Cob foundation cement

[Will Firstbrook WCB of BC]

Hi Ian,

One of the properties needed in a foundation for a cob structure is to
prevent the base of the cob from getting wet. Due to capillary action
cob mortar will likely pass the moisture up to the base of the cob. If
this occurs this is the worst thing you could do to your structure. I
understand your desire to avoid the use of cement. But cement mortar
works quite well and if the stones are arranged in such a way that it is
stable without the mortar you could just fill the cracks with small
stones then use a small amount on the outside. Or none at all if it can
be arranged in a way that the weight of the structure holds it together.
That might be tough to do.

You could possibly use the water glass stuff (silica sealant) to harden
and waterproof cob mortar. This was recommended by Carol Crews as a
sealant that waterproof but still allows water vapor to pass in and out
of the structure. It has been used commercially to seal fireplace
chimneys. There was a discussion on this Dec 97 on the Cob List.

Another possibility is how the earthen floors are hardened and
waterproofed using multiple coats of linseed oil with increased thinning
for each coat by adding pine turpentine to the linseed oil,  for deep
penetration. Then for final coat using linseed oil mixed with beeswax
for waterproofing.

Another option is making a lime based mortar. Not sure of an ideal
recipe but it might work. You might want to make test samples and
perform a torture test on these samples to see what seems to work best
for your situation and climate. For example,  if you live in a desert
cow dung might do the trick by itself.

Good luck in your endeavor.

Regards,
Will

>-----Original Message-----
>From:	vtrac [SMTP:vtrac at alternatives.com]
>Sent:	Thursday, February 26, 1998 3:41 AM
>To:	coblist; WFIRSTBR
>Subject:	Cob foundation cement
>
>Hello everyone,
>
>We are building a small cob meditation space in our backyard and are now in
>the process of building our stone foundation. We already have the rubble
>trench in and are now laying the large stones. Our wall is only about 12
>inches wide which makes it a little harder to dry stack.  We are
>considering using some type of mortar to jam into the stones but do not
>want to use cement. We are thinking that perhaps straight cob would work
>fine, but we are wondering what natural materials could be added to give
>the cob more strength, adhesion and waterproofing properties?  Has anyone
>drystacked a narrow wall (1 stone width in size)? How solid, safe is it?
>If anyone has any experience with this thanks
>
>Ian Marcuse
>
>