Cob: Use of Forms?

[Graham Cooper]

I would just mention that that blackboard chalk is actually plaster -
Plaster of Paris,
otherwise known as gypsum - the same as in sheetrock (called plasterboard in
UK).
Composition - calcium sulphate heptahydrate.

GC

-----Original Message-----
From: owner-coblist at deatech.com [mailto:owner-coblist at deatech.com]On
Behalf Of Darel Henman
Sent: Monday, January 06, 2003 1:14 AM
To: Ray Luechtefeld; cob list
Subject: Re: Cob: Use of Forms?


In reply to myself.

It may be the chalk soil would not shrink like a clay soil would.  Any
experts of U.K.'s version of chalk soil out there in some of your
areas?  Any Brits up to this one?  You can use Imperial units without
fear, we still use them.

Here I have a defintion of chalk as:
A soft, earthy substance, of a white, grayish, or yellowish white color,
consisting of calcium carbonate, and having the same composition as
common limestone.

It would probably need some clay in it to hold it together in an
uncalcined state.

Darel

Darel Henman wrote:
>
> Ray,
>
> Thanks for the source information.  From reading the paper it states
> that the only difference in the shuttered (formed) process and the
> rammed earth (see Techniques 2 page, bottom section) is that rammed
> earth uses mix with less moisture than the shuttered type and requires
> more sturdy forms.  The rammed earth would be stronger then.  The
> introduction section, paragraph one, states that "...Less commonly, but
> still significantly, earth (or perhaps more often chalk) which was
> rammed into place between shuttering can be found."  Indicates a rammed
> earth, but on the the Techiques 1 page, the "shuttering process"
> described seems to be a mix of cob earthen material of a considencty
> between that required for a true monolithic bearing cob wall and the
> light-clay mixture used for partitions or other non-bearing walls.
> Since it is wetter than rammed mixtures, it would shrink more when it
> dries and cures, the shrinkage extent will depend upon the amount of
> straw  mixed in.  This shrinkage might be able to be fixed by a
> subsequent application to fill any cracks and top of the wall.  I will
> read the rest of the report.
>
> It would certainly be easy enough to make some small little test forms
> to make some test pieces in.   Maybe a test size of 1 foot long by six
> inches by six inches.  Maybe a  pound cake pan?
>
> Darel
>
> Ray Luechtefeld wrote:
> >
> > Hi Darel,
> >
> > I don't think so, I believe the paper I'm looking at refers only to
> > cob.  The page talking about forms (called "shuttering") is at
> >
> > http://www.ihbc.org.uk/Cob_Paper/page2.html
> >
> > What do you think?
> >
> > Ray
> >