Full hemisphere arch?

[kebwi]

I've seen a few (two?) ovens in the gallery that have full hemisphere arches. I highly prefer that look. I'm trying to determine if this design is functionally less desirable however. In particular does it constrain the smoke from exiting the dome into the vent area?...or does it cause any trouble and is perfectly acceptable?

Could I make such a design with the top of the arch a little higher than 63%, assuming that the lower corners would average out the smoke's exit-path, or would such compensation not really help the smoke exit and still let too much heat out?

I would love for the people who built full hemispheres to pipe in and say whether their ovens work optimally and whether they have any regrets about the design.

Thanks.

[papavino]

Re: Full hemisphere arch?

Check this thread:
http://www.fornobravo.com/forum/f8/v...vice-6698.html

There are probably others you can find by searching a bit.

[kebwi]

Re: Full hemisphere arch?

Thanks for the ref. FB's forum search is a little weak. It won't accept close-quoted multi-word phrases for one thing.

Thanks again.

[GianniFocaccia]

Re: Full hemisphere arch?

All you have to do is Google Forno Bravo vent arch advice and you go here:

http://www.fornobravo.com/forum/f8/v...vice-6698.html

[sjmeff]

Re: Full hemisphere arch?

I went the full hemisphere route on my oven mostly because I liked the look of it. Also, since I'm building an igloo, I thought it would be stronger and preferable to having some kind of brace on the sides.

I created three full hemisphere arches--interior, exterior, and finally a decorative arch. I haven't had any problems with smoke coming out the front and don't have any discoloration on my decorative arch. I don't do anything special to warm up the flue, either, it just works great as-is.

I think the transition and chimney size (I have a 8" x 36" Duratech) are much more critical to it drawing well. I don't think the hemispherical arch has much of an impact at all in this regard.

If I had to do it over, I'd still go this route. Cutting and assembling the arches was one of my favorite parts of the build.

If you can find tapered bricks in your area, you could create this kind of arch very easily. I couldn't find them so I cut them with the HF saw.

-Stephen

p.s. Here's a search trick. I always use this method rather than the forum search. Type this into google: vent arch site:fornobravo.com/forum

[kebwi]

Re: Full hemisphere arch?

@Gianni: thanks. I had noticed that if I do general Google searches on these topics, sometimes FB forum results pop up in the results, but I admit, it never occurred to me to put "Forno Bravo" in the google search query along with the terms of interest.

Sorry for the trouble, I appreciate the tip.

[kebwi]

Re: Full hemisphere arch?

By the way sjmeff, how did you taper across the wide face of the bricks? Did you make every taper with two cuts, turning the brick over between cuts? I don't think my HF 10" will cut 4.5".

[GianniFocaccia]

Re: Full hemisphere arch?

I too like the looks of the rounded arch and have noticed that most commercial ovens have a more eliptical-shaped entry to them. It appears this proportion provides a wider entry for access in and out of the oven and a lower arch height for heat retention.

Regarding Pompeii oven design, do most builders construct their entryways so the front arch is slightly (1"?) lower than the vent arch to help prevent smoke from exiting the front of the oven?

Thanks,

John

[sjmeff]

Re: Full hemisphere arch?

how did you taper across the wide face of the bricks? Did you make every taper with two cuts, turning the brick over between cuts? I don't think my HF 10" will cut 4.5".

For my inner arch, I cut full-sized bricks in half and then tapered the sides by raising the saw so that the brick would just fit under it (careful, the saw sometimes "catches" on the brick since it's not cutting all the way through). I then had to make two passes for each taper, turning the brick upside down and around for the second pass. The inner arch had 19 bricks and it probably took me 90 minutes to cut them all.

For the outer arch, I lopped off an inch of the brick so that it would fit under the saw. I created a jig to tilt the brick, then cut all of the bricks. This was much faster and they were by far more exact than the ones I cut for the inner arch. Since I left a 1" reveal for the door, the inner and outer arches both lined up on top (the side where the chimney plate sits).

do most builders construct their entryways so the front arch is slightly (1"?) lower than the vent arch to help prevent smoke from exiting the front of the oven?

I agonized over this, thinking that having the decorative arch lower would prevent smoke from discoloring the arch, but I thought it would restrict the opening too much, so I put the decorative arch flush with the outer arch. Fortunately smoke coming out the front hasn't been a problem.

[Dino_Pizza]

Re: Full hemisphere arch?

