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  • Building a 42" oven mainly for bread

    Hi All,

    New user, first post, so please excuse me if I am doubling up on previous posts (I have tried searching).

    I've started building a 42" Pompeii Oven using the Pompeii Oven Plans Version 2.0. These instructions are awesome, but I am struggling to work out a small part regarding the hearth. My wife and I are big bread makers, and plan on making high volume breads with this oven along with pizza.

    On page 28 of the instructions, it show how you could add extra heat mass by sinking a brick island into the insulating layer. It says to take care not to reduce the your insulating layer to less than 3.5 inches.

    Does this mean that the hearth is comprised of the 3.5 inches of structural concrete, then a 3.5 inch insulating concrete layer, followed by the layer of firebricks that make up the extra heat mass under the cooking floor which could then be surrounded by normal concrete?

    I can't imagine that the extra firebricks being set into the 3.5 inch insulating layer would be safe, as it would only leave an inch or so of insulating concrete before the structural concrete layer. If I built the hearth layer as a 9.5 inch rather than 7.5 (being 3.5 structural concrete, 3.5 insulative concrete, 2.5 firebrick surrounded by either insulative or structural concrete) would it be a bad, unthinkable, or unrecommended configuration?

    Thanking you in advance!

    Matt


  • #2
    If you are worried about thermal mass you could set the floor bricks on edge so you will have 4.5"on the floor. You could also render a layer of home brew mortar on the outside of the dome about 1-1.5 "thick. That should give you plenty of bread output. Also do 4" or more of ceramic board insulation under the oven, and don't bother with the vermicreete it is not as good a insulator and is a lot more work. Also do at least 3"ceramic blanket over the top of the dome. Then if you do a enclosure then fill the void with vermiculite as losses fill and you should have tons of thermal mass. It will take more wood and time to heat up, but that is the price you pay for very extended baking. Or if you want to cut it back a little then do the standard oven and a 1\2" cladding and the same insulation and have very good heat retention for baking.

    Randy

    Comment


    • #3
      Hi Randy,

      Thank you so much for such a detailed response.

      I apologise for such a late reply, as I've been very busy building this oven.

      For the oven floor and first round of bricks, I was able to get a really thick rectangle fire brick so I don't think I'll need to lay them on their side - each brick is roughly 11cm high/deep. I've taken on your idea of outside mortar.. I'll use the refractory stuff I'll use to secure the dome around it too. For the dome, I was able to get tapered kiln bricks, these are new and left over bricks from a local brick manufacturer after they made their kilns, so should make an awesome dome.

      I'm up to the part where I need to decide on hearth insulation. My plan was to use vermicrete, but I'm hearing more and more that I should stay away from it, and your post is added to the list. I really like the idea of fibre board, but I didn't account for how expensive it is. I've managed to find and cost up 2" ceramic fiber board, but it's going to cost more than $600 !!!! I don't have $600 for the insulation layer on a top of the structural concrete. There has to be a cheaper way (sub $500). A friend of mine said to use hebel bricks... they're damn cheap and easily sourced, but I simply don't know if I should be doing that. I've very conscious of heat escaping through the floor, and I don't know how that will stack up. There's a little about hebel on this forum, but Im not finding enough conclusive information to make me go down that way. What do you think?

      I will post a picture of my progress after this post.

      Apologies again for the lateness of this reply.. I've been hard at work building this lovely oven!!!

      Regards,
      Matt

      Comment


      • #4
        As you can see, it's quite messy. This was my first ever attempt at brick laying, so i've got a lot of clean up to do. Pretty happy though

        Comment


        • #5
          Vcrete or perlcrete are economical options. Either of these options are about 0.5 as effective thermally wise than cf board, IE 6" v or pretend equals 3" cf. DavidS has a table showing the K of vcrete by ratio of cement to vermeculite. If you are on a budget and your build can accommodate the extra height and OD then these type of insulation are an option.
          Russell
          Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

          Comment


          • #6
            Hi Russell,

            Not really on a budget, but saying that I've already spent over 5k on materials and would like to see things slow down a little but still have a chimney, heat mortar, and blanket to buy. I didn't put $600 down for hearth insulation, but if if costs that much then I just need to spend that. The oven doesn't have too much a height restriction so I could go the vermicrete, though I did read several bad things about it I wanted to avoid.

