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Refractory Motar Cracking - Forno Bravo Forum: The Wood-Fired Oven Community



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Refractory Motar Cracking

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  • Refractory Motar Cracking

    Hi Guys,

    I am currently in the process of starting the second course of my oven.

    I noticed that when I was constructing the opening that the motar was cracking after approximately 1 hour of setting.

    I have tried soaking the bricks in water prior to laying, adding more water to the mix, but neither seem to work. The brick work in some instances does not stick at all.

    I don't want to risk anything colapsing, so I would greatly appreciate any help regarding this.

    Also, how long should I wait until removing the form I have used to construct the opening. Considering that the average temp here at the moment is 32 degrees celcius.



  • #2
    Re: Refractory Motar Cracking

    Hi Ric

    Try adding less water to the mortar, not more. I found a tiny hairline crack would develop during drying if the mortar is too moist. Also I kept the area that was drying damp with a wet towell during warm days to slow the drying down.
    I found brickies sand shrinks alot compared to plastering sand, i wonder if a mix of 50/50 would shrink less. I didnt want to risk it on my dome but could be a good experiment if anyone has some firebricks left over!


    • #3
      Re: Refractory Motar Cracking

      The larger the aggregate (and better graded), the less shrinkage cracks.
      The dryer the mix, the less shrinkage cracks*.
      The smaller the joints, the less shrinkage cracks.

      So, if your joints are larger, use larger aggregate and less water. For thin joints, use more water and a finer aggregate.

      *provided the units you are laying are not bone dry.


      • #4
        Re: Refractory Motar Cracking

        An old mason's trick we use here in the Carolinas is a small amount of dish soap in the mortar to slow curing.This does not affect the strength or color.


        • #5
          Re: Refractory Motar Cracking

          Hi Guys,

          Thanks for the kind reply.

          I am using a motar mix which requires only the addition of water.

          I will try firstly adding less water and wetting down the bricks.

          Would you suggest utilising the same refactory motar to coat the bricks, 1 inch, as illustrated in the pompei oven instructions before applying the insulation. My only worry is that I will still get te same hairline fractures I have been getting in the brickwork.