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Record of my 32" Homebrew cast oven, on a brick base - West Midlands, UK

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  • Record of my 32" Homebrew cast oven, on a brick base - West Midlands, UK

    Here, I will record my progress in making my first cast oven, having spent many, many hours exploring this forum and others in order to identify the best construction methods....

    My plans are for:

    A concrete slab, with a brick built base, supporting a 100mm (4") reinforced concrete slab

    A 100mm (4") thermolite block floor insulation layer, surrounded by concrete to the edge of the formwork

    A 1" ceramic board (CaSi) insulation layer above the thermolite block (to reduce direct heat on thermolite which has an unknown extreme heat tolerance)

    An oven base constructed of 64mm deep firebricks, insulated peripherally with vermicrete, with concrete to edge of base formwork

    A heat retaining dome constructed over a sandcastle of 800mm internal diameter (32"), approx 5cm thickness, (sand, fireclay, lime, cement, polypropylene screed fibres, stainless needles)

    A flue gallery, entrance of 450mm width, 250mm height (62% dome radius), with flue partially in dome area, and ridge for internal door to sit against positioned to enable flue to be isolated from oven when door closed.

    A stainless flue 150mm (6")

    A 1-2 inch ceramic blanket, possibly secured with chicken wire

    A 5cm vermicrete layer

    Render

    Waterproofing (paint or tiles) to dome

    Waterproofing to top of base (tiles)


    Oven Costings:

    Oven:
    • Stainless flue 6" 1m + 0.5m = 58 (ebay)
    • Fireclay 25kg bag 30 (inc delivery) valentines clays
    • Firebricks x 30 75 Vitcas
    • ceramic blanket 7.3m x 1" 65 Vitcas
    • ceramic board 1x1x1.2m 60 Vitcas
    • delivery from Vitcas (pallet) 40
    • Cement 25kg x 2 10 Selco
    • hydrated lime 25kg x 1 12 Selco
    • Sharp sand (from bulk bag costed in base)
    • Stainless needles 3kg 50 Rockbond (inc delivery)
    • Polypropylene fibres 900g 11 ebay
    • vermiculite 100l 24 ebay
    • perlite 100l 24 ebay
    • blue quartz glass (arch) 4 ebay
    Total Cost Dome 470 sterling or $580 USD


    Brick Base (concrete foundation, brick wall and reinforced slab with thermolyte topping):
    • Cement 25kg x 10 40 Selco
    • OSB board for formwork 20 Selco
    • builders sand (mortar) 6 bags 15 Selco
    • Ballast (for concrete) 1xbulk bag 50 Selco
    • Sharp sand (for sandcastle/dome casting) 50 Selco - used less than 1/2 and got most back after sand castle
    • blue bricks 100
    • thermolyte blocks x 10 12 Selco
    • rebar mesh 3.6x2m - 20 Selco
    Total Cost for both Dome and Base: 790 sterling or $985 USD


    Timelapse of building sand-castle, cast dome, and vermicrete insulation



    Time lapse of scratch coat and final render top coat

    Last edited by Nick J C; 08-07-2020, 02:26 AM.

  • #2
    Building the Oven base

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    • #3
      Reinforced slab construction

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      • #4
        Great start, it is nice to see someone doing their homework and thinking things out rather than build on the fly. The brick base looks good. Just a couple items of to clarify. Floor ratio for vcrete 5 to 1., Dome ratio for vcrete 8 -10 to 1. You should consider at least 3 maybe 4" of vcrete on dome, that is equivalent to 2" of ceramic fiber insulation which is the min recommended. Thermalite block are new to me but by protecting them with the CaSi and the boards fully supported by the concrete hearth IMHO you should be okay. They appeared to have good K value and compression. Drill or form a couple weep holes in the concrete heath due to your weather and the Thermalite blocks sitting in a tub or basin.

        PS you may want to add some more outside supports to the forms so the side walls do not blow out.

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        Last edited by UtahBeehiver; 06-04-2020, 06:37 AM.
        Russell
        Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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        • #5
          Up to date today - Reinforced slab completed, now need to wait for it to cure, so will move to preparing dome gallery formwork....Had time to do a bit of concrete mixer maintenance!

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          • #6
            Here are my provisional plans for dome (not yet done so feel free to advise)

            Thanks UtahBeehiver , will take your advice on vermicrete ratios, and have taken note of 'weep hole' advice

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            • #7
              Hi, nice build so far, though I'd just say hi also from West Midlands
              Looks like I am about a week ahead of you I'll be following this closely!

