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  #11  
Old 04-02-2012, 11:27 AM
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Default Re: Apagios's 42" Pompeii

Apagios
Nice build - well finished off with that octagon.
Quote:
I would HIGHLY recommend building the doorway arch right after your soldier course is done, and before you do the 1st course on the dome.
I agree that building the arch should be he first thing. I laid out the soldier course dry then built the arch. If you get the arch right everything else just follows naturally.

Is your arch a perfect semicircle - in a hemispherical dome?

btw nice simple idea using those two bricks to reduce the opening size - I will try that also - since I have a few left over. How good is that quickcrete finish. Wonder would it survive Irish rain?

Last edited by Amac; 04-03-2012 at 09:42 AM.
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  #12  
Old 04-02-2012, 11:58 AM
Peasant
 
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Default Re: Apagios's 42" Pompeii

I mortared my soldier course in before my arch, but that's cause my arch sits on it:



And yes, my arch is a regular semi-circle, in a hemispherical dome. I hesitate to say "perfect" as I think saying "my perfect arch" sounds a little presumptuous But hindsight being 20/20, I can look back at my arch and see ways I could have made it a little cleaner, gotten my cuts to line up better. Though as my Dad kept telling me, and has rung true, none of my friends have ever noticed the "flaws" that I can see. And honestly in the past 6 months, the "flaws" i saw during construction have all but faded in my eyes.

I honestly prefer my oven with those bricks to a smaller opening, or having them mortared in. I like that the arch is entirely load bearing (stronger then vertical sides with a modified arch top), and the flexibility of being able to remove those side bricks. I have yet to try a turkey in my oven, but without those side bricks a small fire will keep the oven at right around 650*F, which is very close to what we roast Turkey's at on a Weber grill. So I prefer the flexibility of using my bricks as an add-on option, rather then having a smaller opening an no way to run lower temps with a fire inside.

The Quikcrete SBC with the scrylic modifier should be fine for an Irish rain. I would definitely add that acrylic modifier/fortifier to make it more waterproof. I found a .pdf file somewhere online from Quickrete, which showed a water storage "tank" made by a drystacked block wall, and the walls and floor inside coated with SBC! I figure if the product is designed to make a water storage vessel similar to a pool, it's probably good for a heavy rain!

Oh I also keep a small plywood "door" in my red brick facade arch, just to keep out splashing and wind blown rain. ON the inner arch doorway to the dome I have a piece of styrofoam with a brick sealing the dome from weather. I found even with chimney cap, some moisture or rain can drip black soot onto the fire brick landing under my vent arch, so I just leave a piece of foil between the doors when it's sealed up to keep my cooking floor cleaner.

Next I need to make a good insulated door to the oven and a nice door for the outer arch to seal everything up!
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Last edited by apagios; 04-02-2012 at 12:02 PM.
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  #13  
Old 04-02-2012, 01:02 PM
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Default Re: Apagios's 42" Pompeii

Thanks for that quick reply. I am just at the stage where I have cut the vent arch bricks and dry stacked them. I actually went to the trouble of tapering the firebricks this time so I hate the thought of not seeing them - even though I feel that they will absorb rain like a sponge. What do you think?

Quote:
Next I need to make a good insulated door to the oven and a nice door for the outer arch to seal everything up!
You should have a look at jcg31's doors - he has actually made two - the one for the outer door is glass with adjustable air control - both impressive pieces.

Quote:
I mortared my soldier course in before my arch, but that's cause my arch sits on it:
I see - I just used arch bricks where you have those three soldiers at either side of the door. I cut three and sliced one in half. One and a half either side to the top of the soldiers. Interesting to see the similarities and variations

Quote:
And yes, my arch is a regular semi-circle, in a hemispherical dome. I hesitate to say "perfect" as I think saying "my perfect arch" sounds a little presumptuous
I agree with the concept of the "perfect" semicircle in a hemispherical arch and the resultant feeling that it adds strength in comparison with straight sided - or angle iron lintelled doors. This to the extent that I think FB should incorporate it to their plans.

