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Strength vs Simplicity - Forno Bravo Forum: The Wood-Fired Oven Community

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Strength vs Simplicity

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  • Strength vs Simplicity

    I have spent the last two years reading this forum and making notes. I am in the process of designing my build and have a basic question that I can't seem to find a direct answer to.

    I understand that the smaller (thinner) the mortar joint, the stronger it is. I will be using either the FB mortar or HeatStop 50 (opinions as to which is better?).

    Being a closet perfectionist and often making things harder just for the sake of doing so, I am inclined to angle cut the tops and bottoms of bricks in the dome chains to decrease the size of the mortar joints.

    Please correct me if I am wrong or making this too complicated:

    I assume:
    1. less mortar = less of a chance of dome cracking during curing
    2. less mortar = greater overall strength in the dome due to the smaller joints
    3. angled tops and bottoms = smaller exposed front faces = more brick
    I have designed the dome (42" oven) with 1/4" mortar joints on the outside of the dome, tapering to 0" on the inside. That is the interior faces of the brick will touch without (or at least a minimal) mortar joint.

    This means an angle cut of somewhere between 3 and 4 degrees on the top and bottom face of each brick, leaving a face height of about 2.25" if a 1/4" rear mortar jount is used. Is this common and/or acceptable? Is it worth the extra effort?

    If using a similar bevel cut on the sides, is the same wedge shaped mortar profile acceptable (1/4" at the exterior, tapering to 0" where the interior sides of each brick touch)?

    Thanks in advance! Once I get past this design sticking point, I feel I can move forward rather quickly and get my foundadtion ready.
    Attached Files
    Last edited by BeanAnimal; 03-23-2011, 10:05 AM.

  • #2
    Re: Strength vs Simplicity

    It sounds like you're on the right track. Although not a specific answer, I'd suggest using an existing thread in the forum rather than starting a new one when you have questions. I recently started one and was dissapointed how few responses I got. I'm not totally sure how everyone uses the forum, but it seems to me the hot threads get more responses and hits.

    Cutting each brick perfectly and minimizing mortar is a better way to go. My build was quicker and I cut each brick in half with a chisel then tapered them with an angle grinder. works, but not perfect and left bigger gaps. I knew this going in, but I wanted to get it done in my lifetime. I used heatstop 50 and really like the product.

    With mine, because there's alot of mortar, I tried to cure very slowly. I kept wet towels over it for days and weeks as I built the dome to keep it damp and to keep the sun from drying the mortar. I would suggest that method for any build. Also, I soaked bricks overnight, left them out to dry for a while before setting them the next day.

    It was a gradual build over about 6 weeks. After the last brick was in, I let it air cure for 10 days before building a small curing fire. I've been having daily fires for over a week now, and this weekend I'll finally try a hot one.
    Last edited by Paul in Rockwall TX; 03-23-2011, 10:25 AM. Reason: i erased half of my post

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    • #3
      Re: Strength vs Simplicity

      Originally posted by BeanAnimal View Post
      That is the interior faces of the brick will touch without (or at least a minimal) mortar joint.
      Bean,
      I did not taper the brick on the top and bottom, I just made a wedge shape and tapered the sides. I was able to achieve what you are talking about.

      Originally posted by BeanAnimal View Post
      Is it worth the extra effort?
      I think it was - you won't see any cracks.
      Check out my pictures here:
      http://www.fornobravo.com/forum/f8/les-build-4207.html

      If at first you don't succeed... Skydiving isn't for you.

      Comment


      • #4
        Re: Strength vs Simplicity

        Building a perfect fit dome is a matter of some difficulty. It doesn't result in a dome that cooks better: It's an exercise in pure craftsmanship. Remember, that if you are building a dome of the proper thickness, the long cut on the face of the brick will be more than you can cut in one pass on a ten inch wet saw. This is slightly less difficult if you can get tapered refractory bricks, but they are considerably more expensive. In any case, you will end up with mountains of brick slivers and go through a lot of diamond blades.

        We've learned that the best commercial oven builders in Italy only trim the inner edges of the bricks so that they appear to fit on the visible side, and fill the outer gaps afterword.

        The issue of cracking and the particular mortar you use has not been fully answered. Some builders claim no cracking with the home brew mortar, and some, like myself, have lots of cracks despite thin joints and heatstop50 use. (In my case I think that's because I tried to build a thin dome). I think the best plan is to resign yourself to a few cracks no matter how you build your dome.

        The main question you have to ask yourself is whether you want to spend your time building, or cooking. If you want to build something fabulous, you'll get lots of applause from the sidelines.
        My geodesic oven project: part 1, part 2

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        • #5
          Re: Strength vs Simplicity

          Not to diminish the hard and beautiful work that dozens of folks here have done, but I can't bring myself to fill such large gaps with mortar. I almost feel I MUST bevel and taper (angle?) each brick for a better fit.

          You mention numerous blades? Are new fire brick really that hard on the blades? WOW!

          I have the 10" HF saw so it would appear that I am going to have some trouble. I have a few fractional HP motors laying around, I may have to throw together a makeshift saw with a 12" or 14" blade for the deeper angle cuts, or put a diamond blade in one of the cheap $99 14" cut-off saws.

          LES:
          I don't follow. You ONLY cut the sides and left the top and bottom parallel? So in effect you reduced the vertical joint size and decided to live with the larger horizontal joints. I presume due to the limitation of the saw's depth of cut as well?

