[Cob] Re: Lime/clay plaster courses

[ocean]

Expansion/contraction is caused by having too little sand in your 
plaster.  If you make both lime and clay plasters sandy enough, you can 
minimize shrinkage.  Try the "crunch" test, where a ball of plaster in 
your hands makes a crunchy sound when squeezed.  Of course, nothing 
replaces the need to make several test panels, with different clay/sand 
and lime/sand ratio.  Plaster them onto a plywood scrap, with the joint 
between each pair, then speed bake them dry in a low heat over 
(200°F?).  You'll see where you have too little sand (cracking occurs) 
and where the plaster is too sandy (plaster crumbles).  Also, fiber 
ratios can help control cracking, though too little sand is the most 
likely culprit.

I wouldn't worry about water wicking up the lime course, as long as you 
don't put the plaster in direct contact with the soil or too close the 
splashing rain gutters, etc.  Lime plasters aren't as hydrophilic as 
cement, and they breathe so well they quickly evaporate any blown rain 
which they might absorb during a windy storm.  (1000 year old cottages 
in rainy, windy Wales have proven the test of time!)

Good luck, have fun, and please post pictures of your test patches on 
your website.  Be sure to report the results to the list; we want to 
know what you discover.  I just looked at your website ( 
http://www.blogas.lt/RB ), and you're building a beautiful house!

Ocean Liff-Anderson
http://www.intabas.com

ps:  If you do notice a different expansion coefficient between the two 
courses (as evidenced by a horizontal crack between clay and lime 
courses) then, why not build an artistic feature into this area, like a 
wavy horizontal groove?  This would serve as an expansion joint, like 
the ones commonplace in concrete sidewalks, slabs, etc.


On Sep 9, 2006, at 12:40 PM, RB wrote:

> Hello everyone,
> I have an idea to use different plasters on the same wall. Lime 
> plaster on
> the lower part of the wall where for shure it will receive rain. And 
> clay
> plaster on the upper part of the wall (protected by roof). Clay plaster
> seems more friendly for my hands and is better for breathing wall. But 
> I
> have some doubts about possible cracking between two different plasters
> (different expanding rate?) and about wetness travelling up the lime 
> plaster
> and destroing clay plaster (we have a lot of "rain during the day- 
> cold at
> night").
> The wall is more light clay then cob. The wall and the house under
> development can be seen at:
> www.blogas.lt/RB
>