USG Casting Fireplace Surrounds - Manual (Page 20)

Drying To achieve uniform results and optimal physical properties, all gypsum cement castings must be properly dried.
This is probably the most important step in the entire process of fabricating fireplace surrounds. Make sure parts
are completely supported after demolding so that the shape does not change. Proper drying will allow for:
– Maximum strength development
– Uniform absorption
– Increased production capabilities
– Reduced efflorescence (soluble salts that migrate to the surface of the part and cause chalky spots on the
part – see photo below)
– Mold and mildew prevention
– Better paintability/finishing
Efflorescence on the Surface of a Mantel
Gypsum cements require approximately 19 lbs. of water per 100 lbs. of powder by weight for complete hydration
in the setting process. However, additional amounts of water are needed to obtain a mixable slurry. After the
material is mixed, cast, and has set, any water in excess of the 19 parts needed is considered excess or “free” water
and must be removed by drying. In addition, moisture left in the part may lead to the growth of mold or mildew.
Production
Process Flow
20 Casting Fireplace Surrounds