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Pdx 42" update

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  • I thought it would be a challenge to measure the OD, but I'm doing pretty well with a yardstick and "compass" attachments. 4 bricks in, it seems pretty consistent that the OD intersects the outside of the dome at the depth of 4.5", which happens to align with the depth of the oven wall bricks. Pictures to illustrate...

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    My build thread: https://community.fornobravo.com/for...-pdx-42-update

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    • Should be close enough for "brick work"
      Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build

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      • At the end of the day I've been able to mark up all but the last 2 blocks in the arch. The 5 blocks at the peak of the arch, including the keystone, were cut two deep on the overcut so will have to be redone. The rest can be salvaged.
        My build thread: https://community.fornobravo.com/for...-pdx-42-update

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        • Remember that home brew mortar is fairly forgiving if the gaps can be filled with mortar. A half to 5/8ths inch is ok imo. Anything over can sometimes be stretched with a sliver of waste cut placed in the middle of the bed joint.
          Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build

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          • Good point, I was concerned about shrinkage and cracks but based on your guidance I can get away with a little extra mortar or using some waste material.

            In addition to the days getting shorter, weather is getting cold now with snow possible later this week. In anticipation I've taking the canopy down and covered everything with tarps. Will have to see if weather cooperates.

            Cheers,

            Mac
            My build thread: https://community.fornobravo.com/for...-pdx-42-update

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            • Originally posted by Gulf View Post
              Remember that home brew mortar is fairly forgiving if the gaps can be filled with mortar. A half to 5/8ths inch is ok imo. Anything over can sometimes be stretched with a sliver of waste cut placed in the middle of the bed joint.
              Commercial refractory mortars recommend that only really thin joints are suitable. I am not totally sure why, but suspect they are covering themselves for warranty issues due to failures caused by steam spalling. As the same issues arise with steam spalling in a thick homebrew joint a simple remedy used for castable refractory would seem a sensible solution. The addition of fine polypropylene burn out fibres to both commercial refractory mortar as well as homebrew mortar will certainly reduce the risk of steam spalling, The drawback is that they require extended mixing time for proper dispersal. insufficient dispersal results in the fibres clumping and does not produce the adequate network of mini pipes that allow moisture to escape. This is something that does require some experience regarding dosage and mixing times.
              Kindled with zeal and fired with passion.

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              • Macrinehart
                ......I was concerned about shrinkage and cracks but based on your guidance I can get away with a little extra mortar or using some waste material......
                From what I can see in your pic, the top arch bricks shouldn't be very far off from the taper. I agree with JRPizza in his post #176. I too like a thicker mortar joint on the outside of the dome. You will probably find that you can compensate when cutting the dome brick to intersect the arch.

                Commercial refractory mortars recommend that only really thin joints are suitable. I am not totally sure why, but suspect they are covering themselves for warranty issues due to failures caused by steam spalling. As the same issues arise with steam spalling in a thick homebrew joint a simple remedy used for castable refractory would seem a sensible solution. The addition of fine polypropylene burn out fibres to both commercial refractory mortar as well as homebrew mortar will certainly reduce the risk of steam spalling, The drawback is that they require extended mixing time for proper dispersal. insufficient dispersal results in the fibres clumping and does not produce the adequate network of mini pipes that allow moisture to escape. This is something that does require some experience regarding dosage and mixing times.
                Thanks david s,
                I wouldn't want anyone to confuse my advise for a home brew mortared brick dome with the refractory calcium aluminate cements which have a 1/8th" tolerance. I'm also sure that 2"+ home brew castables need a little more attention to their ingredients. However, as far as steam spalling is concerned for a home brew mortared brick oven......... that is what the curing and drying schedule is for.
                Last edited by Gulf; 11-29-2022, 06:48 PM.
                Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build

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                • Originally posted by Gulf View Post
                  Macrinehart


                  From what I can see in your pic, the top arch bricks shouldn't be very far off from the taper. I agree with JRPizza in his post #176. I too like a thicker mortar joint on the outside of the dome. You will probably find that you can compensate when cutting the dome brick to intersect the arch.


                  Thanks david s,
                  I wouldn't want anyone to confuse my advise for a home brew mortared brick dome with the refractory calcium aluminate cements which have a 1/8th" tolerance. I'm also sure that 2"+ home brew castables need a little more attention to their ingredients. However, as far as steam spalling is concerned for a home brew mortared brick oven......... that is what the curing and drying schedule is for.
                  Given that most castable refractory products contain burnout fibres as well as instructions for first firing schedules is an admission that steam spalling is a common problem for thick joints and why slow firing schedules alone are insufficient. When firing with wood a slow controlled rise is almost impossible to achieve unlike gas electricity or oil that can be finely metered. Why else would refractory mortars limit the size to 1/8"? A small brick oven will have gaps on the outside of the dome that may well exceed 2". The larger the radius the smaller the gap, so the problem is reduced, but certainly always exceeds 1/8" Filling these gaps with brick wedges is a way of reducing the thicknesss of the mortar joint as well as utilising waste offcuts.
                  From my experience the homebrew recipe's high clay proportion sometimes leads to shrinkage cracks, so I halve it. I know the clay imparts a degree of refractoriness, so while the lower clay proportion fixes the shrinkage problem, I'm not sure if it has other drawbacks, so don't take my lead on that idea.I do however, always add some burnout fibres which also hold the mortar together well.
                  Last edited by david s; 11-30-2022, 04:20 AM.
                  Kindled with zeal and fired with passion.

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                  • Folks - I've been looking through threads on building opening arch and trying to decide, should I go ahead with mortar for the entire opening arch first, and then build an mortar the oven dome layer by layer? Or should mortar the opening arch at the same level as my dome? I've seen pictures both ways but not sure if there is a preferred approach.

                    Cheers,

                    Mac
                    My build thread: https://community.fornobravo.com/for...-pdx-42-update

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                    • It's six one half a dozen the other imo. The geometry doesn't change. I like completing the arch first as it allows for the arch form to be dropped much sooner for clean up. Even with completing the arch first, two or three dome courses should be partially completed on the arch side. That is done to give the arch stability since the upper arch brick protrude into the dome making it extremely top heavy.
                      Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build

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                      • Either way works but if you do the arch in layers be sure to keep the arch course ahead of the dome courses, it is easier to bring and dome into the taper arch rather than trying to cut a complex mating brick with the dome ahead of the arch. I built the arch complete to make sure is all came together. You risk the top of the arch not fitting due to mortar joint thickness creep.
                        Russell
                        Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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