howto cobstruction

[M J Epko]

Shannon Dealey wrote:

>... before starting work each day, check how firm the walls are.

        How firm should it be? Just firm enough so that it doesn't dent much
with the heel of your hand (or foot)? Yet loose enough to gouge with a blunt
one-inch thick stick?

>You should also check how plumb the walls are before starting each day...

        The Backwoods article recommended taping an angled piece of wood or
foam to a long level to match the exterior taper. That way the flat side can
be used for the interior, which should be plumb(ish) and the angled side for
the outside, which tapers toward the top.

        and later Shannon said:

>we put in lots of holes here in Western
>Oregon, and leave it uncovered so it will dry enough to support the next
>day's layer of cob.  It should be noted here that it is important that the
>cob not dry to much between applying layers, since new cob does not bind
>nearly as well to dry cob.  If the cob dries out, you can soak the top
>of it several times over a period of a few days prior to adding the next
>layer, this will improve the bond, but it will never be as good as it
>would have been had the cob been added while the previous layer was still
>a little soft.  For this reason, it is important to always leave the top
>of an unfinished wall rough with lots of holes in it, since this will 
>provide a mechanical bond between the layers regardless of how dried out
>the wall gets before the next layer is added.

        Which causes me to again beg for additional clues on how to guage
whether it's too one or the other.

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        I wrote:

> (I have an adobe book from
> the late 40s that says that adobe test blocks should be plunged in water as
> an additional test. Of course, they recommended stabilizing the mix with
> asphalt emulsion, too...)

        Shannon responded:

> I am not sure what the water test was supposed to prove.

        They were trying to waterproof and stabilize the adobe mix by adding
asphalt emulsion. The submerse test was to see if it worked or not. I'd
probably throw a hunk o' cob in the water just to see what happened. I don't
think it would prove anything, really.

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        Shannon:

>You don't have to plaster, though the wall will probably look better to most
>people if it is plastered, and weather will erode the wall some if it isn't,
>(let your grand-children build their own walls :-)

        Rob Tom's post to the SB list today about lime plaster mixes is
great; I'll be keepin' that one.
        David Eisenberg had some comments (they're in that huge Cob
Gleanings From The Strawbale List post) about plasters, some of which spoke
to how cement plasters seemed to be causing failure in old adobe buildings.
Hmm. I really like the sound of lime rather than cement.

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                INGREDIENTS?

        Shannon says, about the mayo jar soil composition field test:

>It should be noted here that the clay layer should be at least 10% and
>no more than 30% (20% would be better) for the soil used in the mix.  Since 
>soils often don't meet this requirement, it may be necessary to find soil 
>somewhere else (possibly elsewhere on the lot you are building on) which 
>can be mixed with the soil at the site to get better proportions.  At Cob 
>Cottage Company's site, they have a very high clay content and mix three 
>parts soil to two parts sand.  On the other hand there is virtually no clay
>in the soil where I am.

        Andrew Alcorn wrote a long time ago on the strawbale list:

>"Cob" as far as I am aware, correct me if I'm wrong, originated in England
>where the word originally refered to the handful of mix (soil, water,
>chopped straw. horsehair, gravel, whathaveyou) that was placed on the wall
>as a `cob'.

        Sam wrote on the strawbale list:

>What about a cobblestone and cob mix?

        and Shannon replied:
 
>I'm not sure what you mean here, if you are talking about embedding stone 
>in the Cob to reduce the Cob required, I don't think this would be a good 
>idea, since this would tend to disrupt the matrix.  Think of cob as a 
>really crude form of fiber glass, the straw is the fiber, and the 
>clay/soil is the resin.

        And Ann Edminster wrote:

>Ensuring that there is enough straw (organic rebar) in the mix is crucial to
>its crack-resistance.

        Should the soil used be screened for rocks? Why would a few rocks
hurt things? Seems to me they might actually help (I also couldn't get a
grip on that mass bit either, though.) Hmm.
        If glass panes can safely be set directly into the cob (with the cob
built into a bearing arch around the top of the window), wouldn't it stand
to reason that kinda big rocks could also be used (allowing for substantial
cob-mortar: like, foot-wide 'joints') with no ill effect? The cob would arch
over the top of the stone, which would be stonger than the cob itself
anyway... that would seem to be very stable and reduce the amount of cob needed.
        Anybody care to speak to this (please)?

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M J