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 evaporating moisture. As evaporation continues,sufficient water does not move to the surface to keep it cool and the surface 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
What is Best Dryer Temperature? The main physical limitation in drying a plaster cast is the maximum temperature at which the dryer can operate
and not calcine the cast. Recommended temperatures are 110 to 120 °F for USG
®
White Art Plaster, No. 1 Casting
Plaster,Moulding Plaster,Pottery Plaster,H
YDROCAL
®
Brand White and HYDRO-STONE®Gypsum Cements; from 125
to 130 °F for Industrial Plaster PC. Operating much above these temperatures will result in surface calcination;
that is, surfaces of the casts, especially those in front of hot-air ducts,will become soft and powdery.
In the chart on the next page air volumes through the heater were calculated so that hot air entering would be
about 30 °F higher than the temperature in the dryer.This gives an adequate safety margin.
Air Circulation Within the Dryer Drying studies of various materials show that increasing air speed over plaster casts reduces drying time. An air
speed of 15 fps (ft. per sec.) is recommended, and speeds up to about 30 fps are desirable.At speeds over 30 fps,
cost of moving the air can be greater than the value of time saved.
Here is an example of a drying problem in a 3,200-cu. ft. dryer. It was loaded daily with about 4.2 tons of casts
containing some 1.2 tons of water to be evaporated. Although the furnace supplied enough heat to evaporate the
water,and exhaust air volume was adequate to carry out the water vapor,casts were not drying. The solution was
to improve circulation in the drying chamber to 15 fps by adding four 20-in.-dia. supplementary fans. This provided a temperature differential of 2 or 3 °F throughout the dryer,indicating a uniformity of air flow.
Air must flow around each cast.(See figure 2.) If the casts are crowded together,they dry very slowly. They
should be placed on racks or separated by runners so that water vapor is able to escape.
Following is a summary of important points in designing and using equipment:
1. Is the burner large enough to supply necessary heat energy? (Use chart on next page for size.)
2. Is sufficient humid air being exhausted from the dryer’s system?
3. Is the outlet of the exhaust duct well away from the fresh air intake?
4. Is hot air entering the dryer chamber at 150 °F or below? That is the maximum for USG White Art, USG No. 1
Casting, USG Moulding, USG Pottery Plasters,H
YDROCAL Brand White or HYDRO-STONE Gypsum cements.Use 160
°F or below for Industrial Plaster PC.
5. Is air circulation within the dryer fast enough? Are fans within the dryer designed to operate at dryer’s tempera-
ture?
6. Are applicable building and safety codes being met?
7. Can the flame or heating element be seen from inside the drying chamber? They should not be visible.
8. Has production increased since the dryer was designed? If so, larger equipment may be needed.
9. Are spaces between the plaster casts great enough to allow adequate air circulation? (Approx. 1/2 in. on small
casts; 2 in. on casts of 100 lbs. or more.) Consider contacting a heating and ventilating engineer to check sys-
tem for proper air flow and temperature.
Fig. 2—Air-flow pattern for proper circulation in drying chamber.
Designing the Dryer In using the chart shown here, assume that maximum amount of plaster used per day will be 20 to 100 bags; then
design the drying room so that it is adequate for this volume. The chart will indicate how much air must pass
through the heater to supply the required heat energy while keeping the temperature of entering air at a safe level.
The column “Minimum Exhausted Air (B)”indicates how much moist air must be expelled to dry casts in one
day.More moist air can be exhausted if desired, but less than recommended would increase drying time. The volume of air exhausted must be replaced by fresh air introduced elsewhere in the system. Probably the best point to
bleed fresh, cool air into the recirculation system is just prior to the heater. Since total volume of air through the
heater (recirculated air plus fresh makeup air) is large, it needs to be heated only a few degrees to supply the necessary energy to dry the plaster casts effectively. Even with this procedure,additional fans within the dryer will be
required for adequate air velocity of 15 fps.
Plaster Dryer Design Data (assumes 16-hr.cycle for Art & Moulding Plasters)
(A) (B) (C) (D)
Minimum Exhausted Recirculation Total Air
Air & Makeup Air
(2)
of Humid Air
(3)
(Makeup plus recirculated)
Plaster Use Burner Capacity
(1)
Standard Standard Through Heater
(3)
tons/day Btu/hr. cu. ft./min. cu. ft./min. Standard cu. ft./min.
5 600,000 1,800 22,000 23,800
4 480,000 1,450 17,600 19,050
3 360,000 1,100 13,200 14,300
2 250,000 800 8,800 9,600
1 125,000 400 4,400 4,800
(1) Based on a direct-fired heater. If an indirect-fired furnace is used, the stated figure must be bonnet or plenum capacity (usually 80% of the
burner capacity).
(2) Exhaust and dryer interior assumed at 120 °F.
(3) Recirculated and makeup air assumed to enter drying chamber at max. 150 °F.
Faster Drying By using a humidity control as well as temperature control, drying ratio can be increased. Fig.3 shows a curve
relating relative humidity (R.H.) and air temperature. As long as these two measurements give a point to the right
of this curve, drying will proceed without excessive burning of the plaster cast. The closer the control can be maintained to the line, the more rapid the rate of drying.
For example, 140 °F with 40% R.H. will result in faster drying than 140 °F with 50% R.H.If R.H. is 50%, temperature should be increased to 150 °F.
Fig. 3—Minimum relative humidity at temperature to prevent calcination of gypsum casts.
160
150
air temperature – ˚F
140
130
0
10 20 30 40 60 70
percent relative humidity
SAFE ZONE
50
Warning
When mixed with water,this material
hardens and then slowly becomes hot.
DO NOT attempt to make a cast enclosing
any part of the body using this material.
Failure to follow these instructions can
cause severe burns that may require surgical removal of affected tissue. Avoid
dust. Dust may cause irritation to the
eyes, skin, nose,throat, or upper respiratory system. Wear eye and respiratory
protection to avoid irritation. Product
safety information:(800) 507-8899.
Trademarks
The following trademarks used herein are
owned by United States Gypsum
Company or a related company: H
YDROCAL
,
H
YDRO-STONE
, and USG.
125 South Franklin Street
P.O. Box 806278
Chicago, IL 60680-4124
A subsidiary of USG Corporation
800 USG.4YOU (874-4968)
http://www.usg.com
email: industrial gypsum@usg.com
© 2000, United States Gypsum Company
IG502/rev.7-00
Printed in U.S.A.
United States Gypsum Company
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