Northern Ontario oven build

[Shawnr]

The dome is complete! Yay!

I have a piece of insulated chimney (6" with 2" wall) that I had planned on using for the oven. I would like to set the chimney back up the dome a bit, not quite at the top but not right over the vent landing area. I am wondering how I should do this. I have not seen this done too much. To make the vent (ductwork?) from over the landing to where I want the chimney to sit, is casting it the best way? Or use firebricks, cut to fit as necessary? Or steel ducting? If casting it, use castable refractory with reinforcing ie chicken wire? Would this makeshift ductwork be a good place to use perlite/mortar mix? Is this not often done for a reason? ie complications or just the hassle of building the ducting channel up the dome or just not a look most builders want?

As for a thermal break at the front of the dome where the vent walls meet it, I am wondering if sticking (via mortar) a piece(s) of insulation brick to the front of the dome arch would suffice as a thermal break? It would save a lot of cutting to insert the insulation rope into the vent side bricks and seems to make sense to me, although I have no idea if this is enough of a thermal break. I will go look up the thermal characteristics but wondering if anyone has done this. Considering that some builders use no thermal break, I would think it would be ok and then the fear of the rope fibers falling into the food would be alleviated. It seems like a lot of cutting of bricks to fit a rope in there, although that looks like a nice way of doing it too. Just thinking out loud and looking for input.

Re the comment earlier about sticking the vent lower bricks to the insulation, I mortared the bottom vent side bricks to the hearth instead of the insulation. I am not sure what CaSi board is like as my stuff (K Fac 19) is not that. Despite ordering the CaSi stuff, I was sent this other rigid insulation material that is like a thick buffalo board. Very soft. I did not feel comfortable building on it so cut the insulation back. This should be a solid base for the vent bricks. I ran out of 2.5" firebricks and the local supplier is currently out too so am building the vent out of 2" firebricks. Are firebricks necessary for the vent if a thermal break is incorporated into the design?

Thanks all.

Shawn

[Shawnr]

Thank you JRPizza That helps a lot. I noticed as I was going that some joints were lining up and, initially, cut a few bricks to cause an offset, then a few bricks later, the joints were lining up again. I figure (hoping) as along as the alignment spans no more than 2 rows, the support and strength will be there.

As for the vent, I picked up some of the fireplace rope today and will incorporate that into the vent opening somehow. As for securing it, not sure how I will do that yet. Since the dome bricks are mortared to the floor bricks, I might continue that trend. I am picturing mortaring the vent wall bricks to the floor insulation too though, just the bricks under the vent walls, not the middle ones. It kind of means the 2" rigid insulation is supplying the connection from the dome to the vent but with the weight of it all, I think this might work. Does that sound reasonable?

You have a great write up on your build! Thanks for the info in it. And thanks to those many others that documented their builds too!

Cheers,
Shawn

[JRPizza]

Shawn, I tried to stagger my vertical joints to prevent a crack from running up the mortar, so every row I set had bricks narrower than the row below. When I got close to cutting half the brick off to get the width I wanted I switched to half bricks, then closer to the top 1/3, and at the very top back to 1/2. Whatever you need to do to reduce waste. Staggering bricks may look cool but I still got a vertical crack that actually split a brick or two.
Your vent, especially if you do a thermal break, will be free standing. Some have built part of the vent attached to the dome arch, which might give more stability, but also can subject the vent to thrust loads from expansion of the dome and allow for some conduction of heat to the vent. There is not a whole lot of data (any?) so you just need to pick a method and go with it. I built a freestanding vent with a thermal break. You can't have both - it's either attached or it's not. Most folks have opted for a thermal break and freestanding. I just read a ton of build threads and copied the folks that I was most comfortable following along with. As for vent floor bricks, I (again copying) went with a straight alignment. I figured I would not be catching them with a peel so alignment would not matter (and it has not). Also, because I used bricks and they really absorb drippings etc from cooking, I can easily pull them out, toss them into a very hot oven, and they come out looking like new (thanks Gulf for the tip)
I didn't use any leveler or adhesive under the vent floor bricks, but in retrospect I might have mortared the ones directly supporting the arches to get a little more side to side stability.

[Shawnr]

Getting there. I went from thinking I had way too many bricks to wondering if I have enough..:-O

A few questions
It seems like I am cutting large pieces (wedges) off of the 4 1/2" bricks as I get to the top. Does it make sense to start splitting them now? I am trying to reason it out and it seems like either works, but maybe narrow pieces would be easier to work with. There is one more course that I have since the last photo and it seemed to be quite frustrating. Maybe it was just me. Seemed like my visualizations were whacked. I noticed on some threads that I found with those interior roof shots that it seems like a transition to half bricks (half the halves....?) might be in order now?

