[Cob] cracks in plaster

[Shannon Dealy]

On Tue, 16 Sep 2008, Bob Smolen wrote:

> Thanks Jill and Ron for your thoughtful comments.
> I reinspected the wall and there are only a couple of very minor cracks 
> in the last lime/sand only layer. I am pleased.  The earlier coatings 
> were probably too clay rich as you suggest. I will experiment with a 
> less clay mixture.  I mix the lime and sand and clay at the same time. 
> Is it necessary to soak the lime in water for days before mixing. My 
> last coating seem ok so I believe presoak is not necessary. Can you 
> comment on how you have done your plaster mix?

If you are using "Type S" hydrated lime (this is what is most common, 
though it is often called other things like "builders lime"), then a long 
(multi-day) soak is not needed if you do a reasonable job of mixing, but 
it can take an few hours for the water to completely soak in to the lime. 
If you don't let it soak, the little lumps of un-wetted lime will draw 
moisture from the rest of the plaster causing the mix to stiffen up too
quickly and unevenly (quick/uneven setting is what causes cracking, though 
fiber can help prevent it)

If you use "Type N" which is far less common, then it does need to soak 
for several days before use.

I long ago came to the realization that it was much simpler to keep 
buckets of pre-soaked lime (lime putty) on hand to eliminate this problem 
as well as others - a bag of lime just sitting will typically deteriorate 
over time (picking up moisture/carbon dioxide from the air) where a 
bucket of lime with enough water to keep the top of the lime covered 
and an air tight lid will basically keep forever.  So all of my plaster 
recipes are based on lime putty these days rather than dry lime.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
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