Last edited by Amac; 04-02-2012 at 01:15 PM.
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  #14  
Old 04-02-2012, 01:19 PM
Peasant
 
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Default Re: Apagios's 42" Pompeii

I know what you mean about not wanting to hide your vent arch, I was quite please with how mine turned out. But I would be extremely leery of firebrick exposed to rain, especially right at a point where it transitions to your dome.

Even if you don't get direct rain on your dome, if your vent arch is exposed some moisture is going to seep through to other bricks. If you get a wet vent arch hot, I'd be very concerned about it cracking, let alone having some moisture travel through the bricks to the dome bricks near the vent arch.

As much as it sucks to have to cover up some nicely tapered vent arch bricks, it would suck much more to have them crack due to some moisture in them!

We go through the process of "curing" to cook out the last bit's of moisture from the bricks after construction. Even before the 1st curing fire your bricks should feel "dry" to the touch. What I think you are describing would mean you'd basically have to "re-cure" your vent arch after every rain!

The vent arch is a section of high thermal transition too. the top of your arch leading into your vent has all the hot exhaust gasses, and the higher velocity promotes heat transfer by convection. I did 3" of insulation on my dome, but only 1" on vent arch and chimney, with 2" in the section of transition from dome to vent/chimney. My SBC on the outside hits about 100*F. It's warmer then I would have liked but certainly nothing to worry about.

The point I want to get across is that if this area of your oven is moist, the high convection heating is going cause a lot of thermal changes pretty quickly. It will dry out even faster, and IMHO make cracking more likely due to changes in temp occurring faster.

I even got a gallon of Qwikcrete masonry sealer to waterproof my red brick arch and the entire "bar top" around my oven, just to try and prevent water from absorbing into the red brick facade arch.

I certainly don't want to presume to be an authority, just giving you my entire thought process and opinions on the matter of fire bricks getting rained on and how much water could possibly seep into your dome/transition area. Just seems like something I personally didn't want to take a chance on.

And thanks for the door suggestion, I'll definitely search jcg31's doors out on the forum! I was planning to try and work a double paned glass door for the inner door so I can see bread baking!

Oh and I agree on the suggested edits for Jame's eBook, the "true" arch doorway I think is the way to go!
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  #15  
Old 04-02-2012, 01:23 PM
Peasant
 
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Default Re: Apagios's 42" Pompeii

OH I just searched jcg31 & "door" and found he was the one I had read about last year! I plan to get the neoceram glass cut from the same supplier he provided!

Last edited by apagios; 04-02-2012 at 01:40 PM.
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  #16  
Old 04-02-2012, 01:33 PM
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Default Re: Apagios's 42" Pompeii

Thanks - food for thought apagios. Much as I don't want to I feel you are right about those bricks. Even my wife who is my worst critic doesn't want them hidden.
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  #17  
Old 04-02-2012, 01:39 PM
Peasant
 
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Default Re: Apagios's 42" Pompeii

Well in that case I think you guys can have a perfect arrangement. You don't have to hide them as long as she doesn't mind doing the repair work if they ever crack!
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  #18  
Old 04-03-2012, 04:21 AM
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Default Re: Apagios's 42" Pompeii

Yeah - that'll work OK On second thoughts maybe I'll follow your advice - get some normal bricks and do it the sensible way unless someething like this could be done with the firebricks?

Quote:
I even got a gallon of Qwikcrete masonry sealer to waterproof my red brick arch and the entire "bar top" around my oven, just to try and prevent water from absorbing into the red brick facade arch.

Btw while the arch subject is still fresh, there is something I'd like your opinion on. I originally set out to build a 36" - but to get a semicircular arch with sufficient height - I wanted it to be about 12" - would have made a very big opening relative to the dome and didn't want to go to 42" because of fuel consumption issues.
So I made the following compromise. I increased my dome size to 39"
That still would have left a 24" wide opening which I still felt was too wide so..after a lot of head scratching I came up with this solution:

I decided the opening width I could tolerate (no science here just gut) was 21" and that determined that the arch radius would be 10.5".

To get the extra height to 12" I set the indispensible tool pivot to sit 1.5" above the dome centre. That meant that both my arch and my dome were effectively lifted 1.5" but still maintained the arch/dome semicircle to hemisphere "perfect" relationship.
I used the indispensible tool while building the arch - it has a threaded head so is possible to rotate - to confirm it would not deviate from where the dome walls would meet it.

