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38" Igloo in Houston - Forno Bravo Forum: The Wood-Fired Oven Community

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38" Igloo in Houston

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  • #16
    I like the idea of casting the top of the flue gallery and chimney attachment plate. Casting a few inches of concrete on each side of the landing area would also alleviate my concern about the outward force from the front arch (and I wouldn't have to redo it.)

    I finished the transition and added a few more courses. My transition work was flawed, and I ended up with a bump in the front of the dome that has been difficult to recover from. I'm on the 10th course or so now, and the tilt on the bricks is pretty severe. I'm still not using a form or IT to hold the bricks because they seem to stick okay with mortar, but I have a feeling that the IT would have saved me from developing this bump.
    Last edited by JoelTexas; 04-30-2018, 01:37 PM. Reason: Fixed a word

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    • #17
      I finished the dome. The brickwork got pretty rough in the last two or three courses. I miscalculated the tilt angle on the 10th or 11th course, so I had to re-cut each brick to fit by guesstimating the shapes. After that, my circle was no longer round, so every single brick was custom cut based on my guessing and rough pencil marks. Plus, I could no longer see the inside surface, so the grout started accumulating pretty bad and I couldn't figure out how to clean it (I erred on the side of a smaller entry, so I can't quite get my shoulders through the gap). Maybe someday when the kids get bigger, I'll send one in with a wire brush.

      I bought 200 refractory bricks. The guys shorted me by 8 bricks, so I ended up with 192. After closing the dome, I had 4 bricks left over, so I guess it took 188, not including whatever I end up doing to finish the chimney (probably cast p-crete, but I'm not totally sure yet).

      I bought the bricks and concrete blocks a week ago today, so it took 7 days to go from blocks to dome with absolutely no experience, but a lot of time to spend learning.

      Tomorrow I'll finish the insulation and chicken wire and start on the stucco. Probably. Unless I need to do something about the chimney first. But I don't think so.

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      • #18
        I made some progress this week, but it was mainly about letting time pass while the mortar cures.

        I struggled with how big to make my vent. I had picked up a double-wall stainless vent pipe at Habitat for Humanity Restore for about $3 several months ago because it looked like it would make a great oven flue. It has a 6" inner diameter, and an adjustable length that can vary from 2-4 ft. I'm not sure the inner diameter is enough. I looked at building codes published by several organizations including the U.S. Federal Housing Administration and the International Code Council. The most conservative estimate I found called for a 1:8 ratio of flue size to opening size. That seems to mean that my 6" pipe should be good for up to 150 sq in. Coincidentally, my archway has an area of almost exactly 150 sq in. Other codes or situations called for ratios of 1:10 or 1:12, but that seems to be because they allow for longer pipes (15 feet or more) to move the air faster.

        So, it seems that my $3 flue pipe might work, but there's no comfort margin. So for now I built the flue with a 7" opening, and I fit it with an adapter that chokes it down to 6" to fit my pipe. If I get smoke out the front, I'll try making the chimney taller, and if that doesn't work, I'll refit with a larger pipe.

        Following David S's suggestions, I framed up a box around the flue gallery and filled it with a 10:1 mix of perlite and portland, and I also built a dam of bricks and homebrew mortar all the way around to shield the insulation from standing water soaking through during heavy rain.

        I also wrapped the dome in 4" of ceramic fiber blanket. I actually came up about 3 sqft short for the second layer of 2" blanket, so I patched the gaps with trimmings from the ceramic fiber board then wrapped the whole mess in chicken wire to hold everything in place.

        I was concerned about putting stucco directly on the blanket, because the blanket seems easily compressible. I thought about using more layers of chicken wire, and possibly actual expanded metal lath on top of that. But I decided to go with what seems to be a more tried-and-true approach around here. Even though the 4" of blanket is likely more than enough insulation, I also added a layer of about 3" of p-crete to give a lightweight but stable backstop to spread the mortar against.

        I found the p-crete to be easier to work with than concrete because it's so lightweight, but it was also less mold-able than mortar and tended to fall down easily when I tried to stick or shape it on the vertical sides. of the dome. So I used a roll of aluminum flashing as a mold to hold the bottom 12 inches in place while it set. For the top I found that it worked well to scoop a good sized pile onto a flat piece of plywood or cardboard, then gently press the whole board against the side of the dome and hold it in place for a few seconds. That seemed to allow the stuff to hold together well enough that I could gently remove the cardboard without everything sliding back down.

