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30" cast dome design

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  • #76
    The 10:1 mix is really weak. It is designed as an additional insulation layer. As you are reluctant to give your oven a hard outer shell, the 3:2:1 vermicrete is a suggested compromise that will provide a stronger shell as well as restoring the hemispherical form.
    Last edited by david s; 09-13-2020, 12:52 PM.
    Kindled with zeal and fired with passion.


    • #77
      Ok, thanks! What if I do 5:1 vcrete instead of 10:1? I am curious. How much water to add to the 3:2:1 mix?
      I thought that not doing the outer render will help me do less but it seems to complicate things. What's better overall - single 3:2:1 layer or still 10:1 plus outer render? I think I am leaning towards 10:1 vcrete and then a render which seems to be more conventional and maybe even simpler to accomplish. Thank you, David!

      I am a computer guy - that's why I am asking all these questions. I blame it on the job. They pay me "big bucks" to ask questions
      Last edited by sergetania; 09-14-2020, 12:12 AM.


      • #78
        I think I am going with a 10:1 vcrete(40-50mm thick) followed by an outer render layer. At least I saw it done on tens of pictures here so I should be able to manage. In this case I just need to find out how to do an outer render.

        David or someone else (David, I hope you are OK!!!), please explain to me:

        1. What do I use to put an outer render?
        2. How thick is the layer? Assuming just one layer that will go under the waterproof sealant.
        3. Do I wait for the vcrete layer to dry before applying a render?

        Thank you! These may be the last "major" questions that I need to figure out. Cheers!


        • #79
          HELP!!! Trying to apply a 10:1 perlite layer and it is not sticking to the blanket, mostly on one side of the dome, falling off. What can I do if anything?
          Click image for larger version

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          • #80
            That's the reason that a lot of us used chicken wire or metal lath over the insulation batting...10:1 perlcrete is an insulator...not a structural material as David S noted in post #76. There are several options to deal with this problem. The layer you are applying in your picture is very thin and just won't have enough strength to "hold on" or to support itself. I've listed four options below, since you don't really have enough space to make a thicker layer, option 3 is your best bet with some hints at what might help in the other options.

            1) You can make a thicker layer moving around the base, but only go up a short way. Let that layer/course set and come back a bit later and do another layer on top. The hardest part of applying perlcrete or vermicrete is the lower half of the dome. As you move up the dome, gravity starts to help you instead of beat you up. Since this isn't really an option for you based on the picture

            2) You can get a piece of flex board...really almost anything you can bend in an arc...and create a form around the base. That will hold the perlcrete in place until it sets a bit. Again, you don't have to make a form that goes to the top of the dome...just to where the dome starts to turn inward enough to let the perlcrete lay on it instead of having to support itself.

            3) Since you have an excellent insulator already on your oven (the batting), seriously consider the option David S proposed of making a 3:2:1 mix instead of 10:1...or just changing your current mix to a lower perlite percentage...even 5:1 would provide additional insulation and a "little more" structural support.

            4) Utah Beehiver and others used a curved paddle as a trowel/form, anchored at the top of the dome and rotated as the insulating cement was applied...again, it's worst at the bottom half of the dome where the inward curve starts helping.

            I hope some of this helps...don't give up and remember your oven is fully functional without more insulation over the batting...relax, you're at the stage of just making it "pretty"

            In regards to some of your other might consider using a waterproofing additive to the outer render layer, but nothing will be waterproof forever. Also, you do want to cure your oven BEFORE you start putting on anything like a final waterproofing/render. You've got to let a lot of moisture escape during the curing process, so sealing up the outside dome makes for some problems as that steam tries to escape. (Also a reason we have been recommending a vent be installed towards the top of the dome.)
            Last edited by SableSprings; 09-15-2020, 03:46 PM.
            Mike Stansbury - The Traveling Loafer
            Roseburg, Oregon

            FB Forum: The Dragonfly Den build thread
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            • #81
              Mike, thanks! Still working on this. I think I might have saved it for now by applying a band clamp. It holds the bottom. Also, I think part of the problem is I have added too much water and then overmixed it. At some point the mix has become very dense.

              Applied the the band and mixed another batch with much less water. It is weird to work with now but I hope I will hold after it dries. Hopefully, it is correct consistency. Can you tell from the picture? Will it be together tomorrow? It is kind of crumbly now. Also, do I have to finish it all in one go? I think I will have to leave this and finish tomorrow. Is that OK? Thanks for the help!!!
              Click image for larger version

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              • #82
                The perlcrete layer has two functions. Firstly as an insulation layer. Anything richer than 10:1 will not insulate too well.Anything leaner than 10:1 won’t stick. There are a few tricks you can use, but first make sure the mix is right. Mix 10 parts by volume perlite with 1 part cement dry in a barrow. Add 3 parts water slowly during mixing, knocking down any balls formed with the back of your spade. Don’t use a mixer.
                1. A 50/50 mix of vermiculite, perlite works better than either alone. Medium grade of each also works better than either alone
                2 A handful of powdered clay for every litre of cement added to the mix will make the mix more sticky. Acrylic enhancement will have a similar effect. The addition of more cement will have a similar effect but reduce insulation capacity.
                3 a thinner layer over the blanket can be left for 24 hrs which produces a better substrate to add over than the soft blanket surface.
                Hope this helps you.
                Kindled with zeal and fired with passion.


