Cob: Re: Cobber's Thumb and Knitting cob together

[Frances Grill]

Greetings: In regards to the bonding of one days cob to another, why not use
a key-way  to prevent shearing?  If the wall is 10-20 inches thick, a 4x4 or
similiar sized fence post mudded into the top of a section of wall, then
removed as the cob is drying would provide much greater strength to the bond
joint, be easier to fill on the next days cobbing,,,,of course it is no way
as cool as a cobbers thumb. Obviously, since much cobbing is done in a
curvecd plane, your key-way might need to be flexible to follow the course
of the wall. This key-way also works well to anchor a concrete "cap " to
protect the wall from mechanical erosion from rain, since concrete doesn't
bond well to eathen structures, but is quite protective.Cheers, peter grill
-----Original Message-----
From: Shannon C. Dealy <dealy at deatech.com>
To: coblist at deatech.com <coblist at deatech.com>
Date: Wednesday, July 26, 2000 2:11 PM
Subject: Cob: Cobber's Thumb and Knitting cob together


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