USG Drying Plaster Casts - Manual

To attain uniform results and optimum physical properties, plaster casts must be properly dried. This involves
transferring excess water from the cast to the surrounding air.
Plasters require about 18 parts water per 100 parts plaster by weight for complete hydration in the setting
process. However, in order to obtain a mixable slurry, larger amounts of water must be used in mixing. After the
plaster has been mixed, poured, and has set, any water above the 18 parts is considered excess or “free” water
and must be removed from the cast by drying. (See figure 1.)
Drying equipment can be designed to remove this excess water in a specified time and at a predetermined cost.
Advantages of controlled drying include:
1. Proper strength development
2. Uniform absorption
3. Increased production
4. Mildew prevention
5. Better paintability
Fig. 1—Effect of dryness on compressive strength of Gypsum. Note that strength
increases only slightly until 93% of the free moisture has been removed
How Does a Plaster Cast Dry? To evaporate this “free” water from the casts requires an energy source. For each pound of water evaporated,
slightly more than 1,000 British thermal units (Btu) are required. As a comparison to show the amount of heat
required, the BTU output of a typical home furnace will range from 40,000 Btu per hr. in the South to 100,000 Btu
per hr. in colder Northern climates.
Note: A Btu is the amount of energy required to raise the temperature of one pound of water 1 °F. One pound of
water is about one pint.
The same drying action takes place whether the plaster cast dries in the workroom, outdoors, or in a dryer. Use
of a forced-hot-air dryer speeds and controls the drying procedure. Plaster casts will rarely become 100% dry
without the use of a forced air dryer.
As the cast sets or hardens, a chemical reaction causes the piece to heat slightly. Then, because there is more
water than required for the chemical reaction, this excess water begins evaporating from the cast. When a new,
wet cast is placed in a forced-hot-air dryer, rapid evaporation begins. This initial evaporation keeps the cast cooler
than the air temperature in the dryer. Water from the interior of the cast moves to the surface to replace evaporat-
ing moisture. As evaporation continues, sufficient water does not move to the surface to keep it cool and the sur-
face temperature will rise although the center of the cast is still moist. As the amount of evaporation is reduced, the
cast’s surface temperature approaches the air temperature in the dryer. Once the surface of the cast is up to air
temperature, the rest of the free water in the cast evaporates slowly, coming to the surface as vapor where it is
swept away. As this occurs, the entire cast warms to approximate ambient air temperature further and further
toward its center. When the center of the piece reaches the temperature of the surrounding air, the drying process
is complete.
Drying Plaster Casts A Gypsum Cement for
Every Industrial Use
0
Percent Excess Water Evaporated
Compressive Strength
25 50 75 100