Bentonite clay

[Eric D. Hart]

At 09:13 AM 10/14/96 -0500, Becky Myton wrote:

>Yesterday I was rereading the ingredients on my usual brand of kitty 
>litter, "Natural Choice" (prior to dumping in the box, wise-guys, I *do* 
>have better reading material at home!:) ). This stuff is made with 
>bentonite clay instead of concrete (which a cat-fancier publication told me 
>was in most clumping litters). There's usually a strawbale debate about 
>using concrete in plaster coats, with most opinion holding against because 
>of its unbreathing nature.
>
>In know bentonite clay can be used as a waterproof underlayment to living 
>roofs, but what would it do powdered in the finish coat of plaster? Would 
>there be any advantages, or would this just have all the problems of 
>concrete in another medium?
    I did some research to find a source of powdered clay for natural
plasters (instead of 'mining' your own) and found out that bentonite *was
not* the ideal material for plaster.  It has a high shrink/swell potential
so would soon crack your wall.  I was told that Hawthorne Fire clay is the
best (has the lowest shrink/swell potential of any powdered clay) and also
is the cheapest.  I got it from a pottery supply store in Minneapolis and it
was cheaper than a comparable sized bag of portland cement.  We are going to
try some on a test wall we are doing in the next month or so.  That won't be
outside so will not get the acid test.  I would guess that this clay would
have the same problems that other clay based plasters do, namely slow
erosion off the wall.  Hence you need to put a thin finish coat of plaster
on the wall every 3 or 4 years.  I have also heard that clay dust isn't too
good for you, so it might be advisable to seal at least interior plaster
that is clay based.  Might be advisable to do the exterior also (would also
make it wear better).  I don't know how 'breatheable' the wall would be with
this type of plaster.


Eric D. Hart			
Community Eco-design Network	  
Minneapolis, MN  USA		
(612) 305-2899 
erichart at mtn.org			
http://www.tc.umn.edu/nlhome/m037/kurtdand/cen