[Cob] Re: Screening clay for floors & plasters

[Amanda Peck]

Sounds good.  We stomped for our mixing, so sharp little rocks hurt, and 
it's really time consuming. probably especially with the wood shavings, even 
if it did give us an even mix.  A pool float worked well to give a nice 
smooth float finish.  We had a couple of cracks where we forgot to pound the 
mixture down into the gravel below.  I've repaired those, but not finished 
the design.  One of these years.

Those pictures are lovely.

And happy cob-mas to y'all too.

.............

Ocean wrote:
I've had no trouble making and applying volumes and volumes of plaster/floor 
mix (200 linear feet of wall, 600 sq. ft. of floor) with a very simple 
procedure:

1. Whip beautiful red clay in a 55 gallon drum with enough water to cover, 
resulting in a nice runny clay slip.  I use a 1/2" drill and drywall 
"beater" to whip the slip.

2. Pour the clay through a 1/4" mesh (hardware cloth), screeding a bit back 
& forth with a 2x4 block.

3. I don't do a second screening, just mix this slip with river sand, ratio 
is 3:1 (sand to clay) in an Imer electric cement mixer for plaster.  For 
floor finish I'd go with a higher ration - 5:1 or more (Ianto told me 7:1 
for his floors !!)  I don't worry about rocks in my slip, since the sand 
also has small rocks approaching 1/8-1/4".

4. I spray in enough water to the mixer to make sure the clay slip 
completely coats all the sand particles.  Then I add chopped straw which has 
been screened with a 1/2" mesh.  The straw absorbs enough of the water in 
the clay/sand to make a nice consistency.

5. The trick to getting a smooth finish is in the floating after the 
plaster/floor finish is applied.  "Floating" brings the fines up to the 
surface, and forces all the bigger stuff (rocks, chunks of unmixed clay, 
etc.) deeper down, resulting in a very beautiful smooth wall or floor.  On 
walls we float with stainless steel Japanese trowels, which I love!  For the 
floor a nice cement float works well.

Good luck, but don't worry about screening out those little sharp rocks.  
I've never had the patience for an 1/8" screen, though for very fine 
sculpture work, Kiko uses very fine sand and clay, plus grass-fed cow 
manure.  He has some beautiful bas relief sculptures on exhibit at Intabas, 
you can see pictures of his sculptures on our website:  
http://www.intabas.com