[Cob] Wall building problems

[Shannon C. Dealy]

On Thu, 29 Jul 2004, Mark Thomas wrote:

[snip]
> Deming. The cob dries fast in the dry, hot and often windy weather. I
> have not been able to work the next layer into the previous while it is
> still plastic yet firm enough for support. Tried covering with tarps and
> placing wet hay on the wall.
[snip]

Possibly your clay is not getting completely soaked.  Some clays when they
are really dry/hard can be very slow to absorb water and this can actually
be used to speed the hardening of walls (it won't really dry faster, but
it will set up faster).  When you mix the cob, only some of the clay
absorbs the water, then over the course of the next few hours (or days),
the dry/hard clay in the mix soaks up some of the water from the wall
around it, hardening the soft/wet clay and slightly softening the hard/dry
clay.  This works because you actually have to little water in the mix
relative to the amount of clay, but because some of it hasn't absorbed
it's water yet, the mix remains workable.  A simple test would be to put a
sample of the mix in a sealed container and see how hard it is after a day
or two (it shouldn't harden at all if there is sufficient water for the
clay content).  Straw content of the mix can have a similar effect, since
some straws may be slow to absorb water as well.  If this is the source of
your problem, you may find it desirable / necessary to pre-soak the clay
for a day or two in order to get enough water into it to prevent the fast
setting you are experiencing.  This may also make it desirable to adjust
your mix since you probably have more/stickier clay than you realized, and
are only now fully utilizing it.  To some extent this will happen in all
cob mixes, but you might have a more extreme case than most.

If this isn't the problem, the only thing I can suggest is tighter, better
sealed tarps, it can't dry if the moisture is trapped, so if the moisture
is actually escaping, the tarps can't be that well sealed.

NOTE: Once the walls get tall enough / dry enough, the wall below where
the new cob is added will tend to absorb any excess moisture, speeding the
drying of the new cob, and if that part of the wall is not tarped, it will
provide a large surface area to evaporate off the moisture from the fresh
cob, even if the top of the wall is covered.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 929-4089 |                  www.deatech.com