I like the idea of a thermal break from the dome to the vent walls. How do the vent walls attach to the dome? I mortared the dome first course to the floor bricks so do I just do the same (to the floor bricks) since mortaring them to the dome arch would negate the effects of any thermal break, if I understand correctly? I found post #143 referenced here https://community.fornobravo.com/for...r-build/page10 . I cannot find a method of "sticking" the vent walls to the dome arch. Is there an adhesive here somehow? The tube of high temp or mortar?

I have laid out my vent landing floor bricks in the herring bone fashion but now see that most just line these up with the vent opening? Seems easier. My floor bricks are free to move now as they are only sitting on a sand/fireclay mix, although that is more sand than fire clay as I did not have much clay when I started. Now, after cutting all these bricks, I have lots from the saw. Do I use a "richer" sand clay mix to hold the floor bricks to the insulation board? And then mortar the walls to the vent floor bricks holding it all together?

[MarkJerling]

Yup. I think you'd have to grease me up good to get me to fit! LOL

[Shawnr]

lol...I think that is the photo I was thinking of. I am not a large person but at a 16" x 10" opening, even I would be struggling.

[MarkJerling]

Crawling in like this? LOL



I sent my son. He's thinner than me!

[Shawnr]

A little more progress. Some joints good, some, not so much. Starting to look like a pizza oven though....

Today, I stopped adding courses and spent some time in the dome while the top is open enough to work in. I definitely won't be crawling in through the opening like some photos I have seen. I mortared in pieces of brick around the inside of the opening, maybe adding strength, improving air (smoke) flow, and for piece of mind. I can see where building 2 of these things would be good. You learn a lot by doing. I also did the same small brick inserts around the outside of the opening and started looking at how the vent landing will be. I figured a brick and a half long but now thinking maybe just a brick length for the landing. I think the actual vent will be a half brick wide by the opening width, around 16 inches.

Any input appreciated.
Thanks
Shawn

[david s]

Yes they’re the right fibres and dosage about right, although if using as burnout fibres you don’t need as many as when using them in a standard concrete mix. I don’t know the weight or size of your bags, but if making a batch of mortar use about 2g for every 10 kg of dry mortar. Pull the fibres apart, spread them through the dry materials and mix for double the time you normally would as they tend to clump and resist dispersing throughout the mix.

[Shawnr]

Petter I found Fiberstrand 150 fibres locally, 1 lb bags. It says on the spec sheet about 0.67-1 lb/cu yd of concrete so, as you say, I won't use much. Does that proportion sound about right for my application? The specs on Thermobond 915 do not say how much one bag yields directly, but rather 80 bricks with 1/8" joints. Would I be wrong to use one bag of fibres per one bag of refractory?

[david s]

I believe the thin joint recommendation is a result of the inevitable phases transformation that occurs when the chemically bonded water is released (not steam from wet mortar). This happens over a broad temperature range well above the boiling point of water. The shrinkage caused by the water release may crack or spall the mortar. Thinner joints reduce this effect.

However, you are building a dome, so a crack or two will not affect its stability.

It is the removal of the mechanical water rather than the chemical water that is far more potentially damaging to any refractory material for the temperatures and method to which we fire. Whether mechanical or chemical in nature the removal of both is enhanced if the burnout fibres are included.
Here’s a pretty good document explaining water and refractory material.
https://thinkhwi.com/curing-dryout-refractories/

[Shawnr]

Thank you Petter . I agree, although I don't have anything to base it on. As much as thinner is better, I don't think the dome will collapse on me for that reason, but I will try to get smaller joints on each new level as the vertical forces become a larger factor and my brick cutting skills get a little better.

[Petter]

I believe the thin joint recommendation is a result of the inevitable phases transformation that occurs when the chemically bonded water is released (not steam from wet mortar). This happens over a broad temperature range well above the boiling point of water. The shrinkage caused by the water release may crack or spall the mortar. Thinner joints reduce this effect.

However, you are building a dome, so a crack or two will not affect its stability.

[Petter]

Ok. 3/8" seems to be the maximum recommended thickness. I also used Calcium Aluminate mortar like you. My opinion is that as long as it does not crack while curing, you are fine. I used a wet brush about 10-15 minutes after each batch was mortared into place, 10-20 bricks. This smeard all excess mortar into position, tightened the joints and filled them nicely. Then, I let it set for ~30 minutes before cleaning the bricks on the inside. No cracks after curing. I had some really wide joints... There are several ways to do it, but I did it this way and it worked well.

[Shawnr]

I had the same though about the joint gaps. What type of mortar do you use?

Thermobond 915 is the Mortar