One other thing about drying clay...
slab built pieces should take longer to dry to avoid warping for tiles,
and to avoid corners pulling apart due to uneven drying in box type
structures. I have shelves in my kiln room where I dry my slab pieces.
I use sheets of that rigid white plastic gridded stuff that I think is
used to cover overhead light fixtures. It's good to dry on because air
can get to the slabs from both sides. I prop them up with kiln stilts
in the corners and the middle so they don't sag. At first I cover them
with a plastic bag with some holes in it to let it START to dry out,
then I remove the plastic covering and cover the front of the shelves
with plastic, leaving open at the bottom. This keeps the atmosphere
behind the plastic humid as the slabs dry, and the open gridded plastic
shelving (from home depot) assures that the top and bottom dry evenly.
Another factor to consider when working with slabs....clay has a
"memory". If you take cautions to keep your slab flat from the time you
roll it out, til you finish it, it has a better likelihood of not
warping. If you can design a way to flip your slabs between two boards
if you need to turn it over, that will help keep finger dents out of it,
as well as keeping it flat. When you roll out your slab, slide a board
under the canvas or paper that you roll it out on, put another board
over the top and flip it...canvas, clay and boards, all together. Once
it is flipped, you can remove the paper or canvas just by pulling it
off. If you don't want the canvas texture, use a flat rib or a squeegee
with a little water pulled over the clay to remove the texture and make
it all smooth. A window squeegee provides a wider "rib" to smooth large
pieces evenly if texture removal is desired. ANOTHER TIP....If you want
to smooth your clay while working with it, you can use a flexible metal
rib with little teeth on the curved side (available at all ceramics
stores) to "low the field", or actually rough up the area you want to
smooth. The ribs will ensure that all of the clay is evenly "plowed"
depth-wise. Then you turn the rib over to the flat edge and lay it down
at an angle, pulling it across the "plowed field" yielding totally
smoothed over clay...no finger dents, all level. Neat trick to know
about. What you think you don't want to do (rough it up more) is
actually what you must do to even out the peaks and valleys, before you
can drag a squeegee of whatever flat ribbed surface you want over it.
Works like a charm.