The drawback is that the opening is a bit lower than this so called "golden ratio" of 60% but I think there is more than a bit of mythology about the importance of that. Also the internal dome hieght is now almost 21"

Anyway I would like to hear your views?

Also maybe you (or Dave above) might be interested in how I arrived at the arch brick pentagon shape - you showed something similar with the bricks laid out above. The quality of the pic is not great but you can get the idea:

1. Draw four concentric semicircles (or even quarter circles) - the smaller two represent the radius of the inside and outside of the arch - the larger two, the inside and outside radius of the dome. (these can be full size or to scale).
2. Draw two parallel lines at tangents to the outside topmost point of the arch and the inside topmost point of the arch to cut.
3 Then draw a dome radius line from the center of the circles to the point where the top tangent cuts the outside circle of the dome.
In the sketch the shaded area is the resultant correct pentagon shape for my arch brick. If you measure the angles three of them will always be roughly 90 degrees - the others will vary depending on arch and dome size - and where the arch joins the wall will be 1/2 a brick width.

Transfer this to a wooden jig and use it to mark all the arch bricks for cutting - all bricks should be identical, and all will blend smoothly to the inside of the dome..

If you are not confident about drawing to scale - just do it lifesize on a large piece of cardboard.

The sketch also shows the idea of raising the arch and dome, by dropping the floor (or raising the IT even) 1.5", which I talked about above.

Around post 33 on my thread is where I started building the arch and there is more detail there.
http://www.fornobravo.com/forum/8/39...i-17334-4.html (39" Stargate Pompeii)

Sorry about the verbosity of this post - it is easier in the practise than the description.
Attached Thumbnails
Apagios's 42" Pompeii-photo0269.jpg   Apagios's 42" Pompeii-photo0274.jpg   Apagios's 42" Pompeii-photo0266.jpg  

Last edited by Amac; 04-03-2012 at 06:53 AM.
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  #19  
Old 04-03-2012, 10:06 AM
Peasant
 
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Default Re: Apagios's 42" Pompeii

Wow yeah the pics and explanation are a much better way to to get the shape of the arch bricks. Might be worth a sticky in a separate thread, or possibly send a write up to James for an appendix in the eBook?

My method was similar, but not as exacting or precise. I started with the false assumption that each brick of the arch would be a different shape, I was thinking that the keystone arch brick would have to be almost a full brick and the bottom bricks on either side would be ~1/2 a brick, so the arch would get deeper the higher it got. Turns out that assumption was dumb and by the time I realized it I had cut half the bricks so my arch is not as uniform as I would like, but you really can't tell unless you are in the oven, so obviously no one has noticed my arch bricks are not all perfectly identical! (this is one of my own little nitpicks about my work)
----------------

So my thoughts on door height. I do agree there is probably a bit more emphasis placed on that "ideal door height" ratio then maybe there should be. I would say if you are close then you are fine, which it sounds like you are. As much as there is a science to everything, there's a lot of variables for a brick oven, so IMHO a door height ratio withing say 5% of recommended is probably fine.

What occurred to me after my first pizza & issues with keeping the oven above 700*F, was that it is not simply a height ratio. Thinking of only height ratio (door height/dome height) is over simplifying the problem. Total cross sectional door area and door shape also factor in, so instead of simply looking at 1 factor, you have 3 factors to design for.

I focused on the height ratio & cross sectional area in my design, I forgot to account for shape. When I had trouble getting my oven hot I realized my arch was at the proper height ratio, but because I did a true arch it was much wider.

I designed my opening with 13" arch radius (42" oven), which gave me an opening cross sectional area of 265 in^2, the eBook recommended 20" wide by 12.5" tall, or 250 in^2. I knew before I cut my first brick that I was 6% over sized, but I figured 6% was no big deal since I had not realized the shape was a factor.

What I came to realize was that it wasn't simply that I was 6% over-sized, it was that my bottom width was too wide since my shape was a semicircle rather then a rectangular door.

