slip casting is used in powder metallurgy, but its application in ceramics
shaping is much more common. In slip casting, a suspension of ceramic powders in water,
called a slip, is poured into a porous plaster of paris (CaSO4-2H20) mold so that
water from the mix is gradually absorbed into the plaster to form a firm layer of clay
at the mold surface. The composition of the slip is typically 25% to 40% water, the remainder
being clay often mixed with other ingredients. It must be sufficiently fluid to
flow into the crevices of the mold cavity, yet lower water content is desirable for faster
production rates. Slip casting has two principal variations: drain casting and solid casting.
In drain casting, which is the traditional process, the mold is inverted to drain excess
slip after the semi-solid layer has been formed, thus leaving a hollow part in the
mold; the mold is then opened and the part removed. The sequence, which is very similar
to slush casting of metals, is illustrated in Figure 17.5. It is used to make tea pots,