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Earthbag stand with adobe or clay dome - Forno Bravo Forum: The Wood-Fired Oven Community


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Earthbag stand with adobe or clay dome

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  • Earthbag stand with adobe or clay dome

    Hello everybody. I have read articles, forums, blogs, and then gone back and read and read some more all over the internet! I am interested in cob, adobe, rammed earth, strawbale, basically anything that uses found objects to create something useful. Of course, there is a ton of info out there, and I think I have it nailed down now.

    My idea is to use earthbag construction to make the walls for my oven stand. I can get plenty of free sandbags for walls, and I have a lot of rebar to tie them together. I would prefer not to have an enclosed base filled with aggregate to support the oven, as it woul be better to have a wood cache underneath. I may just create a cubby for wood within the base/stand using removable wood forms.

    I have found a stash of insulating firebrick which I will use directly underneath a medium duty brick hearth or a hard clay brick, if I cannot find the harder firebrick locally. ( I can get used firebrick for 1 buck a brick--but it may have come from ASARCO, and I sure don't want to mess with that stuff in my oven)

    We have clay soils here, but not in my own yard...and I thought of using clay from my cousin's yard, but I sure hate to think of digging and hauling it. I also have a resource for Hawthorne Clay Powder which is so cheap, I might as well go that route, and mix my own mud. I have seen mixed results using these powders for various applications, but I think that has a lot to do with errors in the clay ratio to sand/straw mix.

    My thought for insulating after the first clay layer on the dome is to use brick halves (insulating brick) and then an outer layer of clay mix with a bit of portland or small amount of bentonite in the mix. I also thought I could easily crush some of this brick to create an insulating cement, rather than laying on brick halves.

    At this time I can't decide whether to make bricks for a build (I could make an angled form) or to just blob it on over the sand mold, in true cob tradition.

    Hopefully I will get feedback here in this forum, and you can enjoy the folly of the experiments I plan on going through.

  • #2
    Re: Earthbag stand with adobe or clay dome

    I was helping an architect yesterday with an earthbag structure. It is an interesting method and should work well for an oven base. I assume you are going to plaster it with clay as well?

    FYI, unless I am mistaken Hawthorne clay is a refractory clay, which is good, but is very plastic which is bad because it will shrink (i.e. crack) upon drying.


    • #3
      Re: Earthbag stand with adobe or clay dome

      Hi Tscarborough!

      Yeah, plastered on the outside and inside of the earthbag construction. I need to find some barbwire for that too. I have little doubt it will work well.

      I did have some concern about that particular brand of clay, elasticity is a concern due to shrinkage, but it also provides a strength that less elastic clays won't have. There are many clay powders available..and I've read about fire clay, ball clay, bentonite (which shrinks a ton, but helps with elasticity and when used sparingly, but also helps in slips)...

      The clay mix itself will be my biggest obstacle.
      I would love to make bricks, just to do it...but also to test the drying properties, and stability after they do dry. I welcome any thoughts on the matter, as I am no chemist.
      Last edited by texmex; 08-04-2011, 09:53 AM.


      • #4
        Re: Earthbag stand with adobe or clay dome

        Normally you can minimize shrinkage cracking by using graded aggregates and a a gauging material. I recommended a lime plaster yesterday for that project and will do the same for yours.


        • #5
          Re: Earthbag stand with adobe or clay dome

          Lime plaster (would that be considered a calcite?) I have also read that a lower (less fine) mesh count and grog (aggregate?) additions will bring shrinkage down some. As you can see, I have a lot of terminology swirling around in my head, and haven't totally pinned them down!


          • #6
            Re: Earthbag stand with adobe or clay dome

            It is not so much that you want a certain size as it is that you want a gradation of sizes that will allow the finished material to consist of dense aggregate each particle of which is coated with as thin as possible a layer of paste. Grog in this usage is pieces of the hardened material crushed and added back into the plastic mix.

            For example, if you took bread crust and crumbled it up and added it back into a fresh batch of dough, the bread crust would be grog.
            Last edited by Tscarborough; 08-04-2011, 11:28 AM.


            • #7
              Re: Earthbag stand with adobe or clay dome

              Okay, I understand what grog is....

              Can you tell me about the drying properties of lime? Doesn't it have a short bucket life? I'll be working solo, so this could pose a problem.

              I need to figure out what is a good grog and an overall mix for strength and minimal shrinkage. That's why I would like to make bricks for testing. Crazy eh...but this stuff is just laying around or easy to obtain

              cement chunks, yard sand, clay powder, straw (my dead spanish broom plant), chopped up hemp twine, limestone plaster, dust from brick cutting, gravel.

              Somehow this reminds me of fruitcake
              ...all those aggregates
              ....bound together with a bit of batter
              to form an almost indestructable mass.


              • #8
                Re: Earthbag stand with adobe or clay dome

                The potlife of slaked lime is unlimited. Covered with water it never sets. Once mixed with aggregate to make plaster, it has a very long set time, so other materials are usually added to gauge it. That is something like a small amount (2-5% by volume) of portland cement or brickdust, that will cause a (relatively) early set. Lime plaster sets by absorbing carbon dioxide from the air. It is also caustic to work with, as are portland plasters, so you do not want to use bare hands to apply them like you can with a straight clay plaster.

                Lime plaster with a red brick dust (fines to 1/4") at 2-5% by volume of lime used, as gauge drys to a very nice color and texture, too.


                • #9
                  Re: Earthbag stand with adobe or clay dome

                  Thanks for the info, I surely do appreciate it.