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james 04-16-2007 06:28 AM

A Potential New Low Dome Design
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This is something I have been thinking about for a while, and I just finished a cut at the drawing. This is a design for the low dome oven that uses a full-height upright brick (as a soldier) for the first course, an angle cut brick to set the inward angle of the dome for the second course, and then a series of inward angled brick to finish the dome.

This example shows as 42" oven, with a 15.6" dome, or 37%. You could make the dome lower, and have the same angle by either putting the first course on the hearth, or by cutting the first course of upright bricks.

I am hoping we can use this for discussion before deciding whether this will be the recommended method for low dome construction. The aggressive inward curve will require forms or sand for construction.

You would not need the first course upright, or the angle cut second course with the Tuscan dome design.

Comments anybody?

johnrbek 04-16-2007 07:13 AM

Re: A Potential New Low Dome Design
That's exactly how I did mine... 42" internal diameter, 14" internal height. Standing soldiers around outside perimeter of hearth bricks (not standing on top of ...) with cut partial brick to set angle... 1/2" home depot insulation board use to set angle of this brick...

My dome was completed this weekend and I"m in the midst of touching up and pointing the interior.. Photo's aren't ready for prime time, but will be in the next few days.

Thoughts so far:

1. A diamond wet saw with ability to cut at least 3" deep is a necessity. There are just too many angles to be able to do it with anything less. At some point during dome construction, virtually every side of a brick will have to be cut to optimize shape and form and minize mortar joints.

2. James, as you mentioned a sand or some other type of form is required.. I went with sand and used about 150-200lbs of sand to 1 good shovel full of portland and that held up fine. The whole down side to using a form (which I see no way around due to the agressive angles) is that you have absolutely no control of whats going on on the inside. This results in some joints not being smooth or some not fully filled with mortar.

3. Pour a reinforced insulating butress around your soldiers. With that steep angle, you need some support for those soldiers. They will not hold up under the stress (or so I'm told). I'll disclose more on tips & techniques I used when I post my pics...

4. Cut, cut, and cut some more... Again, because of the low dome and our inability to rely on the catenary arch, you need to ensure you minimize your joint thickness as much as possible. This involves LOTS of cutting. Again your 10" diamond wet saw is a must for this cutting. Kind of hard toward the end, but have patience.

5. I'm not sure how to get around this other than to be really skinny but once you finish your dome and opening arch, especially if your sticking to the Quebec interior height to opening ratio of 63%, many of us, (including myself) will not be able to reach into the back of the oven to touch up the inside... Since you use the Form duing dome construction, the first access to the interior of the dome comes after you've take the Form out... 14" internal diameter @ 63% = approx 9" opening height.. too small for me to fit into... So, now it becomes a remote control effort with wooden paint rod, some duct tape, and your trowels and such.. That is unless anyone has any other suggestions...

Those are the keys things I've run into so far.. Let me know what you guys think...

Minor update: James, after looking at your diagram again, the one thing I would add is that you may as well put your soldiers along side your hearth bricks (not on top)... the cutting of the hearth bricks is nothing compared to the cutting done later for the dome... And with that, you get a floor that can be replaced as necessary...

Once more thought: I cut everything, including soldiers to approx 3"... you end up adding another 1/2" to 1" anyone when you butter the inevitable extra mortar in your bucket over the top with each successive chain, but I wanted a quicker heat up.. doesn't really affect design per se.. just a thought.


james 04-16-2007 07:35 AM

Re: A Potential New Low Dome Design
Eccellente John,
Thanks. Have you been taking photos?

I thought of showing the soldiers on the hearth, around the floor, but my guess is that most builders will construct on top. I could do drawings for both versions, and get them into a new version of the e-book.

Photos John?

dmun 04-16-2007 08:02 AM

Re: A Potential New Low Dome Design
As James pointed out the other day, the strength of the dome depends not on mortar, but on the forces transmitted from one ring of bricks to another, down to the support slab. I know there is a tradition of full standing soldiers in low dome ovens, but there is also a tradition of masonry and metal bands buttressing the upright bricks.

My proposal for a low dome oven involves bricks cut in thirds:

This brick has three square faces. This yields three angle bricks, two nine degree, and one eighteen degree.

