I didn't use anything in the bowls-we just pressed them into the
styrofoam. Yes, you can see an impression on the under side of the bowls
that matches the styrofoam bowls!- but the clay didn't stick to the bowls-I
took them out the next day. I did have a potter-in -residence one year who
did a really cool coil (hard to say) project with the kids using a Kool whip
bowl and pressing coils all around the sides and later adding a slab bottom.
They made banks or pencil holders and he had them put damp paper towels in
the bowls to avoid sticking.
Jeryl in SC