Plaster is great to wedge clay on as long as you don't get any loose
plaster imbedded in your clay. Plaster immbedded in clay causes blowouts
of the clay in the kiln. It's easy to make. You need a wooden board on
the bottom, or a table top that you can nail a wooden frame into. You
should use boards as deep as you want to pour the plaster...but a minimum
of two inches deep. When you are preparing the boards and the base, be
sure to place a damp coil snugly into the crack where the boards join the
base, thus eliminating leakage when you pour the wet plaster. When you
mix the plaster, start with a bucket of water (about a gallon at a time).
Sift the plaster into the water forming a mountain peak above the water.
Let the water seep into the plaster. When your mountain of plaster has
peaked above the top of the water, begin stirring. Doing it this way
helps to eliminate air bubbles. You will also want to hit the side of
your bucket periodically with something heavy to make the air bubbles that
might be trapped in your plaster rise to the top. You want the plaster to
be like THICK cream when you are ready to pour. The mountain peak above
the water line usually gives you a pretty good indication that the ratio
of water/plaster will be right. If you need to add more plaster to the
wooden frame after pouring your first bucket, you can always do that. It
would be good to scratch the surface (gouge, even) to let the new plaster
sink into some grooves to give a better bond when added to the first
batch. I have always used plaster to dry my clay out...works pretty fast.
But don't let it get soaked, and don't use a spatula to remove the clay,
unless if you use a rubber one. Better to just wash off dried on clay
with a sponge so as to avoid loosening plaster bits that could get worked
into your clay and surprise you later when you open the kiln to see your
beautiful piece has exploded! I like wedging on a plaster surface, too.
Hope this helps.