USG Casting Fireplace Surrounds - Manual (Page 11)

Design includes the physical appearance of the fireplace surround
and the mold configuration. Consider the following items when
deciding on the mold design:
– Amount of time required to cast the surround
– Fabrication method (spray, hand-layup, solid-cast)
– Amount of material used per unit
– Type of gypsum cement used
– Depth and configuration of mold and resulting effects from
heat exposure
– Drying efficiency
– Finishing options
– Installation method and procedures
The primary design consideration that affects all of the above is the cross-sectional depth of the mold design. In
other words, how far will the mantel and legs protrude from the wall? A deep and narrow backside mold opening
restricts access for both spray guns and hand-layup methods. Solid-casting a deep and narrow opening will pro-
duce a heavy piece requiring a large amount of gypsum cement material. In turn, the large amount of material
increases drying time, complicates the material’s thermal response, and complicates the installation process.
In general, heavy solid-cast surrounds are undesirable because dissimilar thermal movement of the cast piece
often results in cracking. Also, the weight of a large solid-cast surround can pose life-safety issues during and after
installation. Hollow surrounds are lighter in weight than solid-cast pieces and produce more evenly distributed
wall thickness, thereby producing a more even material response to thermal forces.
Other design factors, such as sharp corners and the depth of detail, influence the types of aggregates used,
the method and time needed to fabricate a piece, dimensional stability, finishing options, and ease of installation.
Generally, less complicated and smooth detail transitions will result in easier, faster, and efficient fabrication and
installation. However, highly detailed and intricate surrounds command higher prices in the market.
Design Considerations
11 Casting Fireplace Surrounds