Cob: RE: Origins of Cement vs Cob

[Michael Saunby]

On 18 July 2099 16:00, H. Wayne [SMTP:hcrowbird at lawtonnet.net] wrote:
>     Wow
[BIG SNIP]
>    From the die hard cobist view, how dare you make cob with cement in 
it.  It is NOT true cob anymore.  Oh well, what can I say.  Some folks are 
just not open to suggestion, no matter how helpful the idea might be.
[snip]

The notes I have from the Devon Historic Buildings Trust (scanned copy at 
http://www.chook.demon.co.uk/cob/page2.gif for now) say: A good, well 
graded clayey sub-soil containing plenty of straw requires no other 
additives.  Given the right conditions a well constructed cob wall will set 
almost as hard as rock, so the addition of lime or cement is quite 
unnecessary.  It is claimed that lime was added to cob, presumably in order 
to achieve a faster set, from the 1850s onwards, though there is no record 
of it having been used in earlier times.

OK, so I know that doesn't prove anything, but it's probably a fair 
statement of an historical truth from my part of the world.

Another reason why cob buildings in Devon might have outlived (there are 
many 400 year or so old) concrete buildings (none) if there had been any is 
that no buildings of that age have foundations.  Therefore the lack of 
complete rigidity (another word for strength?) means that a cob building is 
tolerant of a certain degree of ground subsidence, mine has numerous cracks 
in it that have been filled over the centuries, I've even made a few small 
repairs myself.  I guess that when building directly onto a clay subsoil it 
makes sense to use a material which will accommodate any movement.  Perhaps 
I'm being unfair and concrete buildings would have lasted just as well.

Why no older buildings?  There are very few domestic buildings in Devon 
older that 400 years, perhaps because the introduction of chimneys and 
other social and lifestyle changes here made the earlier hall-houses, etc. 
unsuitable.  It could be that it's just not possible to build a house that 
would be just right for 1000 years except as a monument, perhaps disposable 
(after a couple of centuries) earth building is as good as you can get.  I 
suspect however that the 16th and 17th century builders in Devon did reuse 
quite a lot of the materials from the no longer fashionable houses, or 
converted them to barns, etc.  Certainly the timbers in my house don't all 
look the same age, some are repairs but others look like reuse.

Michael Saunby