Cob: Cobber's Thumb and Knitting cob together

[Shannon C. Dealy]

On Wed, 26 Jul 2000 goshawk at gnat.net wrote:

> it's a stick used to make holes in top of a level of cob so that the next level will hold better. 
> I've not been using that technique on my walls. I generally wet the level I'm adding to if it's dry and 
> sometimes use my fingers to punch a few holes but then more often  than not haven't  and have 
> not had any problems with my technique at the this point. 

The point of using your thumb or a "Cobber's thumb" is to get a good
interlock between the different layers of cob.  In a given lump of cob
mix, the straw fibers are all twisted around each other, making it into a
single integrated piece.  When you place a cob on top of a wall, it is not
tied into the rest of the wall until you repeatedly push your thumb
through the cob you are adding and into the wall below it.  By doing this,
part of the fibers from the cob you are adding become intermixed with the
cob below, binding the layers together.  If you don't do this, you end up
with the layers being held together by friction and gravity, which is fine
until the wall encounters some type of horizontal shear force such as a
car hitting the building, earthquake or other disaster, at which point,
any cob which has not been knitted together by this means provides
built-in fracture points for the wall to break apart.  While some people
disagree that this is a necessay proceedure, my approach is always to
build for maximum structural strength in all directions, so I always knit
the layers together in this manner.

With regard to putting holes in the top of the cob, there are two reasons
that I know of for doing this other than the incidental creation of holes
in the cob while knitting the layers together (making holes is not the
point of the knitting process).  First, putting holes in the cob will
provide more exposed surface area which will help the cob to dry faster.
In warm dry areas, this is not generally a problem, but if you are cobbing
in 60 degree (F) or colder weather or at a humid or damp time of year, it
may be necessary.  The second reason is that if you need to stop work on a
wall for a period of time during which the top of the wall may dry and
harden, in most cases you will not be able to get the wall to soften up
enough for knitting the next layer to the wall when you start work again.
By putting lots of holes in the top of your wall before you leave it, you
provide a mechanical interlock that the next layer of cob can be forced
into which will tie the wall together much better than if the new cob were
simply added to a flat surface.  Doing this will NOT give you as good a
bond as knitting together two layers of soft cob, but it will give you a
good bond.  NOTE: before applying fresh cob to a dried wall, always soak
the surface you are going to apply the cob to with lots of water, possibly
over several days, in most cases, the dried cob won't get soft, but it
should get a little softer, and in either case the new cob will bond to it
much better.

Now to answer one final obvious question, why use a cobber's thumb instead
of your own?  Many people don't, but working with a good high sand cob mix
and pushing your thumbs through it all day long does tend to wear the skin
off a bit :-)  Some people will just grab any convenient stick of about
1" diameter for use as a cobber's thumb, and others will buy or make their
own.  The primary advantages of a cobber's thumb that is made for that
purpose over a convenient stick, is that sticks (at least the ones I find
lying around) often are not particularly hard/strong and a day of cobbing
can quickly grind some woods down to nothing, so a manufactured cobber's
thumb made from a good strong seasoned wood will last longer, and it can
be made to comfortably fit the palm of your hand which may be a little
easier on the hands over time (I find that sticks rarely come with
convenient palm shapes :-)  I personally tend to think that people should
get their hands into the mix at least some of the time to get a real feel
for what they are doing, but when I work with a good high sand cob mix
for any period of time, I will eventually grab a cobber's thumb or stick
in order to leave some skin on my fingers at the end of the day :-)


Shannon C. Dealy      |               DeaTech Research Inc.
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