Mongo's 42" CT Build

[Toiletman]

Then on to the vent brick. I cut an "L" in the end of the vent bricks so they wrap around two sides of the dome arch brick, with the two runs of ceramic rope filling the gap between them. I back cut the end of each vent arch brick at a slight angle so only the inside edge of the vent arch brick touches the face of the dome arch brick.

Originally I was going to just reply on compression to seal the rope between the two bricks. But I ended up running a bead of RTV on the two ceramic fiber ropes as I set each brick, that way I won't have to compress the rope as much. With less compression, It'll allow for more independent movement as the hot dome bricks expand and move next to the cooler vent brick.

I cut and dry fit five courses of brick for each side of the vent arch today. I'll mortar them up tomorrow before lunch, let them sit for a few hours while I do other projects, then come back late afternoon and cut the brick for the rest of the vent arch. That's the plan at least. Anyhow, I'm glad I got a couple hours of work in today. Progress is always good.

Sorry to quote such an old thread mongota, but since it's such a good one with good explanations I hope you don't mind .

Could you comment a bit more on your rationale with the two runs of rope, notches and extra silicone bead? Were you worried that due to expansion contact would develop in time, and hence you wanted to control both degrees of freedom? Ie. forward and sideways/top motion? I'm leaning towards only doing the rope in the 'side' and one bead of silicone on the 'face' side of the heatbreak?

Would you do it again like you have done in this oven? Or could this design be simplified?

[NewEnglandNewb]

Thanks Mongo! I never would have guessed they had firebrick based on their website.

[mongota]

@NewEnglandNewb

Greetings Josh,

I bought them at Westbrook Block in Westbrook Ct. WB has a large manufacturing facility and their website is geared towards that, but their sales yard is at 439 Spencer Plains Rd in Westbrook. It's maybe 5-10 minutes off of I-95.

They had the red and buff (beige) colors in stock.

[NewEnglandNewb]

Hi Mongo,

Would you mind sharing where you bought the red firebricks? I’m fairly local (Westerly, RI), and am looking to start my build this year. I’m currently trying to figure out where I can buy materials locally and really like the look of the red firebricks you used.

Thanks,
Josh

[mongota]

Mongo, as a long time lurker, I thought I was familiar with all the greatest hits of oven builds on the forums, yet only tonight did I discover your build. It's beautiful, precise and the detail and clarity of your write -up is just as impressive! I'm back on here as I prepare to build my second oven with all the lessons learned and in a better place to take the time to do it right. I really appreciate a number of your details, like how you layered the blankets, and isolated the vent arch.
Can I ask, how has your cast concrete work held up over time? I spent a few years pouring decorative concrete in college and with wet cold winters any exposed detailed concrete that wasn't sealed regularly would spall and degrade over time. I'm curious how your experience has been as I contemplate ideas like your 4" PVC pipe slab edges and the beautiful vent cap cast you made. Thanks in advance!

Corsairmo, thank you so much for the kind words! I came on the forum today and am just now seeing your post.

The oven has held up just fine. It has been exposed to the elements for quite a few years now. My initial design was to build a gable roof over the oven itself, but due to how close I built it to my pool pergola I didn't because I thought the two roof lines would be too cramped. Then I thought I'd extend the roof (metal low-slope roof) of the pool pergola over the WFO. That may still happen, but I do like the oven being out in the open, in full view.

As a result, the oven has seen a few years' worth of weather and winter freeze/thaw cycles. This year we've had a lot of rain and sub-freezing temperatures, but still no structural issues like spalling or cracking with any of the concrete. I never did seal the concrete or stone or fill any of the voids with a slurry. During construction, the wire brushing of the concrete for a rough texture was purposeful, and while I vibrated the forms for consolidation, I didn't overly vibrate as I wanted some voids in the surface when I wrecked the forms. I like "visual texture". I do have some efflorescence due to the wetting and drying cycles but to me? It's patina! I may clean it up this year but...who knows? If I were to clean it up I'd do just that, then I'd consider using a colorant to renew the darker colors a bit. They've lightened up due to the efflorescence. I'd then run a nice long fire to dry things, then seal with a vapor-permeable sealer. If I do that remains to be seen.