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              • #8
                Hi Nick,

                Good planning. Have you obtained your burnout fibres yet? They should be finer than human hair. I'm not sure whether polyethylene fibres are correct. I know the polypropylene ones are ok. If not sure place some in your kitchen oven to see if they melt at 160C.If they have a higher melting point then they're not going to work as intended.
                When you say your flue is partially in the dome area, does that mean when your door is in place the flue will be allowing heat to escape from the oven chamber? If so then you will need a flue valve to shut it off. Normally when the door is in place it is on the oven side of the flue. This is a better design which eliminates heat loss up the flue when the door is in place although you lose some floor area for cooking.

                Dave
                Kindled with zeal and fired with passion.

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                • #9
                  Thanks for taking an interest david s - the polyethylene fibres was an error in description - thanks - I have changed it to polyprop fibres.

                  Re: the dome design - I am very much trying to replicate your design, which I havent been able to find plans for (trade secret!) - but I have tried my best based on a number of your posts. The flue is completely sealed off by the internal door in my plans (see post above), but I saw a series of posts from you in which you showed how to minimise the gallery depth by starting it around 20mm inside the dome circumference.

                  How do my plans look? - are there any issues - I want to make the design as good as possible. Should my flue gallery be set higher than the exit arch, which i have seen done in some - presumably to catch as much smoke as possible

                  Also, I was wondering if there was a benefit it not bringing the 'homebrew' cast right to the front of the gallery, perhaps using vermicrete here instead, to allow a thermal bridge between the decorative arch and the hot gallery. I think it may be necessary to full encircle the flue with homebrew for structural integrity but would be interested in your view.
                  Last edited by Nick J C; 06-05-2020, 12:14 AM.

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                  • #10
                    Whilst reinforced slab cures, been thinking about decorative arch..... decided to make a reinforced concrete one a la david s . Made a formwork and covered with parcel tape to make it non adherent...will silicone joints and wax before casting in two parts....first a standard concrete with blue glass spread on the bottom, then a needle reinforced layer....hope to polish surface to expose glass.

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                    • #11
                      Hi Nick,

                      That is the way I cast my decorative arches too. Here is a pic of a cross section drawing of my design detailing the flue gallery placement. I think there may be an error in the drawing as it appears that the outer decorative arch is lower than the oven mouth. It has to be taller and wider of course so the door can fit in against the rebate.

                      For the second and third ovens I built I used a design that had the flue inside the chamber and part of the oven casting, ie not a separate casting for the flue gallery. I had the flue pipes manufactured by a sheet metal shop in 1.2mm stainless which is about double the thickness used by flue manufacturers. I cast directly around the pipe in the belief that it would create a firm secure seating. In the case of both ovens the castings cracked around the flue pipe. Although disappointing both ovens still operate well 13 years later. I concluded that what was happening was that the conductive stainless was expanding well before the cooler refractory that surrounds it and was responsible for cracking the casting. 1.2mm thick pipe would have a lot more power than a 0.55mm pipe, but to overcome this problem I now make sure that the fitting is slightly loose to accomodate the expansion. This is easily done by wrapping some cardboard a couple of millimetres thick around the pipe, cast up to it and then removing it. I then pack around the pipe with 1:5 vermicrete which, because of its low strength has enough give to allow for the expansion. The pipe is also supported higher up if you funnel the flue gallery. I hope this makes sense, it has worked for me for many years. Also attached is pic of how I insert the flue pipe which does not involve an expensive anchor plate requiring drilling into the refractory.

                      Click image for larger version

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                      Kindled with zeal and fired with passion.

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                      • #12
                        Hi NIck,

                        If you haven't cast your arch yet you might want to consider embedding some wire ties so there's good integration with the outer rendered shell. See pic
                        This is probably not absolutely necessary but I've been doing this for years and had no reports of cracks around the arch.

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                        Kindled with zeal and fired with passion.

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                        • #13
                          Getting on with the gallery cast formwork..... Note smaller form on inside to sit against sand dome to create ridge for door to shut against. Top rectangular block will be a hole for flue - I will create a cast funnel separately to surround ss flue....thanks david s for images and guidance
                          Last edited by Nick J C; 06-06-2020, 05:21 AM.

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                          • #14
                            Taped up and ready

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                            • #15
                              Laying out insulating Thermolite blocks with forumwork over to get some idea of where insulation needed..... Surrounded with standard concrete

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