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Advice on pouring the hearth - Forno Bravo Forum: The Wood-Fired Oven Community


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Advice on pouring the hearth

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  • Advice on pouring the hearth

    So, I just put my forms up and i'm finally ready to pour the hearth. I've been combing the forums all day, but i still have a few questions i want to be sure on before i proceed.

    1. since having started this oven, i've poured concrete a couple times (pre-mix) and am confident in my "concrete skills"..but it was just normal concrete. I'm a little nervous about now working with vermiculite. I know that the standard ratio for vermicrete is 1 part portland cememt to 5 parts vermiculite, but how exactly do i go about mixing it? Do i just take 5 shovel fulls of vermiculite and one shovel full of portland cement and put it in a wheelbarrow and mix it together with a shovel then add the water, or is there some technique or something else that i'm overlooking? I might be over-complicating things in my mind, but i don't know how the vermiculite will behave in comparison to a normal concrete mix. It seems like it will be difficult to mix a 6 1/2 x 6 1/2 ft., 4 inch thick slab of vermiculite-concrete slab and ensure that it is all a homogenous mix that will insulate the way that i want it to. Any techniques or tips are more than welcome!!!

    2. I'm planning on ordering some FB Board to lay down directly on the hearth once it is poured I've been reading about how the board will replace some of the insulative powers of the vermiculite. Would there be enough insulation under the oven if i poured 2 inches of vermicrete mix (instead of my already planned 4 inches) and just lay the FB Board on top of that. Also, does the board just lay down on top of the hearth or is there some means of attaching the board to the hearth

    3. The 3rd and final question that has been kind of bugging me...will any vermiculite do? I did some reading (and some google seaching) and it seems that vermiculite is used in gardening/landscaping. I wasn't sure how exactly it made its way from gardening to insulating wood-fired ovens, so are there different kinds of vermiculite? if so, what kind should I use and where might i acquire some?

    I know this is a lot of questions for one post, but i'm not sure I feel comfortable building something unless I know exactly what i'm doing. I guess I just want to quadruple check everything i'm a bit insecure about doing before I pour the hearth. It was easy building the bottom half of the structure, but now that thermodynamics and insulation are added into the mix, it's gotten a little intimidating. Thanks for all the help so far BTW!

  • #2
    Re: Advice on pouring the hearth

    Originally posted by Matt916 View Post
    Do i just take 5 shovel fulls of vermiculite and one shovel full of portland cement and put it in a wheelbarrow and mix it together with a shovel then add the water, or is there some technique or something else that i'm overlooking?
    Not really - that's all there is to it.
    Just be careful adding the water, tip it in gently, otherwise your vermiculite will splash out like breakfast cereal!

    I'm sure someone else will say, add the water first, mix the cement & water first etc etc...
    but if you just shove it all in a wheel barrow, and mix it, the end product is exactly the same.
    My 2nd Build:
    Is here


    • #3
      Re: Advice on pouring the hearth

      It's been proved that if you mix the portland and water into a slurry, you can use less portland, but you need a mechanical mixer for this trick, and for the old fashioned method all you need is a hoe.

      The vermiculite from the garden supply is exactly the right stuff. There is a silicone treated version sold as domestic insulation, and we've been warned off this. You can also use perlite if vermiculite is hard to source.

      Two inches of refractory insulation board is just fine for a domestic oven. Four inches of vermiculite concrete does the same thing. You can do more if you want, or mix and match, but either is fine.

      Your next post will be "Boy this stuff is weird, why can't you float it, will it ever set up?"
      Just to let you know.
      My geodesic oven project: part 1, part 2


      • #4
        Re: Advice on pouring the hearth

        I just finished a 3" layer by about 6' using the paint mixer slurry method from this site with a five to one ratio. The vermiculite was measured by volume into a wheelbarrow and the slurry (also prepared by volume) poured from the bucket in a couple additions and shovel mixed. It took a lot more vermiculite than expected from the volume marked on the bag but I had bought extra from the garden supply store expecting to use it later on the dome--instead five bags (each 2 or 2 1/2 cf?) all went into the mix with just over one bag of portland. It took about five barrows full. I wet the vermiculite until damp with what seemed like quite a little water,added the slurry and mixed again and shoveled it in.
        Like many others, I was convinced then and for the next few days it could not possibly work or support a brick oven but a little more than a week later I`m on the third ring and clambering all over it. I also topped it with FB boards which by all accounts seems like overkill. One of the three board was a little thicker than the others so i used the wet sand/clay layer to even the floor. It is a work in progress so can`t speak to final result but I seem to be headed the right direction. John


        • #5
          Re: Advice on pouring the hearth

          haha, i have a feeling that my next set of questions will be about how the vermiculite sets up too. actually, i feel a lot better about it now. I guess the only way im going to know how it works is if i just go ahead and use it. Now I just have to find some vermiculite...it would probably be too much to hope that Home Depot has it...for some reason they never have exactly what i need, so i may just have to go with the pearlite. I'm going to try and use vermiculite, but if i cant find any, do i use the same proportions of the pearlite (assuming that i can get a good source of it)?

          bernerdog, it sounds like your oven is coming together pretty well. I'm gonna try and catch up to you this weekend...well at least have the hearth laid. Thankfully, from what I understand the 2 layers of the hearth (the first layer of normal concrete and the second layer of vermiculite-concrete) don't have to be poured on the same day, which is good news as it has been consistently 104 degrees and up here in Sacramento


          • #6
            Re: Advice on pouring the hearth

            Mix gently. Don't use a machine - this will break down the vermiculite particles making it more dense and thus less insulating.

