United States Gypsum

Quick set or flash set of plaster slurry

Definition: Plaster sets before sufficiently soaked and completely mixed.

Cause A: Contaminated plaster.

Cure A: Determine if plaster became contaminated after removal from bag. Do this by mixing plaster from the storage bin in one mix, then under identical conditions use plaster directly from the bag for a second mix. Any lumps of set-up plaster in bags or bin should be removed. If both mixes set too fast and do not appear contaminated, any of the following causes may be present:

Cause B: Dirty mixing equipment.

Cure B: Clean mixing equipment to remove set plaster or other contamination. Dirty equipment shortens setting time.

Cause C: Impure mixing water.

Cure C: If excessive alum, chlorine or sulfate are present in the water, plaster mix will set too quickly. Check set by making a mix with distilled water. Plaster should always be mixed with water fit to drink.

Slow set of plaster

Definition: Plaster unusually slow in setting.

Cause: Contamination.

Cure: Confirm that plaster has not become contaminated by sugar, glue or organic acids. Then accelerate set by: mixing faster, mixing longer, using warmer water or plaster to increase temperature of slurry, or all of the foregoing in combination.

Lumpy Plaster

Cause A: Plaster has absorbed moisture.

Cure A: Equip plaster bin with tight-fitting lid to prevent absorption of moisture from the air. Keep bin closed so water cannot splash into bin.

Cause B: Plaster has become wet en route to your plant.

Cure B: Bags showing evidence of water should be examined before carrier is relieved or responsibility. Note: Paper bags are usually wrinkled where wet, and lumps have a smooth side next to paper. Carrier inspection should be initiated as the carrier is responsible for this damage.

Cause C: Plaster has become wet in storage.

Cure C: Store plaster in warm, dry, ventilated place; keep off damp floor.

Cause D: Packing lumps in bags of plaster.

Cure D: Do not store plaster in stacks higher than ten bags. Always rotate stocks, using older plaster first. Packing lumps usually are the result of lengthy storage. Never use lumpy plaster; it does not make good casts.

Soft Casts

Definition: Soft casts after drying

Cause A: Consistency is too high (mix is too thin).

Cure A: For sufficiently hard casts, mix plaster at recommended ratio. See page on Drying Plaster Casts.

Cause B: Casts have been "burnt" or calcined in the dryer.

Cure B: Regulate temperature of dryer at about 120ºF. and remove casts as soon as they are dry. If exposed too long, completely dry casts calcine and become chalky on the surface at temperatures in excess of 130ºF.

Pinholes in Casts

Definition: Molds develop small holes - 1/16 to 1/32 in. dia. - in surfaces after being used for only a short time.

Cause: Improper mixing and soaking of plaster.

Cure: Use a good plaster. Sift plaster into water evenly. Do not allow large quantities to fall into water at one time. Let plaster soak completely so all particles are wetted. Thoroughly mix with a mechanical mixer with the propeller adjusted so that air is not whipped in. Mixer should be high speed (approx. 1,750 rpm). Air entrainment, a widespread defect, should be constantly guarded against.

Rough surface on cast

Definition: Cast feels like sandpaper.

Cause: Large particles in plaster slurry settle out against face of case mold.

Cure: Mix until plaster "creams." Slurry is then sufficiently thick to prevent settling.

Cracked Casts

Definition: Large and/or hairline cracks in casts.

Cause A: Abusive handling.

Cure A: Handle casts properly. Wet casts have about one-half their ultimate dry strength.

Cause B: Thermal shock.

Cure B: Do not expose hot casts to cold air drafts.

Cause C: Shrinkage cracks.

Cure C: Dry at moderate temperatures. Calcination due to drying at extreme temperatures causes shrinking and cracking.

Mildew on Casting

Definition: Black, green or reddish-brown shpts of varying size.

Cause A: Cast not dried properly.

Cure A: Remove spots by washing with water or dilute solution of bleach or fungicide. Oven dry casts thoroughly and, if possible, reduce humidity of air in contact with cast.

Trapped air in cast surfaces

Definition: Voids in cast surfaces

Cause: Trapped air in mold faces due to improper pouring.

Cure: pour plaster while still fluid. Start pouring plaster into the deepest portion of the cast. Agitate cast so trapped air rises and fluid plaster fills all cavities of the cast mold.

Paint peeling

Definition: Paint will not stick or it blisters and/or peels off.

Cause: Plaster cast has not been properly dried.

Cure: Dry casts thoroughly before painting.

Caution:

These products may develop sufficient heat to cause burns if allowed to set in contact with skin. May be irritating to eyes. Wear protective clothing and wash after exposure. Avoid breathing dust or prolonged contact with skin and eyes

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