I'll post some photos from today. I have to say the oven looks just how I like...it's a bit rough, but I'll call the roughness "patina", lol.

We've had some really heavy rains over the past couple of weeks and it's been about 3 or 4 weeks since the oven was last fired. I can tell that the oven is carrying a bit of moisture, but I will be firing it tomorrow.

Photos from today, the first shows the front of the vent cap and the cast arch that surrounds the oven opening. I REALLY thought I'd see degradation of the concrete on the underside of the cast arch from heat as this is where I build my starting fires. But nothing. Not even any smoke stains. The oven draws and burns well.



This photo shows that the corner pillar castings and the decorative detailing on the edges of the hearth slab have held up fine, though there is efflorescence on the leg of the cast arch that surrounds the door used to load firewood into storage underneath the oven.



And this last photo is a close-up of the corner of the hearth slab. The surface is rough, a sandpaper texture, but that is what I wanted.

[Corsairmo]

Mongo, as a long time lurker, I thought I was familiar with all the greatest hits of oven builds on the forums, yet only tonight did I discover your build. It's beautiful, precise and the detail and clarity of your write -up is just as impressive! I'm back on here as I prepare to build my second oven with all the lessons learned and in a better place to take the time to do it right. I really appreciate a number of your details, like how you layered the blankets, and isolated the vent arch.
Can I ask, how has your cast concrete work held up over time? I spent a few years pouring decorative concrete in college and with wet cold winters any exposed detailed concrete that wasn't sealed regularly would spall and degrade over time. I'm curious how your experience has been as I contemplate ideas like your 4" PVC pipe slab edges and the beautiful vent cap cast you made. Thanks in advance!

[Michele Enrico]

Okay, got some uploaded.
Pics of the vent arch and throat.

Aha! This is the detail I have been looking for. Nicely done!
Michele

[mongota]

I did coat the slab under my ceramic fiberboard insulation, bringing the RedGard a few inches outside the footprint of the board. Once the board insulation was in place, I also coated a couple inches of the sides of the 4" thickness of board insulation, lapping that down onto the RedGard already on the slab. I figured that'd minimize water wicking in from the side since I don't have a house over my stone igloo and my hearth slab is exposed to the rain.

It's worked fine for me. I've had no water intrusion since the oven was completed.

The RedGard prevents moisture from coming in through the slab. If it somehow came in from above, the drain holes make it easy for water to get out.

I've not used Foamglas.

[Giovanni Rossi]

Mongo, Thanks for this update. I latched on to your use of RedGard early on in my research. I'm using a layer of Foamglas on the slab then Cal Sil with a smaller diameter on top of that. I want to make sure I have full understanding. I'm planning to coat the slab under the Foamglas with RedGard. I have drain holes. The bottom of the insulation blanket for the dome will rest on the Foamglas. I'm curious what you (or others) think about coating the vertical edge of the Foamglas w/RedGard. I hadn't planned to since Foamglas is hydrophobic. But, maybe it makes sense to have a continuous membrane of Redgard at that seam. I was planning to flash the perimeter of the Foamglas and seal where it meets the slab before applying the perlite/cement render. Appreciate any guidance.

[mongota]

Drain holes in the slab are always welcome. I'm grateful that I've never noted any moisture under mine, even when I taped squares of poly on the bottom of the slab. But you are right, build to prevent water from coming in, but provide a path for it to escape in case it does.

And for anyone new reading this, for clarity by no means am I recommending RedGard on top of the entirety of the board insulation, and setting the OVEN floor firebricks directly on top of the RedGard. I wouldn't want any membrane directly under my oven floor bricks subjected to heat from a high temperature fire built in the center of the oven floor.

