Cob Do I really need sand in my clay?

[Will Firstbrook WCB of BC]

Hi Sam,

I attended a cob workshop with Ianto Evans a few weeks ago and in our
discussions about cob he stated that the clay is the mortar which always
shrinks. Too much clay can cause cracks to develop. Also the sand should
be angular, not smooth, so that the sand can interlock when the clay
dries resisting cracking. The amount of water in the mix also is
important as too much causes air spaces that can lead to cracking. The
fiber in the mix is also very important for tensile strength as well as
regulating moisture of the cob. All of this info relates to cob but I
suspect it is also important to earthen plasters as well.

I talked to Greg (not sure of his last name) who is helping Sun Ray
build one of his new creations. They had Carol Cruse there last week for
a plastering workshop, and she recommended to use white flower paste in
the plasters to use instead of the often used horse or cow dung in the
rainy North West (Washington & BC coastal areas). Adding lime putty to
your earthen plaster could possibly help as it expands and contracts
less than clay when it gets wet/dries which makes it less likely to
crack or peel. Lime also is self healing and increases in strength with
time by the process of recarbonation (absorbing CO2 from the air).

Regards, 
Will


>-----Original Message-----
>From:	FROG at nbs.gov [SMTP:FROG at nbs.gov]
>Sent:	Monday, July 28, 1997 11:15 AM
>To:	coblist at deatech.com
>Subject:	Cob Do I really need sand in my clay?
>
>Hi Folks:
>
>I have done some experimenting with cob/earth plaster mixes and it seems to
>me 
>that there is very little correspondence in how many cracks develop and the 
>ratio of sand to clay.  Does anyone have any reason why I can't therefore
>just 
>dispense with the sand?
>
>Thanks
>
>sam
>
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> Sam Droege  FROG at NBS.GOV                      
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>PWRC, 12100 Beech Forest Dr., Laurel, Md 20708-4038
>Http://www.im.nbs.gov
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