USG Casting Fireplace Surrounds - Manual (Page 19)

The Hand-Layup Process
Demolding When the gypsum cement part has completely set in the mold, it needs to be removed from the mold. Do not
remove the part until it is completely set. Even though it has set, the part is still far from its full strength. Carefully
remove the part without damaging it. Several methods can be used to help demold your parts:
– Gently peel the flexible mold away from the piece around the perimeter of the mold.
– Use wedges placed around the edge of the mold-part interface to separate the part from the mold.
– Suspend the mold and lightly tap around the mold perimeter so the weight of the mold or part helps the separation.
– Use compressed air in the back or side of the mold. This requires the installation of air-nozzles in the mold design,
but can be a good option, especially for large fireplace surrounds.
Consult your USG representative for more information about these techniques.
It is extremely important that you keep your parts fully supported after demolding, since parts that have not
been dried can sag, warp, or move, resulting in a misshaped part.
Cleaning and Trimming After the part is demolded, trim it to the final shape since it is easier to do when the part still contains some free
water. After drying, there is more dust and the edges will be more likely to chip.
After trimming and sanding, the part must be oven-dried to gain full strength and prepare it for painting or
other finishes. If the part is thin-shelled, which is possible with hand-layup, the drying time will be relatively short
compared to a solid-cast part or a part cast with a plug to reduce weight. If no plug is used in solid-casting, drying
times can be very long because of the thick mass of plaster. When the part is fully dry it can be painted, stained or
given a metallic finish or other surface decoration. If it is to be left as cast, the preparation of the slurry and mold
is very important. Undermixing, dirty equipment, dirty molds, or contamination anywhere may result in a visible
defect on the surface of the fireplace surround. Also, if the casting is to be left without further surface treatment,
it may absorb smoke, oil from the hands, and other potential contaminants which may result in an unacceptable
appearance after just a few years.
19 Casting Fireplace Surrounds