        I included a 1/2" vent tube through the p-crete to give the insulation an outlet to vent moisture, as I've seen a lot of people on here have done.

        I think I will wait on applying the stucco until after I have a series of curing fires to really dry out the oven and insulation. I am not sure that the fires will do much to drive moisture out of the p-crete thanks to the heavy insulation, but I want to give more time for the extra water to escape before I seal it in, and also I want to allow more time for the p-crete to harden before I start smashing mortar against it. I also thought to wrap everything in plastic for a day or two to help it moist cure before the drying starts.

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        • #19
          I think you’ll be ok with the 6” flue. The recommendation is that 6” is ok up to 36” diam oven and you’re only just over that. As you say, you could go a little higher to increase draw, see how it goes. Once your vermicrete is dry (24 hrs) you can carve the high spots as well as fill the low spots quite easily to improve the form. Well done, your build is going well.
          Kindled with zeal and fired with passion.

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          • #20
            Today, I kicked off the curing fire schedule with about 13 hours of little smoldering fires keeping the oven at around 300 degrees throughout the day.

            I rushed the schedule slightly by starting on day 6 instead of day 7, but I don't know when I'll again have time to sit and fiddle with the fire all day, and from what I've heard, the first day is the trickiest. Also, I think I saw an old recommendation that 5 days of moist curing should be adequate, so I suppose I split the difference.

            While managing the fire, I was watching and listening for cracks or strange smells or noises that might indicate too much heat, but I didn't notice anything beyond the crackle of the wood and smell of wood smoke. It would probably be hard to see anything short of catastrophic cracks in the dome since I had already covered the masonry in around 8 inches of blanket and perlcrete.

            For the first several hours, I found it pretty challenging to maintain a constant temperature. Then after 8 hours or so it became rock steady, and I'd just add a new stick about once or twice an hour. I suppose that the water was fighting me at first.

            I also noticed that the outside temperature stayed cool to the touch for the first 6 hours, but then a spot on the top started getting warm. The surface temperature of the top of the outside perlcrete peaked at about 150 F. Later in the evening, I could finally see steam emerging from the top, and the perlcrete took on a wet look in one small area. I also sniffed the steam, and it smelled like wet cement.

            I took the opportunity to heat up a pot of my wife's plov at lunch time and again for dinner. The slow fire worked great for that.

            I let the last wood burn down, then sealed the oven for the night with a makeshift door of foil-wrapped FB (board). I'm curious to see what the oven temperature will be in the morning after steaming all night.

            Sitting by the fire all day was less physically demanding than some of the earlier phases (I'll never forget the day I spent mixing all those 80lb bags of concrete with a hoe.) The first curing fire feels like a significant milestone, and I enjoyed the built-in celebration of just sitting and watching the fire. I know some people like to keep working on the finish while the curing fires burn, but I'm glad I did it this way.


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            • #21
              I'm on day 4 of the curing fires. At this point, the top of the dome is no longer hot, but the heat has reached the other side of the concrete slab. In the storage area under the oven, the concrete blocks are hovering around 85 F, but the slab directly under the cooking floor has reached 115F. I am a little bit surprised to see this, because I thought that the 4" floor insulation was over kill. I suppose it means that some water got into the floor insulation.

              I haven't seen any cracks in the dome, but I can only see it from the inside. Some of the perlite concrete shell seems to have cracked because it feels a little softer in spots, like it has broken into several pieces. I think it's still firm enough to spread stucco against, so I probably won't add any more perlite. I did see one new hairline crack in the front arch. I'm not worried about it, because even if all the mortar in that joint fell out, I still have a big perlcrete box around it to carry the minimal weight of the chimney.

              I reused the aluminum flashing from my perlcrete mold to make a door. I had some leftover drawer pulls from a recent kitchen project, so I attached them to a piece of cement board, stacked 4" of leftover insulation board on it, and wrapped the whole thing in flashing. I don't think it will last very long, but it's better than just wrapping the board scraps in foil like I've been doing.

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              • #22
                Day 5 of the curing fires.

                Today I noticed that a section of the outer perlcrete has completely crumbled and opened up a hole exposing blanket underneath. I recovered about two gallons of cement-covered perlcrete that crumbled from the shell. I think that my perlcrete mix was probably too dry when I was applying that section.

