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Has there been any advancement in the basic clay/sand mix for cob?

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  • Has there been any advancement in the basic clay/sand mix for cob?

    I was wondering since, like, over a decade ago when I built my first cob oven if the 2:1 sand/clay ratio is about the main recipe. Have there been any advancements with some kind of additives or techniques to reduce shrinkage and cracking?

    My second oven was a brick dome. No cracking at all but its performance wasn't particularly better than the cruddy one I built first. On the other hand, the front didn't degrade in the rain. So I was thinking for my third build that I'd build a brick front going to a cob dome. I was just hoping there was anything else I could do to improve on the old formula.

  • #2
    Well, there is home brew a mix of cement, lime and clay, try the search button or google adding Fornobravo.


    • #3
      Isn't that just the classic ingredients (including sand) for a refractory castable? I'm asking about cob mixes that I can slap around a sand form with my bare hands.


      • #4
        Well you could slap in on but I would suggest that you at least wear gloves.
        Apart from that… sorry I dont know!


        • #5
          A sand/clay mix usually contains chopped straw, which acts as a reinforcement. It could be replaced with man made fibres like heat resistant AR fibreglass fibres as used to reinforce concrete. These would be a better solution, but not free like the chopped straw. With its extremely small particle size, clay imparts a tendency to excessive shrinkage if the proportion is too high, that’s why the mix uses so much sand. The chopped straw also assists with holding the mix together. The addition of some fine polypropylene fibres would assist in moisture elimination as they melt at 160C, but require a lot of mixing for decent dispersal and that would be a difficult task with the mix in a plastic state. The addition of cementious material like lime or cement while increasing strength, eliminates the ability to apply the mix with bare hands, you must wear gloves.
          Last edited by david s; 04-07-2022, 12:20 PM.
          Kindled with zeal and fired with passion.


          • #6
            Yeah straw would be the classic but I was hoping to avoid the affect it would have on the finish since it would put a bunch of random streaks inside the dome after they burn off. Is the "AR" in the fiberglass fibers for "alakli resistant?" If I'm not technically using cement then would I specifically need a variety like that? Or is that just coincidentally what would work?

            I see some various posts about using some grog. That is, pre-fired clay that is then crushed and sifted. It's supposed to help resist in shrinkage at the cost of overall strength. As opposed to say, the pirate drink.

            I was also wondering about, say, scoring the clay or something to control how cracking happens so that it forms regular shapes.


            • #7
              There are all kinds of fibres commonly used for reinforcing concretes. You could use the industry standard for refractories which are melt extract fibres (stainless steel needles), but that would be overkill for a cheap cob mix as they are pretty expensive as well as hard to handle, they’re not called needles for nothing. Another solution would be coated steel fibres, but they do eventually corrode, particularly in a moist, hot mix. Stiff plastic (nylon or polypropylene) fibres are also out because of the oven’s service temperature and not that friendly to apply by hand. That’s why I suggested the AR (alkaline resistant fibreglass fibres because they have a melting point well above an oven’s service temperature range and at 19mm long and around 0.5mm thick they are reasonably easy to apply by hand and are readily available. Basalt fibres are another alternative although they’re pretty new, expensive and I haven’t tried them. Not really sure what you are trying to achieve with your mix, is it cost saving or a desire to use an age old traditional material. There are issues with a cob mix because it is never truly fired to a temperature to make the material permanent and sufficiently hard enough for wear to challenge its longevity, as well as its ability to absorb water and return to mud. Clay only becomes “permanent” when it reaches a temperature exceeding 573C. This is not possible in a WFO. You might get some of it to reach that temperature, on the surface, but certainly not all of it. Attempts to do so with wood fire are way to uncontrolled to allow the unstable changes to occur gently and safely and always result in severe cracking.
              Last edited by david s; 04-08-2022, 02:02 AM.
              Kindled with zeal and fired with passion.