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600mm oven cast over sand - France

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  • 600mm oven cast over sand - France

    Bonjour encore...

    I can't believe it's two years since I wrote my introduction here https://community.fornobravo.com/for...uild-in-france and received many helpful comments from other members. It's been an odd couple of years and progress has been slow to stopped for great chunks of time. Well I'm at the point where I am pretty much ready to start the actual build and thought I would document my successful (and not so successful) steps here.

    I finally got a local ironmonger to make me a trolley/stand for my oven (pics). He also threw in a 3mm steel plate for me to use as a base. The idea of having a non-fixed base was so that if I ever needed to relocate it, I could employ a few friends to lift the top off and place in a trailer leaving the base separate. A more pessimistic addition could be that if I suffer COF (catastrophic oven failure) I could assemble a new oven and re-use my stand/trolley.

    I am planning to build a small (600mm ID) dome over sand. 50mm castable refractory with ss needles, 50mm ceramic blanket, 75mm vermiculite cement coat, covering waterproof coat. To be built on a base of 3mm sheet steel, 100mm vermiculite cement, 50mm (or 55mm) refractory bricks.


    I am the opposite of Mr DIY, so I have a number of starter questions - some are just seeking confirmation(!), for others I would welcome people's opinions.

    1 - Base
    Do you think this 3mm sheet metal base is usable/sufficient? I was planning to drill a number of holes to assist drainage/drying of a 100mm vermiculite cement base. Do you think I need rebar? or could I maybe drill extra holes and secure in a load of upward-facing 75mm bolts? (I want to do a few of these anyway so that the bolt heads below the plate ensure it is not possible for the whole to slip on the trolley, although with the weights involved this may be overkill!) Should I just create a temporary wooden form to cast the vermiculite or do you think it would be better to mount permanent wooden edges? (I was thinking temporary, and then maybe tile the cement faces afterwards.)

    2 - Door, entry galley and flue
    I am planning to cast the entry arch as a separate piece - 190mm high for the flue (63% of internal height). What width do you suggest for the entry? I guess it needs to be at least 350mm to be able comfortably to work a 30cm pizza - is 400mm too wide? The depth of the entry arch I want to keep as short as possible, mainly dictated by the width of the flue. Here in France we have readily available 80mm or 150mm flues. I recently picked up an extremely cheap 1000mm length piece of double walled flue - internal 80mm - (pic) at a car boot sale - if 80mm is sufficient for my small oven, I'd like to use this. I could then permanently mount a small section of 80mm flue to the oven, and make this 1 metre length removable, meaning I can throw a tarp over the structure in winter.

    3 - Casting
    Vermiculite (or perlite) to cement - recommended 5:1 for the floor and 10:1 for the dome - silly question but this is by WEIGHT isn't it..?

    There will be more!

    Any comments greatly appreciated.

    Rik


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    Last edited by thargog; 11-22-2020, 05:23 AM.

  • #2
    Hi Rik,

    Your plans look pretty good, but those tiny wheels on the stand look somewhat inadequate to me. Your oven will weigh in the region of 300kgs and the very small wheel diameter may present problems running over tile joints. The steel plate for the base sounds adequate, although it will be subjected to heat and moisture which are major contributors to corrosion. I like your idea of the bolts sticking into the vermicrete insulation for location, although with so much weight on top it is hardly necessary.
    It is best to cast the dome first, then cast the gallery ib front of it so you can create the rebate for the door. For a small oven there is more heat loss from the door opening, although a wide opening gives far better access. For my money a wide opening is better. You can easily retire a small oven but you can't increase the door width once it's built.
    My door opening is 400mm gallery opening 440mm wide and actual door 430mm. I like it, good access.
    80mm diam flue is too small, you'll have tons of smoke out the front, you need to go to 150mm. Mine has a 150mm but my oven diameter is only 540mm It works really well.
    Your idea of vermiculite or perlite for the dome casting is a poor one. Even a small amount of either will drastically reduce the strength of the dome. Keep the casting dense and strong to store heat like a battery. Use vermiculite or perlite as insulation only. The ratios are by volume not weight. Interestingly even a !0:1 vermicrete mix has more mass from the cement than from the vermiculite.

    Dave
    Kindled with zeal and fired with passion.

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    • #3
      Hi Dave - thanks for the comments.

      The wheels do look a bit small in the photos but they should be fine. They're 100mm diameter heavy duty double bearing, and the set of four are rated to 700kg so I reckon they should cope. Luckily my terrasse tiles are reasonably well laid and level..!

      Thanks for the comments re the door dimensions, flue and ratios - your door sounds wide for a 570mm oven but hey if it performs well, that's great news for me! Not such good news is I wasted a whole 10€ (!!) on the flue pipe. Oh well it was worth a try - I'll take it back to a car boot...! Re the ratios - cheers - it has reminded me there isn't such a thing as a silly question.

      My casting question about vermiculite/perlite was indeed just for the insulation layer. (Here was my layer 'spec': "
      50mm castable refractory with ss needles, 50mm ceramic blanket, 75mm vermiculite cement coat, covering waterproof coat.") The castable refractory with needles will indeed be the inner ("dense/strong") layer.

      Do you have any opinion on the subject of the need for strengthening the vermiculite base? Either with rebar or a good number of upward pointing bolts? I guess my worry was if I ever did get three burly blokes to help me lift it, whether the fact that the steel flexes ever so slightly would cause any issues or whether everything would be so glued together by that point that it would not be a concern.
      And also whether it would be advisable to drill holes in the steel plate (from reading other threads I believe this would just aid the drying process rather than for any longer term gain...)

      Cheers


      Rik
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      Last edited by thargog; 11-22-2020, 05:29 AM.

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