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Holding it all Together! - Forno Bravo Forum: The Wood-Fired Oven Community


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Holding it all Together!

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  • Holding it all Together!

    Sorry if this has been covered before (which I'm sure it has), but...

    I am soon to start building an enclosure for an Ephrem 2061. (I've already bought the bricks, so there's no going back now!) This is a small domed oven made from cast volcanic rock (or somesuch) and consists of 6 parts I believe - 4 sections for the cooking floor and 2 left and right halves of the dome.

    All this will be supported on a regular concrete slab (suitably held up by load-bearing walls and piers), and covered with a tiled roof on brick walls.

    Now, I am pretty much sold on the idea of mounting the oven on FB insulating board - or insulating firebricks - but I don't know what to fix to what - and if so, with what. The oven manufacturer says to bed the floor on "a thin layer of lean mortar" and to place the dome on that without mortaring. My concern is that the heating/cooling and consequent expanding/contracting of the oven will (1) cause the mortar bed (or oven floor) to crack if it is mortared, or, shuffle away from its original position if not. Ditto for the 2 halves of the dome section. I've seen heat resistant silicone - which would seem to fulfil the promise of holding everything whilst giving flexibility for movement, but it is only good for up to 300 degrees c. Some people say mortar everything, others say just to rest it all in place, but the truth is I just don't know. I'm thinking of perhaps setting it on ceramic fibre blanket with stove rope 'cushion' round the edge. What about keeping it centred and held down with strong springs all round? (Only kidding, but you get what I'm saying!)

    Basically, I don't want anything to crack, but I don't want it to move either.

    Cheers guys...

  • #2
    Re: Holding it all Together!

    It will crack and it will move.

    Given the heating and cooling cycles, this is unavoidable in a brick or ceramic oven. My recommendation is to rely on gravity to hold it together. Mortar is not a "glue".


    • #3
      Re: Holding it all Together!

      Hello Cleverdick,

      A traditional method of bedding floor brick is to place it on a paste of fire clay, fine sand and water. That is typical of what they use in kilns of many kinds around the world. The clay will harden from the heat of multiple fires but not like Portland mortar. Download the free Pompeii oven plans and read the part about floor bricks starting on page 32. There are hundreds (if not more) of successful Pompeii ovens built according to these plans.

      I found a website for the Ephrem ovens and reviewed the construction instructions for model 2061. They are quite interesting in that they recommend placing the cooking floor directly on a thick layer of mortar (sand, cement and water) and also placing a thick layer of dry sand over the dome insulation. I think most folks who comment on this forum would advise against placing the hearth on what amounts to a heatsink made of Portland cement mortar. Your comment about using FB board seems a good alternative and you don't need to bed the hearth bicks. Simply lay them on the FB board.

      I would certainly substitute perlite or vermiculite over the rock wool dome insulation. Sand is less costly than perlite, but you can fill the extraneous voids with other heat resistant materials, even rock wool, then gain the benefit from the dry perlite where it is needed most.

      BTW, some rock wool products have an organic binder that holds the fibers together. Some of these binders give off an acrid smell for the first few fires when they get hot. You might also invest in FB ceramic blanket in lieu of rock wool or alternatively pour eight or more inches of dry perlite around the dome. Perlite and rock wool have similar insulating values and the brick enclosure will hold it in place around the oven. Be sure to get a good layer over the top of the dome.

      As for joining the dome parts, I would follow the manufacturers instructions. You might contact their technical department and ask them about it. They should provide expert advice on their products. I think you will find that heat cycling will cause cracks in mortared joints anyway. Pompeii ovens, with their many mortared joints, crack in many locations to little or no ill effect.

      Best of luck,

      Here is the link to my oven number 1 construction photos!

      Here is the link to my oven number 2 construction photos!


      • #4
        Re: Holding it all Together!

        I think you should follow the manufacturers advice and build to their plans. But make sure you include insulation under the floor. Blanket is not suitable under the floor it will compress with the weight use insulating board or vermicrete.
        Kindled with zeal and fired with passion.


        • #5
          Re: Holding it all Together!

          Thanks guys...

          Yes, I was going to cover the dome with ceramic blanket and and then pour loose vermiculite over that.

          It seems to me that if I mortared the dome joints then (1) flakes of mortar might fall onto a pizza, or (2) the concrete dome might crack if the mortar didn't allow the different sections to move!

          But, If I didn't put something over the joints, then there might be the possibility of ceramic fibres getting into the cooking area which could be hazardous.

          How safe is this ceramic fibre blanket/board? What makes it different from asbestos?


          • #6
            Re: Holding it all Together!

            How safe is this ceramic fibre blanket/board? What makes it different from asbestos?
            Asbestos is a naturally occurring mineral who's fibers have a particularly nasty effect on the lungs. The news is that no dust or fiber is particularly good for your lungs. You should use lung protection when cutting any ceramic material (that's a real respirator, not some ten cent paper mask), and encapsulate any refractory insulation.

            Once again, Sand is not an insulation. Don't try to insulate your oven with sand, or gravel or broken glass.
            My geodesic oven project: part 1, part 2


            • #7
              Re: Holding it all Together!

              The ceramic fibre used is classified as a class 2 carcinogen in Europe and is apparently considered unsafe to breathe, unless you use the more expensive water soluble ceramic fibre which is considered safe. Trouble is you won't know unless you wait for 20 years or so.
              The thermal conductivity of dry sand is between 0.15 and 0.25 (suprisingly low) especially when compared to 5:1 vermicrete 0.72 to 0.75 and dense firebrick around 1.4 Almost any material can act as an insulator if you put enough air through it. An insulating firebrick is made of the same material as a dense firebrick one is a good heat bank the other a good insulator. Glass too is a good insulator when made into fibre blanket. Likewise ceramic blanket which uses clay as its base material.
              Kindled with zeal and fired with passion.