It goes back to my second post in this thread about regulating cold air in and hot air out, your only real control is over the cold air intake. As the hot exhaust gasses will simply increase flow velocity to account for the cold intake air, the by products of wood combustion, and the increased energy (heat). Rate in = Rate out. If you maximize your wood added, you're not really going to make it burn any hotter, so that leaves you with cold intake air as the only "control" left. (Excluding some complex metal damper allowing you to control the exhaust gasses, but it's far easier to passively control cold air intake then actively control hot exhaust gasses!)

So the recommended opening was 20" by 12.5", a rectangle. Because my arch was wider at the bottom I was pulling in much more cold air intake then I needed, and the excess intake air was simply causing the exhaust gasses to move faster, the overall effect was a cooler oven. (Less dwell time of the hot air to get as much heat from the combustion into my bricks as possible, i.e. low efficiency)

My 2 side bricks were cut not only to reduce my bottom width from 26" down to ~20" (I can measure when I get home if needed), but I also sized them so that I reduced my opening cross sectional area back to the recommended 250 in^2. So my side bricks fix both the width of the door at the bottom to control cold air intake, and also correct the overall size of the door.

So 2 simple bricks cut to fit closely fixed all my problems, allowed me to have a more variable oven temp (I can pull them out for a ~600*F roasting temp), and did not change the structure or strength of my true arch.


... However I think you are looking at the opposite situation.

you basically have a 10.5" arch radius, sitting on top of a 21" by 1.5" rectangle right?

so your arch is ~173.2 in^2 and the 21 by 1.5" gets you another 31.5 in^2 = ~204.7 in^2.

The recommended opening area in the eBook for the 36" oven is 228 in^2.

My suspicion is that you should be fine, I would think smaller opening would simply draw cold air intake faster to match the rate of the exhaust gas out. Especially since your arch design favors cold air intake (as opposed to a rectangle which would have the same "inlet" area width as "exhaust" area width. By that I mean if one were to find the line in mid air in your doorway where the air goes from cold to hot. (BTW I love telling friends to find that line, air temp goes from ambient intake air to hundreds of *F for the exhaust gasses in a very short distance, less then say 1/2 inch of vertical movement, as you raise your hand. Just stand back so you don't get elbowed when they yank their hand out of the doorway and curse you for trying to burn them )

If anything, I might expect you may end up with a pretty efficient oven (getting more heat into the bricks from a certain amount of wood).

As a worst case scenario, if you find your opening too small. Since you clearly are not going to grind out your arch, you can simply get a small blower and just put a couple inch diameter air hose resting in your opening during warm up to help heat the oven, think the same concept as the "blast furnace" style doors, expect now with forced induction. Effectively you'd be supercharging your oven during heat up! Once heated it will certainly have the heat capacity to cook pizza for hours with a small fire!

Though as I had the opposite problem I would certainly suggest listening to others advice on the topic as well!

haha and same apology for the verbosity!

Last edited by apagios; 04-03-2012 at 10:14 AM.
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  #20  
Old 04-03-2012, 02:02 PM
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Default Re: Apagios's 42" Pompeii

Quote:
I designed my opening with 13" arch radius (42" oven), which gave me an opening cross sectional area of 265 in^2, the eBook recommended 20" wide by 12.5" tall, or 250 in^2. I knew before I cut my first brick that I was 6% over sized, but I figured 6% was no big deal since I had not realized the shape was a factor.
I was going to do those calculations but I stopped when I realised that as you make your opening bigger you also make your dome area smaller as you are taking over floor space. It may not affect the calculations hugely but it threw another variable in the mix - enough to put me off

Quote:
It goes back to my second post in this thread about regulating cold air in and hot air out, your only real control is over the cold air intake. As the hot exhaust gasses will simply increase flow velocity to account for the cold intake air, the by products of wood combustion, and the increased energy (heat). Rate in = Rate out. If you maximize your wood added, you're not really going to make it burn any hotter, so that leaves you with cold intake air as the only "control" left. (Excluding some complex metal damper allowing you to control the exhaust gasses, but it's far easier to passively control cold air intake then actively control hot exhaust gasses!)
Still struggling with some of this I'm afraid. What puzzles me is why when you restrict the flow of air in to a fire it causes the a fire to blaze up?
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