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cob + firebrick?

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  • cob + firebrick?

    I began my oven with the idea of keeping the total cost below $100. I have very good clay dug from our property, salvaged 4 x 12 fir timbers for a base, and about 40 used firebrick. Following Kiko Denzer's book, I figured I'd build a dome mold from sand and cover it with cob layers as he describes. To date I've built the foundation (wooden timbers) and have laid down a 6 inch layer of bottles and sawdust-clay mortar, topped with 4 inches of clay-sand mortar.

    Before I could begin the next phase, however, I offered to help a neighbor (to whom I owe many favors) demolish a large fireplace and chimney. In addition to profuse thankyous he gave me the firebrick. I don't have an exact count but there's probably 40 or 50 good full-sized brick, giving me a total of about 100 firebrick.

    I'm tempted to use as much firebrick as I can for my oven but I'd like to build this without having to purchase fireclay mortar or doing any cutting of bricks. We live a long way from town so a trip to the hardware store is a pain, not to mention the cost.

    I'm wondering if it is practical to build a Tuscan-style oven of both firebrick and clay, without using "real" mortar, by building over a sand form? Perhaps, for example, by using stacked firebrick halfway up the dome, then going up from there entirely with clay cob?

    If I were to do this, how much insulation would I need for the next outer layer? I have both sawdust and peat moss to mix with clay. (A test brick made with equal parts clay and peat moss turned out to be very light and quite strong.)

    For the entrance I have a terra cotta chimney tile with a 10 x 14 inch opening.

    I would appreciate your thoughts!

    Carl

  • #2
    Re: cob + firebrick?

    Hi, just watch out most fireplace bricks are insulating rather than the heat hold type that are needed in a pizza oven.
    http://www.fornobravo.com/forum/f2/o...-nz-14012.html

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    • #3
      Re: cob + firebrick?

      Originally posted by wemme View Post
      Hi, just watch out most fireplace bricks are insulating rather than the heat hold type that are needed in a pizza oven.
      I had no idea that there are two types of firebrick. How would I tell which I have, since some came from a recycle yard and the others from a fireplace?

      In any case, while waiting for a reply to my question I decided to go ahead with an all-cob oven. Well, almost all cob -- I have a firebrick hearth and a single circular row of firebrick laid on their sides.

      Rather than use clay with sand for the inner dome (about 34 inches ID) I'm going to use a mix that might be called grog -- I have a source for small crushed olivine rock that is heavy with "fines". I'll mix this with clay and apply it 3-4" thick to create the dome (16" high).

      I've also been given a large amount of rigid 2 inch insulation, scrap from a local pizza oven factory. Using a chop saw I expect to piece together a kind of geodesic layer 4 inches thick, coat it with a thin layer of clay, then go over everything with a traditional lime plaster.

      I have no idea how this is going to work out but I'm hopeful!

      Has anyone here ever worked with olivine before? This is incredible stuff but the only oven I know of with olivine was made entirely of large solid pieces. The hearth alone was estimated to weigh 2 or 3 tons and isn't that large. The owner says it takes way too long to heat up.

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      • #4
        Re: cob + firebrick?

        Hi This topic has been talked about to no end on this site.
        http://www.fornobravo.com/forum/f6/d...html#post78618
        but there are more..

        I've been tempted on using clay as fire bricks are hard to get where i am.
        http://www.fornobravo.com/forum/f2/o...-nz-14012.html

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        • #5
          Re: cob + firebrick?

          I had no idea that there are two types of firebrick. How would I tell which I have, since some came from a recycle yard and the others from a fireplace?
          A firebrick is hard, solid and weighs about 8 pounds. An insulation brick is porous, with a pumice like texture, and weighs about two pounds. They aren't used in fireplaces. They are mostly used in ceramic kilns and the like.
          My geodesic oven project: part 1, part 2

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          • #6
            Re: cob + firebrick?

