USG® No. 1 Pottery Plaster 50lb Bag
USG Pottery Plaster Mix For Wedging Tables:
For wedging boards or tables mix 70 parts of water to 100 parts USG pottery plaster. This equals 4.17 gallons water per 50 pounds dry pottery
50lb Bag U.S. Gypsum Pottery Plaster Product Data
– The best material available for sanitaryware and dinnerware casting.
– Formulated for long life and reduced breakage (stronger molds).
– Available with or without thermal shock additive.
POTTERY PLASTER is highly uniform, noted for outstanding performance
and long life. The standard of the industry - this is the best material
available for sanitaryware and dinnerware casting. Used for making
plaster bats, wedging boards, and working molds. The U.S. Gypsum Pottery Plaster is a relatively soft plaster which may be used for carving.
Consistency (parts of water by weight per 100 parts plaster )
1 Hr. Compressive Strength
1,000 psi 6.8 MN/m2
2,400 psi 16.3 MN/m2
Maximum Setting Expansion
69.0 lb/ft3 1.10 g/cm3
Time (Machine Mix)*
14-24 min. 14-24 min.
U.S. Gypsum Pottery Plaster General Directions and Guidelines
Preparing U.S. Gypsum Pottery Plaster: Use potable water at temperatures between 70° and 100° F (21° and 38° C). Since variations in slurry (the plaster and water mixture)
temperature produce variations in setting time, it is important to keep
both the plaster and water in a stable temperature environment prior to
use. The higher the temperature of the water, the shorter the set time.
Weigh both the pottery plaster and water for each mix. The water-to-plaster
ratio is critical because it governs both the strength and the
absorptive capacity of the mold.
Sift or strew the USG pottery plaster into water slowly and evenly. Do not drop
handfuls of plaster directly into the water. Allow soaking for 1-2
minutes. The plaster should be fully dispersed in the water prior to
mixing. Small batches require less soaking than large batches. See
bulletin IG503 for specific soaking instructions.
Mixing the USG pottery plaster slurry is one of the most important steps in producing plaster molds with maximum strength, absorption, hardness, and other important properties.
Mechanically mixed plasters develop uniform molds with optimal strengths. Plasters can be mechanically mixed through
both batch and continuous processes. Proper blade and bucket dimensions are important for obtaining the best batch mix
(see IG503 for details). Longer mixing times result in higher mold strength and shorter setting times.
To prevent air entrainment and provide a uniform, smooth surface, careful pouring of the USG pottery plaster slurry is necessary. Agitation of the
filled case mold is a further step used to prevent air at or near the
mold surface. Whenever possible, the plaster slurry should be poured
carefully in the deepest area so the slurry flows evenly across the
surface of the case mold.
Pouring a large amount of slurry directly on the face of the case mold may result in slight densification of the plaster mold
at the point where it strikes the surface of the case. This produces a hard spot, giving uneven absorption.
All pottery molds should be dried as quickly as is safely possible
after manufacture so that maximum physical properties can develop. Dry
to a constant weight.
The best drying rooms or ovens provide (1) uniform and rapid circulation (minimum of 15-30 fps) of air with no “dead spots”
having little or no air movement, (2) equal temperatures throughout the entire area, and (3) provisions for exhausting a portion
the air while replacing it with fresh air. High humidity surrounding
the drying room or oven inhibits the efficiency of the drying
because the air pulled into the room is incapable of picking up much moisture from the molds.
The maximum temperature at which plaster molds are safe from calcination is 120° F (49° C). With substantial free water in
the mold, higher drying temperatures can be used without difficulty. As drying progresses, the temperature must be reduced to
prevent calcination. The safe drying zone is in the shaded area of graph (below, right). Before removing molds from the dryer,
the temperature should approach that of the area around the dryer to prevent thermal shock.