                I think this hole is really too deep to fill with stucco, so I am considering mixing up a new batch of perlcrete to fill the hole and maybe beef up the whole area that's soft now. I guess it's a bad idea to do this while the oven is hot, because it seems like it might push the moisture out of the perlcrete before it sets up. I am kind of anxious to get a weather-resistant coating on the oven before we start getting more rain, and so I hate to add more perlcrete because it will delay everything while waiting for the perlcrete to moist cure and then dry out again with a bunch more fires.

                I am tempted to wait for the oven to cool, then try filling the hole with a fire resistant expanding spray foam (like Great Stuff Fireblock). That wouldn't introduce any water, and I could trim and stucco over it the next day.The safety sheet says it shouldn't be exposed to temperatures above 240F, but since it would be isolated from the brick surface by 4" of blanket, it shouldn't ever see significantly high temperatures. It may not last forever, but I think the stucco should be self-supporting once it cures even if the foam eventually decomposes leaving a void. Anyone know if this is a terrible idea?

                Today, the temperature in the hole is about 140 degrees F while the temp inside the oven is around 500. The temp under the floor is currently about 102 (yesterday it reached 120). The rest of the outside shell is approximately ambient temperature. I think this means that the oven isn't dry yet, and the water vapor is escaping through the huge hole instead of continuing to the top where it was previously hot and damp.

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                • #23
                  You can easily mix up some more perlcrete and fill those holes and cracks. When it has set (24 hrs) you can also easily carve it back. The correct amount of water is 3 parts by volume for every 10 parts perlite or vermiculite and 1 part cement. When mixing it up it should just start to pool in the bottom of your barrow or bucket. Too much water washes the cement off the grains. I’ve found a mix of half perlite and half vermiculite makes for a more workable mix than using either of them alone. You can also add some sand to the mix which makes it easier to apply by trowel, but will reduce its insulating value.
                  The under slab temperatures you report indicate there’s still moisture in the floor insulation. It will improve as you continue using the oven.
                  Last edited by david s; 05-11-2018, 03:16 AM.
                  Kindled with zeal and fired with passion.

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                  • #24
                    Yeah, you're right. This isn't the time for short cuts.

                    The oven is at 350 F now and cooling, so I went ahead and mixed up the rest of my perlite (18 quarts) with some sand (4 quarts) and a little extra portland (3 quarts). This mix was a little stickier and easier to work with than the 10:1 mix I was using before. I worked it into the holes and used the rest to even out the shape of the dome. I'll give that a day or two to set, then start the stucco.

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                    • #25
                      I made a stucco mix with 4 parts sand, 1 part lime, 1 part white portland, and a pinch of fiberglass fibers in each batch. I applied the scratch coat and the brown coat. Then I wrapped the whole thing up in plastic to cure for a few days. I am pretty happy with how the stucco looks, so I won't feel bad leaving it like this for awhile (until it's very dry). I want to eventually seal it (Redgard, maybe?) and then put tile over it, which is why I skipped the finish coat.

                      After applying the brown coat, the dome is within 1/4 inch of the edge of the block wall on both sides. I suppose that's okay, but I'll have to figure out how to do the transition in that area since I'm planning to do a stone veneer on the dome and a different stone veneer on the base.

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                      • #26
                        Very nice finish on the stucco.
                        joe watson

                        "A year from now, you will wish that you had started today "

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                        • #27
                          That dome is going to look great with a stone finish!
                          joe watson

                          "A year from now, you will wish that you had started today "

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                          • #28
                            Thanks, Gulf. I thought about your amazing revolving trowel a lot during this process, wishing I had the skill to rig up something like that without levering the top of my dome off. I'm happy with how it's turning out just by eyeballing it, but I really wasn't sure if it would be okay or not.

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                            • #29
                              I decided to go ahead and stucco the base so that it will look nicer until I get around to doing the stone veneer. For now, the backyard has a definite Tatooine feel.

                              We are having afternoon temperatures in the mid 90s now, so I have the whole thing wrapped in Costco cling wrap to help with the moist curing, and I'm keeping a tarp over it since I couldn't get the cling wrap to stay on the top.

                              I don't think I've shown my steam vent yet, so I'll put it here. The PVC penetrates the stucco and perlcrete layers and rests on a seam between two pieces of blanket. I just used PVC for the vent, since I don't expect the shell to get very warm.

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                              • #30
                                PVC is fine for a vent. Do you have plan for capping it?
                                joe watson

                                "A year from now, you will wish that you had started today "

                                My Build
                                My Picasa Web Album

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