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High Mass, Low Mass? - Forno Bravo Forum: The Wood-Fired Oven Community

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High Mass, Low Mass?

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  • High Mass, Low Mass?

    Folks,

    I'd like to understand a bit more about the difference of high mass and low mass ovens in a practicable construction sense. I'm reasonably comfortable that I understand the difference in terms of how the oven performs, but I'm not sure of the real life construction differences.

    What construction characteristics constitute a low mass, medium mass, or high mass oven?

    For example, if i build an oven using 115mm (4.5 inches) wide fire bricks as the internal wall, followed by a 25mm (1 inch) insulating ceramic blanket, followed by a 40mm (1.5 inch) insulating castable render, and then either more render, or a second skin of solid red bricks, then;

    What sort of oven have I built in terms of heat mass? High, Medium, or Low.

    To change the heat mass rating and move it up, what would I do? Just add more to the insulation layers? Add a layer of something (not sure what?) before I lay the insulating layers?

    I guess that while I generally understand the difference between the heat mass layer and the insulating layer, I'm not sure exactly how they work hand in hand, given that I guess at some point the insulating layer should be contributing to the mass layer by holding the heat. Also I'm not sure how to directly increase the heat mass layer apart from another layer of fire bricks, but I'm sure there are other more practicable and affordable ways to do this.

    I guess to add some context, I have this plan to build twin ovens that currently stands at one smaller sized dome oven, and a larger tunnel design. The small one primarily for pizza's, and small roasts that is quick to heat, and the larger one for big cook ups, potentially in tandem with the smaller oven. At this stage apart from basic design shape differences, construction plans are similiar in terms of bricks and insulation. My prefrence would be lower mass for the smaller oven (quicker to heat, not required for extended periods), and higher mass for the larger oven, but I'm not sure where my basic plan that I described above sits in terms of mass rating, so i don't know how to fine tune the construction plans to get what I think I would like.

    All advice/explanation appreciated.

    Cheers,
    Peter.

  • #2
    Re: High Mass, Low Mass?

    I built my oven with firebricks on edge, for a thickness of 2 1/4 inches, which apart from Christo, who followed my plan, may be the thinnest oven built here. This is covered by a one inch blanket, and a fill of the enclosure with vermiculite concrete, which is as little as an inch or two at the edges and about eight inches on top. My theory was that this was more or less the thickness of the pre-cast refractory ovens, and would behave in more or less the same way. It also had more or less the same wall and floor thickness. This is clearly a low mass oven. It seems to work fine, although I'm a little nervous about driving enough heat into the firebrick layer for the hours-long retained heat turkey cooking. Other makers have gotten a low mass approach to the dome by using 1/3 bricks instead of half bricks.

    Most of the attempts to increase mass in the pompeii design have been by adding more mass to the floor. Originally the insulating layer was put under the support slab, so that all four inches of concrete was absorbing heat. Experience showed that this wicked way too much heat away from the cooking floor, and now people who want more mass put a slug of concrete or another layer of firebrick below the cooking floor only, surrounded by insulation. Search "island hearth" for more on this approach. You can also increase the mass of your dome by covering it with a layer of refractory concrete like stucco, before insulating.

    The true high mass oven uses a full eight inches of solid refractory material in both the dome and the hearth. This is the classic Alan Scott design, and is mostly used for bread baking, where commercial bakers want multiple bakes from a single firing Canuck Jim has an oven like this, and uses it for multiple bakes of bread, from high heat to low heat, year around.

    All forms of oven should have as much insulation as you can reasonably plan for. This keeps the heat in the oven, and the cooking, instead of heating the enclosure and surroundings.

    It all depends on what kind of cooking you want to do. The conventional wisdom here is that a low mass oven is for quick heat-ups for Pizza, and a single bread bake. A high mass oven is more useful in an environment where it is fired every day, and the heat retained for the day before is re-used.
    My geodesic oven project: part 1, part 2

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    • #3
      Re: High Mass, Low Mass?

      What do you mean by big cook ups...how long do you want to be able to cook in the oven? I have a barrel vault that is 4.5 inches brick thickness followed by about four inches of concrete for a total of about 8.5 to 9 inches.
      I can certainly help you decide...
      Dutch
      "Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity. " Charles Mingus
      "Build at least two brick ovens...one to make all the mistakes on and the other to be just like you dreamed of!" Dutch

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      • #4
        Re: High Mass, Low Mass?

        The easiest way to explain thermal mass is to picture a sponge. The refractory material (brick, ect) is like a sponge. The more mass, the more heat it will hold, the thicker the sponge the more water it will hold. The insulation layer has nothing to do with heat mass, it just reduces the rate at which the heat leaves, like a layer of plastic around a sponge drastically reduces the amount of water from leaking or evaporating. Also like a sponge the higher the mass, the more heat (ie. water) it will take to saturate (fill/soak) it. That is why high mass ovens take longer to heat.
        Wade Lively

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        • #5
          Re: High Mass, Low Mass?

          I have built, owned and used a number of high mass barrel vault bread ovens as well as traditional round dome ovens. From my experience, I have a strong opinion that you should only build the higher mass oven if you want to start a commercial bread bakery.

          There are some good threads on this topic throughout the forum, but the major issues are:

          1. Holding high heat for pizza and other high (750ºF) temperature cooking. It is very difficult keeping high mass ovens hot in both the floor and dome.

          2. Heat up time. A dome oven can be ready to cook in 45 minutes, where it takes 2-3 hours to really fire a high mass oven.

          3. Shape. The dome oven works well for fire in the oven cooking, where the barrel vault oven does not leave space for both pizza and the fire.

          It's an interesting topic. Here are some other postings to read:

          http://www.fornobravo.com/forum/f2/w...s-round-2.html

          http://www.fornobravo.com/forum/f2/h...-heat-684.html

          And a couple of fun grahpics:

          http://www.fornobravo.com/forum/atta...works-heat.jpg

          http://www.fornobravo.com/forum/atta...n-hearth_2.jpg

          http://www.fornobravo.com/forum/atta...n-hearth_1.jpg

          Have fun working your way through it.
          James
          Pizza Ovens
          Outdoor Fireplaces

          Comment


          • #6
            Re: High Mass, Low Mass?

            Guys,

            Thanks for the responses. I will ponder the info for a day or so and be back looking for more.

            Cheers,
            Peter.

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