USG Plasters and Gypsum Cements for the Ceramic Industry - Manual

Plaster molds are a vital to producing many types of ceramicware. Along with a few other materials, plaster
is the basic modeling material for tooling in the ceramic industry. Consequently, a review of recommended
procedures and the relationship between plaster tooling and hand and mechanized clay fabrication may
prove helpful.
Following are definitions of common technical terms mentioned in this discussion.
Block Mold. Original mold made from a finished model. Used to make check casts before case molds are
fabricated.
Case Mold. Made from the block mold, it becomes the die for fabricating working molds.
Working Mold. Sometimes called a production mold, it can be one of three types: casting mold, jigger mold, or
press mold.
Consistency. The water-to-plaster ratio or number of parts of water, by weight, mixed with 100 parts of plaster,
by weight.
Alpha Hemihydrate. Gypsum calcined under pressure, producing a lower-consistency material with different
crystalline structure than ordinary plaster. It produces harder and stronger casts with limited absorptive power.
Alpha hemihydrate can be obtained in different formulations to control expansion, consistency and strength.
Also known as gypsum cement.
Beta Hemihydrate (also known as plaster of paris, kettle plaster, kettle stucco). Gypsum calcined in a kettle at
atmospheric pressure. The individual particles are highly irregular in shape, which causes high water demand to
make a workable slurry.
Snap-Set of Plaster Slurry. The condition when plaster sets before it is sufficiently soaked and completely mixed.
Slow Set of Plaster. Plaster unusually slow in setting.
Absorption: The absorptive property of plaster molds makes slip casting possible. The mold can be likened to a rigid sponge,
Key to Slip Casting drawing water from the slip, retaining it and gradually passing it through the mold to the back, where it evaporates.
Maintaining a desirable rate of casting depends upon establishing equilibrium between water absorption from
the slip and drying out of the mold. The factors governing this relationship are:
1. The water-to-plaster ratio (varying from 68 to 90 parts of water to 100 parts of plaster, by weight) governs the
initial absorptive power of the mold. In large, multi-piece sanitaryware molds, some parts of the mold require a
higher water-to-plaster ratio than others to give greater absorption to selected portions of the cast.
2. Homogeneity (freedom from pinholes and bubbles) governs to some extent the even capillary movement of water
through the plaster mold. Uniformity is a function of mold-making technique and attention to detail in the mixing
operation.
3. The rate of absorption over a broad range of water content in the mold governs the long-term casting quantities of
the mold. At what minimum moisture content is the high absorptive rate too great? At what maximum moisture
content is the low absorptive rate enough to do the job?
Best results generally are achieved when the driving power of a mold is “in balance” with the layer of slip that
has built up on its working face. The ability of a mold to transport water from the ware surface should have some
relation to the rate at which water reaches the interface between ware and mold.
If the mold absorbs water from the slip too quickly, the result is surface shrinkage of the body immediately next to
the mold. This produces a reduced rate of casting, surface cracks, case hardening or premature release of the
ware. If the mold removes water too slowly, the cast does not build up at a satisfactory rate, is likely to be soft or
“mucky,” and the ware tends to stick to the mold. Once a state of approximate equilibrium is reached, continuous
capillary columns exist from the slip-mold interface entirely through the mold to its exterior.
Plasters and
Gypsum Cements Product
Application
Plasters for the Ceramic Industry