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80 cm cast oven in the Netherlands

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  • #16
    Your build is going really well. Regarding the vermicrete, if you have plenty of blanket then you are not relying so much on the vermicrete layer as an insulator, so it can be made richer to make its application easier. The stuff can also be easily carved back if the form is a bit uneven. The smoother it is the less render is required to make the final surface perfect. As an example the strength of vermicrete can be altered easily by the addition of cement and sand and its consistency made more workable with the addition of powdered clay. I've made lots of these with students, it can be carved and other bits added on (yes the drain is out his mouth)

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    Kindled with zeal and fired with passion.

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    • #17
      Thanks David. That's a very fine piece of sculpture!

      Thanks for your advice on making it stronger. However, would you advice to add a second layer of vcrete to make it thicker and in a better shape?
      Or does it do no harm to have some blanket visible in this stage, and will the final render layer fix everything?

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      • #18
        The render layer(s) will also even out lumps and bumps, but the smoother it is the easier the render application becomes. Just yesterday I shaved back some high spots on a customer’s oven to make the job easier. A bit of exposed blanket won’t trouble you much at all. After driving out all the moisture, before applying the outer render, you will be left with a dry substrate, so to counter this it is prudent to cover the finished render as soon as it’s finished off with a wet sponge.
        Last edited by david s; 09-10-2022, 07:13 PM.
        Kindled with zeal and fired with passion.

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        • #19
          I ran into a rainy problem.

          After successfully burning my oven for seven consecutive days, I managed to bake my first pizzas and bake some bread. The vermicrete layer dried and I covered it with a plastic tarp to protect against rain. I plan to finish it off with stucco during the Christmas holidays if the weather allows for it.

          However, I forgot to fix the tarp and the wind blew it off. For two quite rainy weeks my oven was exposed to the seasons, until I found out about it.

          How should I continue? Should I start over with seven days of slowly increasing fires? Or can I move faster now in order to drive the water out? How do I know when the drying process is finished?

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          • #20
            I guess that all depends on how wet it got. Both the vermicrete and the blanket can suck up a lot of water. If it were mine I'd be repeating the 7 fires in 7 days. I'm assuming you got to the point where all the carbon burnt off the entire dome internal surface.
            Kindled with zeal and fired with passion.

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            • #21
              ​​(Regarding my last message: after firing the oven slowly up again over some days, I didn't notice that much vapour coming off the dome. Apparently the rain didn't do much harm.)

              Last week I finally got time to finish everything with stucco. I think it looks great.

              I used ready made outdoor stucco, and I added Compaktuna Pro for the dome in order to hopefully make it waterproof. I also covered it with a wet blanket and plastic for three days to make sure that it hardened enough before drying.

              I'll let it dry a few days more and then fire it up!

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              • #22
                That looks very good. If it were mine I'd be cooking a few chickens before attempting pizza temperatures. Better to be safe than sorry.
                Kindled with zeal and fired with passion.

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                • #23
                  I've been heating/curing my oven today, and a minor crack appeared in the stucco, in front of the chimney. I now realize that I gave the chimney some room to expand in the insulation vermicrete layer, but not in the outer shell (vermicrete and finished off with stucco).

                  With such a crack, the outer shell is less waterproof. Is there a way to seal this crack so to make it waterproof again?
                  And should I do something to prevent further cracks in case I raise the temperature?

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                  • #24
                    It is better to do the drying fires before the outer stucco layer is applied because steam pressure from excess moisture can cause the outer shell to crack. The other problem is that your stucco is hard against the stainless pipe. The more conductive steel will expand first, placing stress on the stucco that surrounds it. Which ever was the cause of the cracking, it would be better not to repair it yet. Continue driving out the water with more fires. At least there is now an exit path for the water to escape through. To create an expansion joint at the pipe to stucco join you wil need to cut the stucco away around the pipe. A diamond blade on an angle grinder and a masonry drill bit would be my preferred tools. It will be pretty messy but when you’re don wrap some corrugated cardboard around the pipe, then wrap some plastic around the cardboard so it will be easy to remove. This should be around 4mm thick.Then stucco the gap up to the cardboard, wait 48 hrs, remove the cardboard and wrap the repair in damp towels for a week to enhance its strength. When it’s all dry, fill the gap with high temperature silicone.
                    Kindled with zeal and fired with passion.

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                    • #25
                      Thanks David. I have already done the drying fires and even cooked some pizza, although that was two months ago. After that, I covered it in plastic and only last week applied the final layer. So there may have been some moisture but probably not much; I also made a vent pipe on top. So I assume that it is the expansion of the pipe and not the steam.


                      Would such an expansion joint still be necessary or advisable now that it already cracked? Could I not treat the crack (a mere 10 cm) as a natural expansion joint, and simply fill this with silicone? Or would that lead to other problems in the future?

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                      • #26
                        You could try that, but every time you fire the pipe will still expand and opening the crack, allowing water entry where water sits around the pipe.
                        Kindled with zeal and fired with passion.

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