More by chance than skill, these angles form approximately the edge of an elipse, formed with the locii at the top of the half bricks.

I think this is a stronger system than the full soldier system with one wedge. Since you have to cut the angle bricks anyway, why not cut them to build a stronger curve?

james 04-16-2007 08:18 AM

Re: A Potential New Low Dome Design
As always David, this is a great posting. I particularly enjoy your band and butressing comments. Funny and true -- all at the same time. Did you see the photo of the oven where the brick fell in?

Thanks for the insight.


johnrbek 04-16-2007 10:04 AM

Re: A Potential New Low Dome Design

My thought on putting the soldiers around the hearth instead of on it, is that the benefits outweigh the effort and time required to cut the hearth bricks. Your feeling that most builders will build on top of the hearth is interesting... My recommendation would be that they don't. Cutting the hearth floor was maybe 10 or 15 cuts and less than 2 hours time... It's just inconsequential when compared to all the other cuts that need to be made to the dome. Again, with the agressive angle of the dome, at various stages in my build, I had to cut all 4 sides and the inside face of some of the bricks... As dmun pointed out (that you pointed out the other day), it's the forces of one brick against the other the create strength in the dome. To minimize mortar joint size is key here... The 10" diamond wet saw was invaluable here.. and at a cost of less than $200 on ebay.. more than worth the price of entry if you want to build the dome right...

The only other thing I can think of that, in my mind anyway, would make going with the soldiers advantageous would be that you have a little more height along the permiter of the oven which might make it a bit easier to build a fire and have it breath a little better... Remember your already in a low dome and that arc is agressive.. if you start arc'ing in right at the sides, then you've got very little vertical space along your perimeter and less vertical room for your wood, coals, flames... just a thought...

James.. I have taken many pics and will post a few shortly..


maver 04-16-2007 09:12 PM

Re: A Potential New Low Dome Design
Johnrbek - congratulations on finishing your dome - I'm looking forward to pics. As far as pointing the interior in such a small space, it sounds like the perfect job for child labor ;) . I look forward to seeing your butressing system. I suppose with enough mass insulating concrete (like perlcrete) should be able to provide enough buttressing to overcome the stress at the angle where the dome meets the soldiers.

Regarding starting the fire at the periphery in an oven with an arch through the sides as David proposed vs with soldiers, I usually don't start the fire at the edges anyhow - I start in the middle and push it to the side when it is close to pizza time. I wonder if the gentler angle of an arch would improve the reflection of heat in the oven - I doubt it is significant.

I really looking forward to hearing how both yours and David's ovens perform once they are insulated and fully cured.

carioca 04-17-2007 01:38 AM

Re: A Potential New Low Dome Design

just a bit apprehensive about those 'flat' arches - I prefer the load transmission along the catenary! (The spelling checker flags catenary - hasn't it hear of a catena = chain?). Here's a link to a site where I dug for the 'perfect' curve for my dome: Auroville Earth Institute is a research, design and developing agency for vaulted structures, construction of various Vaults, Arches, Domes (VAD).

And further to the questions of soldiers, sleeping or upright: my supplier of taper-cut firebricks insisted that their 'kit' called for the first ring to use 11 'sleeping' bricks - albeit angle-cut ones...

I've amended that design recommendation slightly to put down a first course of sleeping straight bricks, so that my hearth bricks don't intrude into the rising curve of the dome.

Waiting for more calcium silicate board before I place the first brick, though...



Hendo 04-17-2007 05:29 AM

Re: A Potential New Low Dome Design

That link is very interesting. With a Tuscan hemispherical design, I'm glad I decided (without knowing any better) to go with a thick 4" wall thickness. Perhaps it won't fall down after all!

Cheers, Paul.

maver 04-17-2007 12:29 PM

Re: A Potential New Low Dome Design

Originally Posted by carioca (Post 9641)

just a bit apprehensive about those 'flat' arches - I prefer the load transmission along the catenary!

Consider that JB loaded the outside of those standing soldiers with insulating concrete - this effectively places the load through a more appropriate arch. I think a slight angle at the top of the standing soldier would contain the outward force of the dome above, and a buttress around the standing soldier would allow proper direction of force through solid material. It looks like JB has thought this out well.

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