USG No. 1 Casting PlasterProduct Data
— Industry standard for figurines, plaques and lamp bases.
— Hard working surface with reduced paint absorption.
#1 CASTING PLASTER is similar to white art plaster, except mixed at a lower use consistency to increase strength, chip-resistant, and to further minimize paint absorption.
Technical Properties English Metric
Use Consistency (parts of water by weight per 100 parts casting plaster) 65 65
1 Hr. Compressive Strength 1,200 psi 8.27 MN/m2
Dry Compressive Strength 2,400 psi 16.55 MN/m2
Maximum Setting Expansion 0.210% 0.210%
Density Wet 100.0 lb/ft3 1.60 g/cm3
Dry 72.5 lb/ft3 1.16 g/cm3
Set Time* 27-37 min. 27-37 min.
General Directions and Guidelines
Preparing the Mix Use potable water at temperatures between 70 and 100 °F (21 and 38 °C). Since variations in slurry (the casting 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.
Measuring Weigh both the casting plaster and water for each mix. The water-to-plaster ratio is critical because it governs the strength
and the density of the final cast.
Soaking Sift or strew the 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 Mixing the plaster slurry is one of the most important steps in producing plaster casts with maximum strength,
hardness, and other important properties. Mechanically mixed plasters develop uniform casts 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 cast strength and shorter setting times.
Pouring To prevent air entrainment and provide a uniform, smooth surface, careful pouring of the slurry is necessary. Agitation of the filled mold is a further step used to prevent air at or near the surface of the cast piece. Whenever possible, the slurry should be poured carefully in the deepest area so the slurry flows evenly across the surface of the mold.
Pouring a large amount of slurry directly on the face of the mold may result in slight densification of the cast at the point
where the slurry strikes the surface of the mold.
Drying All plaster casts 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
of 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 casts are safe from calcination is 120 °F (49 °C).With substantial free water in the
cast piece, 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 casts from the dryer, the temperature should approach that of the area around the dryer to prevent thermal shock. See IG502 for more details on proper drying.