                • #83
                  David and Mike, thank you for the timely help during a crisis! Considering this is the first ... major hiccup!?.... of the build I think I am doing pretty good overall!

                  DO NOT USE A MIXER! Definitely a great advice! We figured it out, just a bit too late! So we did finish it in the dark using a flashlight. Fingers crossed, it will look and feel OK tomorrow. The band clamp saved the day. If you look at the picture in the previous post the lower part is dense, probably not too much insulation but it is very solid. The rest is still crumbly. We have maintained close to 10:1 ratio though only used 2 parts of water(not 3, way too runny).... mixed with a drill-type mixer... yeah... We have tried to mix as little as possible, very carefully, after overmixing the first batch. It is what it is. Do you think I have a problem with the layer? I will post more pictures tomorrow. Hoping what we did tonight is good enough to put a render over.

                  David, good to hear from you, I worried when you went dark for a little while. If nothing else we need you here!!!

                  DEFINITELY DOING A RENDER OVER THIS CRAPPY LAYER... if it is good enough!!!
                  Last edited by sergetania; 09-15-2020, 10:38 PM.


                  • #84
                    Sorry I’ve been out of action since being admitted to hospital on Aug 17 with a staf infection in my blood after a new aorta valve via open heart surgery and long term antibiotics it looks like I get out of hospital tomorrow.
                    To rewind a bit the 100% acrylic render will set you back more than $100/10 litre bucket and that may be enough to change your mind to use a cement based render over which you can waterproof.
                    I am mystified about your 10:1 vermicrete mix. You refer to it as perlcrete and again as vermicrete, which is it? Also what grade did you use? The medium grade for both perlite and verrmiculite ie what i’ve found best the coarse grade for each making a looser mix more difficult to apply. The fine grade holds together better but requires more water which needs removing anyway. I’ve mixed literally tons of this stuff and never have I found 2 parts water/10 parts vermiculite or perlite sufficient. You haven’t got the silicon coated perlite by any chance?
                    Kindled with zeal and fired with passion.


                    • #85

                      I didn't realize you are still in the hospital. That's never good. Hopefully, it's behind you! Get well!

                      I have used perlite, Therm-o-rock brand, the only one they had at Home Depot. It does not have grade on it but the content is Perlite only. That's all I know. For the render, I was looking to use base/brown layer concrete mix from Home Depot with acrylic bonding agent. Do I need to wait for the perlite layer to dry first?

                      Now, below is the mess I have found this morning. Some parts of it crumble, at least on the surface. Is that normal? Do I need to knock off crumbling pieces before applying a render? The lower band is rock solid, of course, probably gives it strength but no insulation. It is mostly below the hearth floor though and I have the blanket under it. So, basically, is what I have good enough to put a render over? Can anyone tell?
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                      • #86
                        Have you cured yet?? Your the pcrete has a lot of water in it and once you stucco the water vapor is locked in and will build up pressure and possibly crack stucco.. So you need to let dry out, then cure, then stucco, Water sublimates to steam increases volume a factor or 1500 or more.
                        Last edited by UtahBeehiver; 09-16-2020, 01:49 PM.
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                        • #87
                          Got it, will wait, cure,then render. Thanks!

                          What about some crumbling pieces of the perlite layer? Knock them loose? How do I know if it's good enough? What if I knock off too much? Should I add more on top? Or do I fill any voids with render applying the next layer?
                          Last edited by sergetania; 09-16-2020, 10:10 AM.


                          • #88
                            Originally posted by sergetania View Post
                            Got it, will wait, cure,then render. Thanks!

                            What about some crumbling pieces of the perlite layer? Knock them loose? How do I know if it's good enough? What if I knock off too much? Should I add more on top? Or do I fill any voids with render applying the next layer?
                            Definitely no expert here but having got through stage recently thought I’d chip in. I used vermicrete for the insulation layer. Yes it was crumbly. Once it once on and dried for a day it didn’t feel like it would fall apart but equally I could fairly easily flake small pieces off if not careful. For me the key thing was to protect this layer whilst getting through the curing - hence the use of plastic covers and later on a temporary roof shelter. It was several weeks in the end before I started the render layer and indeed I haven’t fully finished yet because there’s no longer a rush (temporary shelter) and every fire is just serving to drive more moisture out.

                            I personally don’t think the integrity of the layer you’ve built is a big problem - but you probably do need to keep it protected from the elements while it’s drying out.
                            My cast oven build thread



                            • #89
                              Mullster Thanks for the encouragement! Sometimes it's all it takes
                              This build was going way too smoothly till this step. So this is a good learning experience, hopefully,and nothing that needs to be redone.

                              Wondering if I need to patch the layer in a couple of places where I had the strap or just do it with the render.

                              Definitely have a plastic cover. It's a hurricane season and while they reach us weakened they still dump a few inches of rain in a day. Cheers!


                              • #90
                                Sergetania, don't worry about patching the strap've got great insulation. The perlcrete is primarily to give you a more solid base for the render and to help give you a nice, smooth dome shape. As Mullster & UtahBeehiver have noted, you have a long way to go to drive out all the making sure it's securely protected from rain is really important right now. Also, when you put a tarp over the oven, make sure your base hearth is covered so water doesn't seep in at the base perimeter seam of your oven.
                                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