            Thanks, Matt

            Comment


            • #7
              Originally posted by UtahBeehiver View Post
              Vcrete or perlcrete are economical options. Either of these options are about 0.5 as effective thermally wise than cf board, IE 6" v or pretend equals 3" cf. DavidS has a table showing the K of vcrete by ratio of cement to vermeculite. If you are on a budget and your build can accommodate the extra height and OD then these type of insulation are an option.
              This info suggests a lean (10;1) vermicrete has a similar thermal conductivity as ceramic blanket and calcium silicate board at 200 C The addition of cement to hold it together quickly reduces its insulating capacity. I also suspect the blanket performs better as the temperature climbs above 200 C Also any moisture present in any insulation layer drastically reduces its insulating capacity. Whilst a 10:1 mix is workable over the dome and is strong enough to act as a suitable substrate to render over, it is too weak and crumbly for underfloor insulation. A 5:1 mix is required there and as Russell has stated is around 1/2 as good as the cal sil board. So to get the equivalent insulation with 5:1 vermicrete as cal sol board you need to make it twice as thick. i.e. 4" of 5:1 vermicrete.
              http://www.dupreminerals.com/downloa...de-screeds.pdf
              http://www.morganthermalceramics.com...ty_sept_14.pdf
              Calcium silicate insulation brochure.pdf
              Kindled with zeal and fired with passion.

              Comment


              • #8
                Thanks David.

                This is kind of off topic, but does anyone know of experienced oven builders in Adelaide? I wouldn't mind having someone come out to give me advice on a different issue I'm facing.

                Comment


                • #9
                  I think you are on the right track trying to find some more oven owners near you. They will have insight on where to get stuff and what to use. I did not realize you were down under. I gave advice thinking you were in the USA . There are probably other options that might be a better value and more available. I don't know how much of the supply you have so far, but I spent roughly $6000 us to build my oven. I know fire bricks cost more in your area but it sounds like you have most of that stuff already. Good luck. I know you will find some good help on here.

                  Randy

                  Comment


                  • #10
                    Thanks Randy.. this forum has been pretty awesome so far

                    Comment


                    • #11
                      Originally posted by matthanna View Post
                      Thanks David.

                      This is kind of off topic, but does anyone know of experienced oven builders in Adelaide? I wouldn't mind having someone come out to give me advice on a different issue I'm facing.
                      Try giving Nissanneil a PM. He's always been keen to help Adelaide folk
                      Kindled with zeal and fired with passion.

                      Comment


                      • #12
                        Thank you. I will

                        Comment


                        • #13
                          Originally posted by RandyJ View Post
                          If you are worried about thermal mass you could set the floor bricks on edge so you will have 4.5"on the floor. You could also render a layer of home brew mortar on the outside of the dome about 1-1.5 "thick. That should give you plenty of bread output. Also do 4" or more of ceramic board insulation under the oven, and don't bother with the vermicreete it is not as good a insulator and is a lot more work. Also do at least 3"ceramic blanket over the top of the dome. Then if you do a enclosure then fill the void with vermiculite as losses fill and you should have tons of thermal mass. It will take more wood and time to heat up, but that is the price you pay for very extended baking. Or if you want to cut it back a little then do the standard oven and a 1\2" cladding and the same insulation and have very good heat retention for baking.

                          Randy
                          Randy,

                          This home brew mortar, is there are particular thread I can find the recipe? I've been searching, but no successfully.

                          Thank you,
                          Matt

                          Comment


                          • #14
                            Belated welcome to the forum Matt. Out of curiosity, in your first post you mentioned that you and your wife were looking at baking mainly bread...what kind of volume (number/loaf size) are you thinking about with this oven?

                            I bake mostly bread in my oven (39") but only once per week . I tend to make from 15-20 loaves every Friday and give them to my neighbors. My oven only has 3.5"-4" of perlcrete (1:5 mix perlite and cement) for its insulating slab, but holds heat plenty well for quite a few loaves of bread. The oven is covered with about 4" of 1:10 mix perlcrete. Our "record" is 30 loaves one afternoon-one firing, but the real limitation has been prep room/proofer space for us. I generally like to do a baguette style loaf (sized at 400 g) for the neighborhood and pop them in the oven (loaves - not the neighbors ) starting at 575F for about 15 minutes.

                            Anyway, just curious about your bread making projected quantities. Again, welcome to the forum and good luck with the Adelaide contact via David above.
                            Mike Stansbury - The Traveling Loafer
                            Roseburg, Oregon

                            FB Forum: The Dragonfly Den build thread
                            Available only if you're logged in = FB Photo Albums-Select media tab on profile
                            Blog: http://thetravelingloafer.blogspot.com/

                            Comment


                            • #15
                              Hi Mike,

                              Thanks

                              The wife's a solid baker and works in a bakery. She bakes a lot daily, but is hoping to start her own wood oven bakery/outlet here in adelaide. My guess is we'll be looking produce 40/50+ loaves per day though not all traditional loaves (she's very experimental and creative).

                              I'm guessing that this oven will have loads of mistakes and will likely need redoing, so while I haven't been even finished my first build, I'm trying to learn as much as possible for a second oven. I'm guessing that once the first is finished, we'll be learning what works, and what's broken. Then either tear it down or build another oven with fixes to problems. If the first doesnt need replacing, we'll be improving only with the second.

                              I have very little knowledge and skill in this area, so all this is quite hard and very timely.

                              Regards, Matt

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