          Comment


          • #6
            Re: Strength vs Simplicity

            "but I can't bring myself to fill such large gaps with mortar. "

            Fill the larger gaps with "extended" mortar. Anything over a 1/4 inch, extend your mortar by mixing 2:1 with mortar sand (in effect have two batches of mortar). Also use bits of wedge shaped bricks left over from your cutting. All this will reduce shrinkage cracking.

            Comment


            • #7
              Re: Strength vs Simplicity

              Originally posted by BeanAnimal View Post
              LES:
              I don't follow. You ONLY cut the sides and left the top and bottom parallel? So in effect you reduced the vertical joint size and decided to live with the larger horizontal joints. I presume due to the limitation of the saw's depth of cut as well?
              Correct. There was zero mortar on the sides and interior of the oven. But, I had about 5/8 of an inch between bricks on the exterior. As David pointed out, all that work adds nothing to the end result - food.

              Also, I recall going into my third blade (cheap ones). I was cutting paver's as well and they will eat up a blade a LOT faster then the fire brick.
              Check out my pictures here:
              http://www.fornobravo.com/forum/f8/les-build-4207.html

              If at first you don't succeed... Skydiving isn't for you.

              Comment


              • #8
                Re: Strength vs Simplicity

                LES:

                I had wondered if the dome bricks could be cut to dry fit well and in essence only mortared on the outside. It would make the vertical cuts MUCH easier due to the simple single angle that reference the center point of the dome. Attempting to create touching vertical joints on the dome side and a constant 1/4" gap on the outside introduces odd angles that don't reference the center point of the dome and change for each course.

                Are you the only builder who has used no mortar in the vertical joints and instead dry fit them?

                To the other experienced builders: Thoughts on this method? I sure like the way Les's dome turned out.
                Last edited by BeanAnimal; 03-24-2011, 04:06 AM.

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                • #9
                  Re: Strength vs Simplicity

                  You mention numerous blades? Are new fire brick really that hard on the blades? WOW!
                  I think blade number five is on my HF saw. I lost count. Like Les, I built more than my dome, but still. The secret to blade longevity is use LOTS of water (more than that micro pump can move) and very little cutting pressure.
                  put a diamond blade in one of the cheap $99 14" cut-off saws.
                  It can't be said often enough. In any sort of dry cutting PROTECT YOUR LUNGS.
                  My geodesic oven project: part 1, part 2

                  Comment


                  • #10
                    Re: Strength vs Simplicity

                    dmun,

                    Thank you for adding the very important safety precaution that I overlooked when mentioning using the dry saw. Silicosis is not a pleasant ailment and certainly dry cutting brick, concrete and/or pavers is certain to release very fine dust laden with silica! I spent many years working in the mining industry, some of it in rock mines. I spent a great deal of time drilling in limestome mines (roof bolting) and it was not uncommon to see guys drilling without water and without respiratory protection!

                    Do you see an issue with dry fitted vertical joints?
                    Last edited by BeanAnimal; 03-24-2011, 04:56 AM.

                    Comment


                    • #11
                      Re: Strength vs Simplicity

                      Originally posted by BeanAnimal View Post
                      Do you see an issue with dry fitted vertical joints?
                      I didn’t see an issue with it. I thought of it as an advantage. With all the expanding and contracting going on, I felt it would minimize cracking. I have no clue what is happening at the back since it’s covered.
                      Originally posted by BeanAnimal View Post
                      Are you the only builder who has used no mortar in the vertical joints and instead dry fit them?
                      I am not aware of any - I know a few have come extreamly close. I think if someone had the right saw and time you could dry fit the entire oven other than the entry.
                      Check out my pictures here:
                      http://www.fornobravo.com/forum/f8/les-build-4207.html

                      If at first you don't succeed... Skydiving isn't for you.

                      Comment


                      • #12
                        Re: Strength vs Simplicity

                        I had thought that a total dry fit would work as well, but the angles just get so complex. I have laid out the entire dome in autocad and can see that one would need a CNC machine to cut anything after about the 4th chain. Even the bottoms of the 1st-3rd chains would need a bit of easing to flit perfectly flat.

                        I am going to go ahead and attempt to work with dry fit sides and attempt to taper the tops and bottoms for a cleaner fit with mortar.

                        Is that 4" of FB board you used on th hearth?

                        Comment


                        • #13
                          Re: Strength vs Simplicity

                          Yes, I used 4 inches of board under the hearth. You mentioned a CNC machine. I think the ticket would be a water jet.
                          Check out my pictures here:
                          http://www.fornobravo.com/forum/f8/les-build-4207.html

                          If at first you don't succeed... Skydiving isn't for you.

                          Comment


                          • #14
                            Re: Strength vs Simplicity

                            I think the ticket would be a water jet.
                            For the cost of water jetting 20 different styles of bricks in two planes you could buy multiple modular ovens.
                            Really, look into tapered firebricks. They're worth the cost if you're going to cut that third plane for perfect fit.

                            Harbison Walker has a branch near you. Call them before you commit yourself to months of work. Everyone's time is worth something.

                            Buncher Industrial District Michael Stillwagon
                            Ave. B Bldg #14-A
                            Leetsdale, PA 15056-1390
                            My geodesic oven project: part 1, part 2

                            Comment


                            • #15
                              Re: Strength vs Simplicity

                              I used the large saw to make rough tapered and/or compound cuts and then used an angle grinder on each brick as I went along laying them. Not precise, but it minimized joint gaps on the inside surface. I left the gaps large on the dome exterior and filled them with home brew mortar.
                              George

                              My 34" WFO build

                              Weber 22-OTG / Ugly Drum Smoker / 34" WFO

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