that putting a flat disc shaped thing up there wouldn't be "dome" anymore, but it certainly would force flame and heat to make that change to a more horizontal path, a (more) circular movement then just going up. The only other ideas we had were to have a metal worker we know make a new dome top in pieces of half inch steel, then crawl in, stuff insulation up and bolt the metal dome insert together in place in there. That's metal, is expensive, and could warp. I do not see how brick could be used, or even a patching fill-plug of clay-cement that holds to bolts we drill in first to stick it onto. That would not stay up in the making without building an elaborate form...then there would not be room to get in and put the material in..scratch that one. Back to the shelf idea...anyone have a better idea? At least by doing something related, we could get this 63% going.

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