[Cob] crack in plaster

[Mahavir Mahedu]

I read a very interesting anecdote about the famed natural material architect "Laurie Baker". 

While in Himlayas, he came across a village where the people mixed a mystery liquid in their plaster mix and the result was crack free plaster.

The people did not reveal the secret liquid even after coaxing.

Later, Laurie Baker found out that the mystery liquid was "Pig Urine" !

Upon testing various other urines (human, cow etc.) they found that "Pig Urine" had the highest content of Urea.

Urea is known to be a binder of clay. 

How about testing addition of urea to the plaster mix ?

Mahavir.



----- Original Message ----
From: Bob Smolen <boka at mwt.net>
To: coblist at deatech.com
Sent: Tuesday, 16 September, 2008 11:51:09 AM
Subject: [Cob] crack in plaster

I have a small section of cob wall that I have covered with 2 layers of a coating of 3 parts sand , 1 part clay, and 1 part lime. I spread this on in a fairly dry mix and let dry for a week during a hot , dry spell 80+ temps. I sprinkled the wall with water each day but the plaster developed many thin cracks (spider web like). These cracks formed almost as soon as the wall dryed,in a day or so. Nothing fell off and the coatings seem to be adhering well.  Sunday I plastered with a mix of 3 parts sand to 1 part lime. The coating was not real thick, say 1/8 in. in most places. This ccoating has developed a few spider cracks. Should I be concerned or is this normal? Will an all lime plaster be a good final coat?
The sand I have is fairly coarse but I have screened it down to 1/8 size. 
I forgot to mention that I had straw/dry grass mixed in the first coats but nothing in the last 3/1 mix.
Thanks for your comments,
Bob
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