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Wood fired oven in Ontario

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  • Wood fired oven in Ontario

    Downloaded plans to build a double chamber cob oven last year but since having being exposed to so much info on other builds I don't feel like it is going to suit my needs.I built a dry solid stacked stone base of 5x5 up to 24 inches and have framed out the top for 4 inches of a reinforced concrete base on top.Now am trying to decide on what to use to insulate floor......I should mention that I am trying to use as much recycled material as possible such as the angelstone from my old fireplace and insulating material from the inside of a large kiln, and clay from the foundation. What's the difference between using vermiculite and clay slip compared to cement and vermiculite? The oven will be between 34-36 diameter.

  • #2
    Cement and vermiculite won't return to mud if it gets wet. I actually use a little powdered clay with the cement in a vermicrete mix because it makes the mix more workable. This is important if you are making a lean mix, say 10:1. A 5:1 mix is still pretty workable with cement alone.
    Kindled with zeal and fired with passion.

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    • #3
      I would like to use the clay that I dug up but am wondering if it shrinks will the dome sink.

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      • #4
        Clay on its own shrinks considerably (around 10%). Cob mix is normally approx 2 parts sand 1part clay. This reduces the shrinkage considerably. You can test the clay/sand content of the clay you've dug by the following method. Take a sample approx half a litre, dry it, pulverise it and wet it down with around one litre of water. Mix thoroughly to a thin slip and pour it into a jar. Leave it overnight or two and you will see distinct layers of aggregate, clay, water. You can then determine how much sand you need to add to the mix. Adding straw to the cob mix, around 15% by volume, helps provide reinforcement, An excellent insulate mix for over the dome can also be made using chopped straw with just enough clay slip to make it stick together.
        Kindled with zeal and fired with passion.

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        • #5
          Thanks,
          For hearth insulation I'm thinking of using glass bottles (either beer or mason jars) with clay slip/vermiculite for one layer, followed by one layer
          Of vermiculite and Portland, followed by a layer of 2 inch insulating firebrick from a kiln that I dismantled.

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          • #6
            Can't see why you want to use so many different types of insulation. Glass bottles are not that great as an insulator but they are free if you don't count the contents consumed. I just weighed a 750 ml wine bottle and it was 550 g. Taking the thickness of the glass into account the bottle volume is about 800 ml which gives you a density of 688 g per litre By contrast the same volume of 5:1 dried vermicrete has a density of around 250 g per litre and that's without even factoring in the extra mass of the sand you'd have between the bottles. Glass has a thermal conductivity of 1.05 compared to vermiculite's 0.065. I know what I'd prefer to be using.

            http://www.engineeringtoolbox.com/th...ity-d_429.html
            Kindled with zeal and fired with passion.

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            • #7
              I appreciate your knowledge and experience David and thanks for the chart, it's very interesting. With regards to the bottles, I think it's the air trapped inside that does the insulating, I'm assuming that according to the chart, Air, atmosphere (gas) 0.024, Means that air is very low on the scale of conductivity.

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              • #8
                Yes it's the air that does most of the insulating, but if the material that surrounds the air cells is dense and conductive, then you are merely adding thermal mass. That's why the total density of the layer is important. An example of this would be to replace the glass in a double glazed window with the same thickness of steel, a denser and more conductive material. The window would not insulate too well. If you're trying to do a cheap build then use the bottles, but if you're after performance use a less conductive material.
                Kindled with zeal and fired with passion.

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