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Crack during early stage curing - Forno Bravo Forum: The Wood-Fired Oven Community

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  • Crack during early stage curing

    Just got my dome closed and I fired my oven after about a week of letting it sit / running a space heater in it for a couple hours here and there.

    Unfortunately I just re-read the sticky in this section about just hitting the temperature and then dropping things down - I did NOT do that and had my oven at the curing temperature for a couple hours which probably explains my result.

    My first fire was small and contained but certainly not just newspapers - kindling and charcoal (I forgot about the advice of this sticky and instead had a memory of an article I read strategy mentioned to fire slowly but keep the temperature pegged for a few hours at a time). Anyway, I only brought the fire up to 100C - approx 200K and held it there for about 2-3 hours. The charcoal and a make-shift door really helped keep the temperature steady.

    At this point I hadn't noticed any cracking - although I didn't inspect the dome. It's worth noting that my dome is NOT insulated yet.

    My next firing was roughly the same although I brought it up to 150C but it didn't stay at that temperature for too long. The temperature stabilized around 100C again for a couple hours. It was during this burn that I noticed a crack (see photo). I wasn't too upset to see the crack - it was certainly discouraging but I had read enough posts to know it wasn't uncommon. I am concerned that 1) it showed up so early 2) it may grow to be structurally relevant as the temperature increases.

    My third fire - around 200C - only lasted about 45-60 minutes but the crack grew to about 1/8" in the biggest spot - the attached photo is from this burn.

    I have a few questions:

    1) At what point should I start becoming concerned?
    2) Should I keep firing it or try to elicit some repair? My thought / concern with repair is that there is obviously a stress at this part of the oven so any repair will either break under that stress or another stress fracture will develop elsewhere.
    3) Given the expansion seems to be across my inner arch, how will this affect my chimney stack? My worry is that if the middle of my inner arch is expanding and contracting by 1/16" -> 3/16" each time, I would expect the mortar holding the chimney stack to break pretty quickly as I assume it will be attached to the inner arch. Is there anything I can do about this?

    Thoughts?
    Stephan
    You can see (sort of) that the temp is about 175C

  • #2
    So it would seem the crack is happy to grow when things are getting hot. I have a couple more pictures that hopefully show things as they are.

    Again, I'm still not sure what's going to happen when I add the chimney components. If someone has some insight into my questions from the previous post I'd really appreciate it.

    Thanks,
    Stephan

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    • #3
      There are two types of ovens. Those with cracks, and those with owners that lie about the cracks...

      That crack probably won't cause any problems. If it bothers you, you could knock out the top two arch bricks and reset them. It's unlikely to crack again once you do that. I think I had a crack in my front arch and did that.

      But I've seen much worse, and the mortar is really meant to hold the bricks apart, not hold them together.
      My build progress
      My WFO Journal on Facebook
      My dome spreadsheet calculator

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      • #4
        looks like a very nice oven - beautifully laid. Mine, on the other hand, is not so beautiful. I have no idea what's happening under the insulation (at least 6" of percrete). We're in the curing process. I read about just bringing up to temp and then cooling, but then also published by FB it says to fire each time for at least 6 hours, and that longer is better! So two questions: which method is the best? We've been going for about 1/2 hour-45 mins, mainly b/c the thought of sitting next to the oven for 6 hours is unfathomable. Next question: is it possible that the percrete is helping to keep the oven from cracking, how in the world would we fix it if it does crack (and how would we ever know)? Sorry one more question: I think I understand that the dome will start steaming and getting hot on the outside while the moisture escapes, but so far there's no heat outside, no steam - and I've gone to 300 degrees. Thanks for advice,
        Linda

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        • #5
          If you are seeing steam off the pcrete then you are going to hot. Be patient, slow go. The pcrete holds a lot of water so it takes time. I have seen some builds where the water pools up on the hearth from the pcrete during the curing phase. Remember one or two pieces of wood added to a fire can really spike the temp. so don't over load.
          Russell
          Build Link............... Picassa Photo Album Link

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