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Variable Thermal Mass?? - Forno Bravo Forum: The Wood-Fired Oven Community



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Variable Thermal Mass??

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  • Variable Thermal Mass??

    Most of us know that thermal mass is a double edged sword.

    On the one hand, all that retained heat makes for the ability to have a very long coast (cool down) phase which allows for things like multiple batches of bread from a single firing, and other types of retained heating.

    The down side is that all that thermal mass takes a long time (and a lot of fuel) to heat up.

    Has anyone attempted to vary thermal mass by adding or subtracting firebricks? It seems as if one could drastically change the amount of thermal mass by simply varying the number of firebricks depending upon what you want to do in a particular fireing.

    I'm in the middle of my first build. My oven is a vault style and is constructed with zero cladding and lost of insulation. The oven interior as built will be 40 inches long by 37 inches wide. (10.5 square feet). For comparison purposes, the square footage of a a forty-five inch pompeii style is also 10.5 square feet. The height of the vault interior is 19 inches.

    Here's a pic of the partially completed vault interior on my oven.

    The side walls and roof vault are bricks laid sideways (4.5 inches), the floor is bricks laid flat (2.5 inches). There are going to be approximately 190 firebricks in the oven by the time I'm done. (Approximately 155 in the vault, call it fifteen in the entryway/transition, and call it amother another 20 in the chimney/transition.) As noted previously, zero cladding. As such, the thermal mass is effectively identical to that of a 44 inch pompeii style.

    In other words, fine for pizza and causal bread baking...but insufficient thermal mass for multiple batches of bread from a single firing.

    But what's to stop someone from doubling up the floor mass on a vault or pompei style oven. Similarly, what's to stop someone from stacking brick along the walls? For example, if I doubled floor thickness (to five inches) by adding a layer of firebricks...and stacked three firebricks high running the half the length of each sidewall, my oven interior would still measure 16.5 inches high, forty inches long, and have an average width of 32 inches...and it would have an amount of thermal mass approaching that of a similarly sized, fully cladded, Allan Scott oven. The floor would be fiven inches thick and the walls would have an average thickness of seven inches. (The roof would remain at only 4.5 inches.)

    It seems like this would be handy when you wanted to do some serious retained heat cooking. And when you didn't want to have all that thermal mass, you could simply pull the bricks out and stack them behind the oven.

    So does anyone vary the thermal mass of their oven depending on what they want to accomplish on a given day?

    Last edited by WJW; 03-13-2012, 11:32 PM.

  • #2
    Re: Variable Thermal Mass??

    I think this is quite possible, but by filling your oven with bricks you are reducing its area that you could use for cooking. An Italian bloke was telling me they used to use a couple of truck axles thrown into the oven to achieve the same purpose. Why don't you try it to see how it goes? How much bread are you intending to cook? I sometimes do two batches of bread without refiring in between, in my little 21" oven so I'm sure you could do so in yours too.
    Kindled with zeal and fired with passion.


    • #3
      Re: Variable Thermal Mass??

      If I were you I'd start with what you have and find out if it's good for your needs. If the floor is loose you could run your bricks so you get 4.5 rather than the 2.25 thickness. Take your time, cure the oven, and see where you really are with the oven before making changes.



      • #4
        Re: Variable Thermal Mass??

        I agree with seeing what my needs are before doing anything...I'm just thinking out loud. The reality is that I'd be surprised if I ever want to do multiple bread batches in light of the size of the oven (what would I do with all the bread )...but I was just wondering if people do that at times.

        I only bring it up because it seems like a lot of effort is devoted to the question of thermal mass. In fact, people choose a particular type of oven based in large part on how much mass it does or doesn't have... and it occurred to me that this should be something which is easily manipulated on a temporary basis any time one chooses.

        And no matter what, I don't foresee making any reeal "changes" other than temporarily setting bricks in there and seeing how it affects the saturation and coast time. I plan to put three thermocouples in so it'll be interesting to experiment and see hard numbers on the results of this stuff.



        • #5
          Re: Variable Thermal Mass??

          ... Hey, is it just thinking out loud??
          Here we go!
          The starting point could be design from the begining thinking in terms of variable mass, the easy way I can imagine is use sand, desert sand. Just as seen in old egiptians movies, it can flow like a liquid filling cavities around the dome or hearth and remove it as desired. The empty space, once it removed is filled with static air, the best isolating material after the vacuo. This could be done using also some liquids, but expensive and intolerant to cracks...