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Castable Refractory Products (VT600) - Forno Bravo Forum: The Wood-Fired Oven Community


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Castable Refractory Products (VT600)

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  • #31
    Thanks again Dave, those are great references.
    I've placed the insulating Vermicrete now and am pretty happy with it, I'm a bit concerned that the corners are a bit crumbly so am planning to skim coat it with a Mortar layer around the edges, do you see any issue with that? Is it normal to be crumbly? I'm concerned I may not have used enough water and the mix is off? Right now I have it covered as it is supposed to rain today. Going to attach a picture now that I have all the forms off

    If I understand correctly the thermal break is either blanket material or Vermicrete between the actual Dome and the Flue gallery?

    Is there any type of height to diameter ration I should be looking for with the dome?

    I also pulled a bunch of old bricks from an old fireplace, they certainly look like fire bricks but is there any way to tell for sure? Guess next step is to start cutting these.



    • #32
      The usual ratio of water to vermiculite is3:10 by volume, but if you are using the finer grade it requires more water. Troweling the surface brings more cement to the top to produce a harder surface. Too much water in the mix washes the cement off the grains resulting in an inconsistent mix.I find some powdered clay added to the mix makes it more workable. Too late for that now so trowels skim coat on the edges to protect them. You do want the stuff to dry out though so plenty of sun and wind on it for the time being is good. Keep the rain off it though.
      Probably not a good idea to have exposed blanket in the entry. Weak vermicrete mix or ceramic rope would be a better choice or seal over the exposed blanket somehow.
      Dome height to diameter is usually 2:1 (hemisphere) but some folk like it lower (Neopolitan dome) where the roof is closer to the food, for faster cooking pizzas. The ratio of oven door height to internal dome height is not unsurprisingly, the Golden Mean (1:1.618) but 2 or 3% higher or lower will make no difference.
      Last edited by david s; 08-13-2019, 01:01 PM.
      Kindled with zeal and fired with passion.


      • #33
        OK I think I have my calculations done for the amount of castable I'll need. Maybe someone can Check them?

        Basing on a 32 inch dome. Calculating volume of sphere that's 4/3 Pi R Cubed. 4/3 x Pi (16 cubed) that gives me 17157 divide by 2 for half a sphere is 8578 Outer sphere is 4/3 x Pi (18 cubed) = 24429/2 = 12215
        12215-8578 = 3637 inches cubed
        Convert to Litres 3636/61.024 = 59.58 litres
        Saw Dave S say 55lb bag coverts to 16.37 litres but my bags are 50 lbs which makes it 14.88 litres then reduce by 15% when mixed gives me 12.65 litres
        59.58/12.65 = 4.7 bags Add the flue area and I need at least 5 bags maybe 6 to be safe

        Next question is the stainless needles and PVA fibers how are those calculated based on these quantities. I thought I saw needles were 3% by volume so 5 bags x 50 = 250 x .03 would be 7 pounds of needles?
        I have not been able to find a % for the fibers, any suggestions.

        3:10 was what I saw you had posted somewhere else David so that was what I used on the Vermicrete.



        • #34
          Your calculations sound about right. The burn out fibres you need are polypropylene not PVA. You need way less than you’d think. For every 20 litres of dry castable a handful is plenty, but make sure they’re well mixed in the take a while to disperse completely.
          Regarding your question re firebricks, it’s not easy to tell. Firebricks come in many different colours but are usually a creamy colour and not completely vitrified. They will not have a glossy appearance and should be more porous than a normal house brick. If out of a fireplace you’d expect them to be true firebrick.
          Kindled with zeal and fired with passion.