Found this:
http://listserv.repp.org/pipermail/strawbale_listserv.repp.org/2007-March/000699.html "Basically it's a paint rather than a plaster and if you translate the
word correctly it simply means "smooth."
So a smooth clay paint is what you're after. Depends upon the soil
how you mix it. If you go to a ceramic supplier and buy bagged
powdered clay you will typically need to mix it with some type of
filler. In the SW a lot of people have used mica flakes, but could
be anything like marble powder, whiting, depends what you have. Add
silica sand if you want some texture. Usually this works somewhere
around 1 part clay to 1/2 part filler. If you simply window screen
some type of soil most likely you want need any filler. From there
you usually benefit with some type of binder. Depends upon what
you're going over. If it is a clay plastered wall you might get away
with just the clay. If something else then most likely you will need
to add something. Keeps it from dusting and falling off the wall.
Popular one for a lot of people is some kind of starch paste as in
wheat. Easy recipe is mix 1 part white flour with 2 parts
water,
boil 1 1/2 parts water and then add the 1 to 2 mix. When it thickens
dilute it with 2 parts water to 1 part paste. Then add your dry
ingredients to the point that it resembles a thick latex. You want
to brush it on thick so that it covers, going over drywall and smooth
surfaces will take more coats (typically.) Once it starts to set up
take a tile sponge or something similar (damp) and work out the brush
marks gently. If you have mica in the mix it will help bring it
out. Other glues will work by the way, you would have to
experiment. This oughta get you started."