Plastic pressing is a forming process in which a plastic clay slug is pressed between
upper and lower molds, contained in metal rings. The molds are made of a porous material
such as gypsum, so that when a vacuum is drawn on the backs of the mold halves,
moisture is removed from the clay. The mold sections are then opened, using positive air
pressure to prevent sticking of the part in the mold. Plastic pressing achieves a higher
production rate than jiggering and is not limited to radially symmetric parts.