CALCIUM SULPHATE (#1M) PLASTER

Mix Preparation
Use potable water at temperatures between 70 °F (21 °C) and 100 °F (38 °C). Because variations in slurry (USG No. 1 Moulding Plaster and water mixture) temperature produce variations in set time, it is important to keep both the USG No. 1 Moulding Plaster and water in a stable temperature environment prior to use. The higher the temperature of the slurry, the shorter the set time. Conversely, the lower the temperature of the slurry, the longer the set time.

Weigh both the USG No. 1 Moulding Plaster and the water prior to use for each mix. The water-to- USG No. 1 Moulding Plaster ratio is critical because it governs the strength and the density of the final cast.

Soaking
Sift or strew USG No. 1 Moulding Plaster into the water slowly and evenly. Do not drop large amounts of USG No. 1 Moulding Plaster directly into the water as proper soaking of the USG No. Moulding Plaster may not occur. USG No. 1 Moulding Plaster should be fully dispersed in the water prior to mixing. Small batches require less soaking time than large batches. See USG IG503 Plaster Mixing Procedures for specific soaking instructions.

Mixing
Mixing USG No. 1 Moulding Plaster slurry is one of the most important steps in producing USG No. 1 Moulding Plaster casts with maximum strength, absorption, hardness and other important properties. Mechanically mixed slurries develop uniform casts with optimal strengths. USG No. 1 Moulding Plaster can be mechanically mixed through both batch and continuous processes. Proper blade and bucket dimensions are important for obtaining the best batch mix (see USG IG503 Plaster Mixing Procedures for details). Longer mixing times result in higher mold strength and shorter set times.

To prevent air entrainment and provide a uniform, smooth surface, careful pouring of USG No. 1 Moulding Plaster slurry is necessary. Agitation/vibration of the filled mold is a further step used to prevent air at or near the mold surface. Whenever possible, USG No. 1 Moulding Plaster slurry should be poured carefully in the deepest area so that 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 USG No. 1 Moulding Plaster mold at the point where it strikes the surface of the case. This produces a hard spot, giving uneven absorption.

All 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 (4.6-9.1 mps)) 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 drying efficiency because the air pulled into the room is incapable of picking up much moisture from the molds. The maximum temperature at which USG No. 1 Moulding Plaster molds are safe from calcination is 120 °F (49 °C). With substantial free water in the mold, a higher drying temperature can be used without difficulty. As drying progresses, the temperature must be reduced to prevent calcination. Before removing molds from the dryer, the temperature should approach that of the area around the dryer to prevent thermal shock.

When properly used, USG No. 1 Moulding Plaster is easy to work with and complies with the federal Labeling of Hazardous Art Materials Act, 12 U.S.C. Section 1277 and ASTM D4236. Keep indoors at temperatures between 65 °F - 75 °F (18 °C - 24 °C) and 45% - 55% RH. Do not stack more than two pallets high. Keep from drafts. Rotate stock. USG No. 1 Moulding Plaster should be used within 6 months of the manufacturing date located on the package. Always follow handling and use directions and safety warnings on the package.

36 months after the date of manufacture in original unopened packaging, stored in a cool and dry place and away from sunlight.