You have a few things to consider here. 1st do a forum search on “parabola” & “ ellipse” and “squished ellipse.” I don’t recall your future ovens size but a 42” diameter oven that is half spherical can be quite big depending on how high your soldier course will be. A half sphere is beautiful and strong but a parabola will transfer the loads from the weight at the top better. However, no one ever had a dome collapse over these dome shape preferences.

Also, for pizza or flat bread, you want the heat lowered in the center of the dome closer to the floor, not up at the peak of the half-sphere. The flatter the top, the more heat reflected down. The taller the top, the more brick and the farther away the heat and the more wood to burn. This is all incremental though. I think a half-sphere is easier to build with half bricks without tapering much. You could build a lower dome (squished ellipse) but then build up the insulation on top into a half-sphere and by the time you stucco the outside, it’s the shape you want with an efficient lower dome within.

Some have found that lowering the front arch can make putting in the oven door a more careful process as you have to tilt it in. Remember, you can always use the decorative tile or brick you put on the outside of your finished outer arch to drop down a 1/2" to hold some smoke back if that became an issue (ie: you did not have enough front landing or tall enough vent chamber to have made a sufficiently drawing oven).

BTW: if I'm way off about any of this stuff, please don't hesitate to say so .
-Dino

[kebwi]

Re: Full hemisphere arch?

Thanks as always. I'm certainly meticulous enough to cut and shape my bricks more than is required...but I was hoping to get away with no double-cuts, which means no wide-axis tapers. I like the trick of diminishing the brick width by an inch, which just brings it within the saw's reach. Doing so has another advantage too. It permits you cut the inner and outer arch bricks at the same angle since they correspond to the same radius of curvature. The alternative is to have the outer arch slightly larger to create a reveal, in which case the outer bricks must actually be tapered at a different angle than the inner bricks.

...my only concern is whether thinning the bricks to 3.5" weakens the arch. The thinner the arch-bricks are along the radial axis, the more precisely shaped and placed they must be in order to preserve a true self-supporting arch.

Cheers!

[sjmeff]

Re: Full hemisphere arch?

I would highly recommend leaving a reveal for the door. You'll need it if you want to do any baking in the oven and it's obviously not something you can add later (not easily).

I think it would be fine to have both the inner and outer arches 3.5" bricks. They'll be much easier to cut, and I can't imagine that they would be less structurally sound (my exterior arch is 3.5" and very solid). Depending on the type of chimney you're building, you'll have to compensate for the slightly higher exterior arch, but you have plenty of options there.

I used the "Angleizer" software to calculate the tapers on all three of my arches. Worked great. Here's a screenshot of the calculation I used on my inner arch. My inner arch is 20" wide and rests on untapered half bricks to raise it to 12.5" (or maybe it was just 12", can't recall exactly off the top of my head). But in short, it's not difficult to change the jig to compensate for the different angle of your inner/outer arches.

The more time consuming part is creating good forms on which to build the arches. I was more meticulous with the form for my outer arch and it saved me time in the long run (see my pictures in the gallery section comparing the two). I reused the second form for my decorative arch.

Anyhow, if it would be helpful, I'd be happy to mail you my Angleizer CD now that I'm done with it. Or I can run the calculations for you.

S

[dmun]

Re: Full hemisphere arch?

You could also cut (in two dimensions) every other brick, to make an arch with seven keystones, doubling up the keystone bricks:



This arch is drawn with a ten inch inside radius, and standard brick sizes.

[kebwi]

Re: Full hemisphere arch?

That certainly is cute dmun. It violates arch-theory in that not all of the bricks are wedge-shaped and those that are not can theoretically slide radially and fall out...but I admit that in practice these ovens seem to hold together regardless of theory. :-)

[Lars]

Re: Full hemisphere arch?

Oh Dmun,
You may want to rethink that arch design. Arches are a continuous compression design. Think of squeezing the banana out of the peel. An arch like that relies only on friction and the sheer strength of the mortar to stay put. Masonry's strength is compression, its weakness is shear.

Keith - do you mean half circle arch? A full hemisphere is how the main body of most of the ovens are shaped. A full hemisphere is the rotated half-circle arch.

Just throwing in my two cents. That much closer to my forno bravo pizza cutter!

Lars.


WAIT--- I will leave my comment up there... but I may have been fooled by an optical illusion. DMUN, are those straight pieces? They look 'opposite wedge' shaped!