[Re: Cob: Couple Of Questions]

[Shannon C. Dealy]

On Mon, 22 Jul 2002, Sarna Salzman wrote:

> When bonding clay for ceramics and pottery, I use a vinegar-slip to help
> connect two pieces of drying clay together. I'm not really clear on the
> chemistry of it, but it is supposed to be more effective than a water-based
> slip. Wouldn't the same apply to cob? (Though, I guess that might involve a
> LOT of vinegar!)
> Sarna

I don't know if this would make any difference in the strength of the clay
bonding between fresh cob and dried cob that has just been soaked with
water, but the primary loss of bond strength is from the fact that the
straw fibers from the fresh cob are not going to be mixed with the cob in
the dried layer below.  Basically, what you end up with is two layers
interlocked only by the cob from the fresh layer sticking into the holes
you left in the old dried layer below, with some additional strength from
the clay bonding between the two surfaces.  The result is something like
stacking lego bricks, you have reasonably good strength from the
perspective of horizontal forces trying to push through the side of a
stack of bricks, but you don't have nearly as much strength if you simply
try to pull a brick off the top.  Of course in the ideal approach, all
layers are kneeded into the preceeding layer which binds the straw into
both layers, resulting in a wall which is effectively one giant lego
brick, and which is significantly stronger either horizontally or
vertically than a stack of bricks, unfortunately, this can't be done if
cob is applied to a layer that has already dried.  The only real question,
is how much strength do you actually need?  In many areas I don't think it
matters much, but if I were in an earthquake zone I would be concerned
that, vertical motion of the ground during a quake might break loose an
upper layer along the line of a wet/dry cob bond.  If I were in a tornado
or hurricane zone, I might be concerned that high winds might be
sufficiently strong to take the roof off along with the top section of the
wall it was attached to if there was a wet/dry cob bond high up on the
wall.  Other areas may have similar hazards that could be a problem for
cob walls with this type of bond, or some sections of your structure
depending on the design, might not be structurally sound using a wet/dry
cob bond to hold it together (like built-in book shelves).  Ultimately,
this is one more consideration to where and how you build your cob house,
but with good planning, in most cases you should be able to keep the walls
from drying out between layers of cob, even if you must leave it for days
or weeks between layers.

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) 451-5177 |                  www.deatech.com