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Drying Plaster Casts

For uniform results and optimum physical properties, plaster casts must be properly dried, done by transferring excess water in the cast to the surrounding air. For complete hydration in the setting process, plasters require about 18.6 parts water per 100 parts plaster. To obtain a mixable slurry, larger amounts of water must be used in mixing. After the plaster has set, any water above 18.6 parts is considered excess or "free" and must be removed from the cast by drying. Drying equipment can be designed to remove excess water in a specified time and at a predetermined cost.

To evaporate "free" water from casts requires an energy source. For each pound of water evaporated, slightly more than 1,000 British thermal units (Btu) are required. A Btu is the amount of energy required to raise the temperature of 1 lb. of water 1ºF. (One lb. of water is about 1 pint.) As a comparison to show amount of heat required, the Btu output of a typical home furnace will range from 40,000 Btu per hr. in Southern to 100,000 Btu in Northern climates.

The same drying action takes place whether the plaster cast dries in a workroom, outdoors or in a dryer. Use of a forced-hot-air dryer speeds and controls the drying procedure.

As the cast sets or hardens, a chemical reaction causes the piece to heat slightly so that excess water begins evaporating from the cast.

When a new, wet cast is placed in a forced-hot-air dryer, rapid evaporation begins. Initial evaporation keeps the cast cooler than 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 The surface temperature rises, 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 where, as vapor, 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.

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 120ºF. for: USG White Art Plaster, No. 1 Casting Plaster, Moulding Plaster, HYDROCAL White, Statuary HYDROCAL and HYDRO-STONE Gypsum Cements; from 125 to 130ºF. for Industrial Plasters PC and PC-1. Operating above these temperatures results in surface calcination; that is, surfaces of the casts, especially those in front of hot-air ducts, become soft and powdery.

In the table below, air volumes through the heater were calculated so that hot air entering is about 30ºF. higher than the temperature in the dryer. This gives an adequate safety margin.

Plaster Dryer Design Data (assumes 16-hr. cycle for Art and Moulding plasters)
Plaster Use tons/day	(A) Burner Capacity Btu/hr.	(B) Minimum Exhausted Air & Makeup Air² Stand. cu. ft./min.	(C) Recirculation of Humid Air³ Stand. cu. ft./min	(D) Total Air (Makeup plus Recirculated) Through Heater³ Stand. cu. ft./min.
5	600,000	1,850	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 120ºF.
(3) Recirculated and makeup air assumed to enter drying chamber at max. 150ºF.

Drying studies on 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.

	Air must flow around each cast. Casts that are crowded together, dry very slowly. They should be placed on racks and separated by runners so that water vapor is able to escape.
Air-flow pattern for proper circulation in drying chamber.

To cover important points in the use of drying equipment, review this checklist of questions.

In using the table from above, assume that the maximum amount of plaster used per day will be 20 to 100 bags, then design a drying room adequate from this volume. The table indicates how much air must pass through the heater to supply the required heat energy while keeping 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 flow increases drying time. The volume of air exhausted must be replaced by fresh air introduced elsewhere in the system. Probably the best place to bleed fresh, cool air into the recirculation system is just before the heater. Since the total volume of air through the heater (recirculated air plus fresh makeup air) is large, the air needs to be heated only a few degrees to supply the necessary energy to dry gypsum casts effectively. Even with this procedure, additional fans within the dryer are required for adequate air velocity of 15 fps.

By using a humidity control as well as temperature control, drying rates can be increased. Graph to the right 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 surface calcination of the plaster cast. The closer the control can be maintained to the line, the more rapid the rate of drying.

For example, 140ºF. with 40% R.H. will result in faster drying than 140ºF. with 50% R.H. If R.H. is 50% temperature should be increased to 150ºF.

Minimum relative humidity at temperature to prevent calcination of gypsum casts.

Minimum relative humidity at temperature to prevent calcination of gypsum casts.

Stored plaster should be kept in a warm, dry place away from damp, cold floors and walls. Stacks should not be higher than ten bags so as to avoid packing lumps, as well as to improve safety. Air should circulate around each pallet.

Obviously, leaky roofs cannot be tolerated. The storage area should be clean. Broken bags should be neatly tied or put in slipover bags. Failure to follow good practice is likely to result in set-up chips or lumps that can accelerate set, cause hard spots, produce low-quality casts and increase waste.

Often plaster is stored in a "cul-de-sac" - a square or rectangular area with access from one side only. These conditions make rotation impossible because new loads are necessarily placed on top of, and in front of earlier shipments. Eventually, it is necessary to clean out the whole area. Then it is not unusual to work with material from five or more shipments - with resulting variations in plaster performance and properties of casts.

Normally plaster should be stored no longer than 90 days before use. A code showing date manufactured is stamped on each bag to help in rotating stock and regulating usage.

Plaster Master Supply
4308 Shankweiler Rd. Orefield, PA 18069
Phone (610) 391-9277 · Fax (610) 391-0340