            As for curing; protect the initial pour from direct sunlight if it is hot. After about 3 hours you can start misting it, after about 8 hours or so you can apply the water directly (if it starts "washing away" go back to misting it for a while). Covering it with wet burlap bags or wet old towels is also useful. You need to ensue that the vermiculete has plenty of water to cure uninterrupted for at least six days.
            Last edited by Neil2; 07-29-2009, 11:50 AM.


            • #7
              Re: Advice on pouring the hearth

              I'm almost done with the hearth. I poured the structural cponcrete a couple days ago and am going to home depot later today to buy the vermiculite and portland cement. Due to budget restraints i don't want to buy too much, so how many bags of portland cement and vermiculite should I buy? i'm not too sure how big the bags of portland are, but the vermiculite comes in 3.5 cubic ft. bags. The Forms I have to fill are roughly 70 in X 78 in X 2 in.


              • #8
                Re: Advice on pouring the hearth

                You need about 6.3 cubic feet. Buy 3 of the 3.5 bags. The loose vermiculite tends to compress a bit when wet. (If you don't need the extra bag take it back for a refund).

                Two bags of portland should do it. One 60# bag is about 5/8 cu foot. Mixed 5:1, two bags will be good for 6 1/4 cu feet of final vermiculete. I would mix it 6:1 (still plenty strong enough) to account for spillage etc. Again; buy an extra bag of cement and take it back if you don't need it. Most places will take building products back if unopened (or at most will charge a 10% re-stocking fee).


                • #9
                  Re: Advice on pouring the hearth

                  Thanks for the advice. I'm looking forward to getting off work so I can go do that. I was worried initially that i wouldn't be able to find any vermiculite, but i talked to someone over at Home Depot and found out that they usually carry four bags per store. They will, however, order it for a customer if anyone needs it, so if anyone has trouble sourcing it, Home Depot seems to be a good place to look.

                  I have one more question for the day: I originally was planning on building a barrel shaped oven because i thought it would be easier since the barrel shape is retangular and the shape of the brick is rectangular, but after reading on the advantages of a dome-shaped oven i've decided that I would rather go that route. Is there anywhere i can go for plans or instructions on how to build the dome out of standard firebrick? I know a lot of people have done it, I have to admid i can't figure out how rectangulat bricks can fit to form a round dome chamber...


                  • #10
                    Re: Advice on pouring the hearth

                    "I have to admid i can't figure out how rectangulat bricks can fit to form a round dome chamber..."

                    Now you are getting into the real interesting part.

                    Check this site - there are numerous threads on techniques and builds using brick domes.


                    • #11
                      Re: Advice on pouring the hearth

                      My technique for mixing vermicrete was messy but quick.

                      Lay out a big waterproof sheet of some sort on flat ground.

                      Pour a 4m3 bag of vermiculite onto it. Put the right amount of portland on. Iused a mix of about 8:1. Making sure to wear rubber gloves and an old long sleeved top get in there! Mix with arms and hands in a sort of sweeping method. Once thouroughly mixed add water and mix again. Add water until it's the consistensy of slightly damp porridge oats.

                      Although messy, the advantage of this technique is you can do very large quantities in one go, as opposed to loads and loads of smaller mixes. I also found it to give a nice consistent mix.

                      Just make sure you cover up exposed skin on arms and hands as Portland is highly irratative. I made the mistake of not doing this and my hands and arms went incrediblly dry and itchy! Not nice!


                      • #12
                        Re: Advice on pouring the hearth

                        I wish I had heard that a couple of days ago. It would have saved me a lot of trouble . I got off work and bought all the materials that I needed. So I came home and thought I may as well finish the hearth, i still had about 5 hours of light and I would only have to pour about 2 inches. I guess I under-estimated how long mixing that stuff takes becuase I barely finished before I couldn't see anything. Hopefully I did the mixing and pouring correctly. It seems like the vermiculite on my hearth is already set up quite solid. I'm going to keep it damp for a few more days per everyone's advice beofre i take away the forms, but is it normal that it sets up so quickly or is that a bad sign?


                        • #13
                          Re: Advice on pouring the hearth

                          "but is it normal that it sets up so quickly or is that a bad sign?"

                          You are OK. You can continue building around or even on it but allow water to get into it on a continuous bases for at least 6 days..

                          (I see you learned the first lesson about concrete pours - allow for lots of time because once started you can't stop.)


                          • #14
                            Re: Advice on pouring the hearth

                            You said to allow water to get into it on a continuous basis for at least 6 days -
                            I read somewhere else on the forum not to let water get into it, because it will get trapped, freeze in the winter, cause problems...

                            I laid most of the insulated hearth Monday evening - covered it because of rain...And yesterday laid the rest - covering it again because of rain.
                            Should I uncover and spray it with water?

                            Also, the first stuff we laid Monday, was with bigger grade vermiculite. (1A, I think) and then I purchased more yesterday, not knowing that I had to check grades, and it was 3A, which is so much smaller. Will it matter?

                            What this stuff reminds me of when it's all mixed is not damp porridge, but damp/wet sand on the beach....it doesn't absorb water like porridge does. And if you get too much water it sort of floats and rinses off the cement from the vermiculite. That was my first experience with it. If you add the water a little at a time mixing it in before adding more, it doesn't separate. Kinda like making pie dough or biscuit dough or something....



                            • #15
                              Re: Advice on pouring the hearth

                              And one more thing...how long should we wait before laying the oven floor after pouring the insulated hearth?