[david s]

Thanks for that report, it’s good to know the Redguard stands up to the heat. Apart from keeping the moisture out, it is also important to provide an escape route rather than locking it in. This is of course weather dependant, but after torrential rain or even continuous high humidity the insulation can pick up considerable moisture. Some drain holes in the supporting slab provide an escape route for moisture as it travels away from the fire.
My approach to preventing water wicking up from the stand is a little different in that I use a sealer on the top of the supporting piers and make the cast slab waterproof with both Xypex and Edencrete concrete additives which also increase strength.

[mongota]

RedGard update...

When I built my oven, I painted RedGard on my slab to prevent moisture in the slab from wicking up in to my 4" of floor board insulation. Once the insulation was set, on top of the RedGard, I then painted RedGard on the 4" edge of the insulation, as well as carrying a bit of it on to the top of the insulation at the landing. I'll try to attach a photo from way back when as pictures are always better than words. Especially when I'm writing the words.

All told, the RedGard has performed marvelously. My oven has been in use for several years.

Here in the New England, we've been hammered with rain over the past few weeks. The last storm a couple of nights ago dropped 7.4" in one night. It was pretty amazing.

Before firing the oven up the other day, I pulled a few bricks up off of the landing just to see how the insulation looked underneath. All looked well. There have been a few questions regarding RedGard and how it holds up to temperature. In the photos below, the front row of bricks that make up the floor of the landing sit right on top of this RedGard. The landing is where I start my fires, so it has seen a bit of heat over the past several years.

I'm happy to report that the RedGard looks just fine. It's intact. No tears. No degradation. No delamination from the insulation below. No bubbling. No defects at all that I can ascertain. I can press a thumbnail in it and it's still "rubbery" so to speak. With this being the part of the membrane most exposed to oven heat, this location would have taken the most abuse from high temperatures. And it looks great.

That's all, folks!

[mongota]

You have made a remarkable oven, well done!

My take on oven moisture: If it is fired once or twice a week, the oven heats more quickly, gets hotter, and stays hotter longer than when Moisture builds up from disuse.

Firing the oven, even to an intermediate temperature goes a long way to controlling moisture. You can't get rid of a lot of moisture with just one firing cycle though. The solution is to use it more often. ;-)

Keep in mind that wood contains at least 8-10% moisture, sometimes a lot more. This moisture makes a more moist heat than in your kitchen's oven. There are pluses to baking with that moisture given off by the firewood. JMO

Agree with you completely, Lee.
I've seen the same, with frequent firings the fire itself burns hotter, the oven heats up faster, and the dome clears more quickly. And the outside surface of the oven (in my case the stone dome veneer) never gets warm.
After a period of non-use, all take longer, and the exterior stone veneer is slightly warm to the touch due to moisture drive.

[Lburou]

You have made a remarkable oven, well done!

My take on oven moisture: If it is fired once or twice a week, the oven heats more quickly, gets hotter, and stays hotter longer than when Moisture builds up from disuse.

Firing the oven, even to an intermediate temperature goes a long way to controlling moisture. You can't get rid of a lot of moisture with just one firing cycle though. The solution is to use it more often. ;-)

Keep in mind that wood contains at least 8-10% moisture, sometimes a lot more. This moisture makes a more moist heat than in your kitchen's oven. There are pluses to baking with that moisture given off by the firewood. JMO

[mongota]

Vent Cap update...

I was a bit skeptical regarding the need for a vent at the apex of my dome, but for a couple dollars in materials and 15 minutes of my time I incorporated one. Why not?

I used black pipe with a threaded cap.

After a few months of sitting dormant, Ifired up the oven yesterday and a couple hours in to the firing, I thought "why not?". So i climbed up and unthreaded the cap. Moisture!

The inside of both the cap and pipe were wet.

I left the cap off and when i put it to bed for the night i reinstalled the cap. Came out today and popped the cap off. More moisture


I have 4" of CF board insulation under the floor. I have 4" of insulation over the dome (four 1" layers of CF blanket).


So, I'm sold. It certainly does seem to be a valid escape path for moisture within the dome structure.