            Rather than use clay with sand for the inner dome (about 34 inches ID) I'm going to use a mix that might be called grog -- I have a source for small crushed olivine rock that is heavy with "fines". I'll mix this with clay and apply it 3-4" thick to create the dome (16" high)
            I used a mix of sand, crusher dust (fine aggregate) and clay, with a small amount of Portland cement, to create a 2 inch thick dome. So far, it's been great - the oven runs at an excellent temperature.

            Cheers,
            Mick
            My Clay Oven build:
            http://www.fornobravo.com/forum/f43/...dah-12821.html

            Comment


            • #7
              Re: cob + firebrick?

              We completed the dome yesterday and I'm anxiously watching it cure. The olivine material is so heavy that we had a fair amount of sag, especially in the lower circumference of the dome. My only concern, however, is that the dome might actually be too dense -- the mix is up to 6 inches thick about one third of the way up, then thins to about three and half inches as it reaches the top.

              It took several hundred pounds of material to cover the dome.

              I'll post photographs once I recover enough strength in my arms!

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              • #8
                Re: cob + firebrick?

                I'd never heard of olivine. It turns out that it may be one of the most common minerals on earth. The only commercial use for it at the moment is green sand, the sand used in the casting industry, meaning that it's certainly not without refractory properties.
                It took several hundred pounds of material to cover the dome.
                The pompeii 90 kit includes 120 firebricks, not including the refractory tiles for the floor. At nine pounds apiece, that's about half a ton.
                My geodesic oven project: part 1, part 2

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                • #9
                  Re: cob + firebrick?

                  Has anyone ever used strawless cob as mortar for the firebrick?
                  Last edited by Jimney; 06-01-2013, 11:00 PM. Reason: More specific.

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                  • #10
                    Re: cob + firebrick?

                    Gudday
                    Why bother to take a extra step in making the bricks when you can build the dome as a single unit in cob.
                    On the subject. Early in my investigation on WFO I was leaning towards building a stabilized clay dome. That's clay with N-type mortar (Portland cement lime and sand) Added an addition of fibers cut off an old material so when they burnt out it would provide an escape for the water from the dome structure.
                    Insulation was to be clay and chopped sugar cane mulch with a layer of fiberglass insulation to then allow me to put a render dome over that the fiberglass would provide an Expantion layer from the dome to prevent cracking.
                    O a common brick or paver hearth before I forget
                    It seemed to me to be a good option at that time considering I live 1 km off the sea and we get some pretty good rain here at times. I considered it the most water resistant option .... But I found the forno plans and decieded that the time and effort would probably be the same. It would take me some time so the money or finding salvage bits could wait as things went along.
                    If would still consider building this way if my climate was a little less wet
                    Anyway hope you find something here to help
                    Regards dave
                    PS knew about the stabilized clay as a friend built a house from them 30 yrs ago . The bricks were pressed in a special machine that he hired special built. Lots of clay and mortar to make a full house worth of brick!
                    Last edited by cobblerdave; 06-02-2013, 12:11 AM. Reason: Add footnote
                    Measure twice
                    Cut once
                    Fit in position with largest hammer

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                    http://www.fornobravo.com/forum/f28/...ock-17190.html

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                    • #11
                      Re: cob + firebrick?

                      Originally posted by Jimney View Post
                      Has anyone ever used strawless cob as mortar for the firebrick?
                      Originally posted by cobblerdave View Post
                      Why bother to take a extra step in making the bricks when you can build the dome as a single unit in cob.
                      I think the question was about using clay for mortar.
                      The English language was invented by people who couldnt spell.

                      My Build.

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                      • #12
                        Re: cob + firebrick?

                        Originally posted by Jimney View Post
                        Has anyone ever used strawless cob as mortar for the firebrick?
                        By cob i presume you mean clay and sand mixed together without the straw.
                        Not so good. It's way too sticky- can't get it off the trowel.
                        It also shrinks too much, you'll have lots of shrinkage cracks.
                        The homebrew mortar is way better 3:1:1:1 sand, clay, lime, portland cement
                        Kindled with zeal and fired with passion.

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