USG Casting Fireplace Surrounds - Manual (Page 26)

Fabrication Parts made for fireplace ornamentation are much larger and heavier than most products made from gypsum
cements, so you need to establish manufacturing procedures that account for this size and weight. The major issues
that can occur with large pieces are expansion and warping. While these potential problems are related they are not
identical, but may produce similar results. Sometimes there is confusion about the cause of these problems.
Expansion is the natural result of the rehydration of gypsum cements. There are products formulated to
minimize expansion, but there is no way to eliminate it without causing other problems. For example, consistency
is a factor in the amount of expansion and typically the higher the consistency, the lower the expansion. High-
consistency plasters produce lower strengths, so they are not a good choice for surrounds.
Expansion occurs somewhat uniformly in all directions but is most noticeable in the longest dimension.
The net effect of expansion is usually not a major concern and is generally considered a benefit, since very
minute mold details will be well-reproduced. However, there are a few situations when it might cause a problem:
– Rigid molds can produce restricted points which may cause the part to bow because of expansion.
– The product design may produce long straight areas where there is a cumulative effect from expansion that cannot
be relieved, even in a flexible mold.
– The product has significant differences in cross-sectional dimensions, causing higher expansion in thicker areas
compared to thinner areas.
– Solid castings with no reinforcement tend to present more problems with expansion.
Warping is due to “cold creep” of the undried part after it has been removed from the mold. The greater the
changes in thickness or in design elements on the part, the more likely it is that the piece may warp. This condition
can usually be remedied by supporting the piece correctly after demolding. With some designs this means placing
the piece flat on a rigid surface so it is properly supported. In other cases it may be necessary to construct a frame
to provide proper support when the part is removed from the mold.
Installation Building codes do not contain much data on fireplace ornamentation made from gypsum cements. Over time, the
code bodies will likely generate data to provide for standards in this area. Currently, codes are primarily concerned
with the safety of the building itself and combustible materials and not too much with the interior finish, but this is
expected to change. At present, each manufacturer is responsible for establishing installation criteria to protect
the product from failure and provide long-term durability.
There are several types of fireplaces that burn fuel, and these are the only units that require guidelines
concerning gypsum cement fireplace surrounds. Purely decorative fireplaces have no potential problems that
require guidelines. Older fireplace designs are vented, which means a chimney or some other exhaust system to
carry the smoke and some heat out of the building. Originally these fireplaces burned wood, but some newer ones
burn only propane or natural gas and some can burn either gas or wood. Because these fireplaces are vented, the
temperature around the fire box opening is relatively moderate depending on the construction of the fireplace and
the location of the burning spot as well as the vent or chimney. With this type of fireplace, placing the fireplace
ornamentation directly next to the firebox is probably not going to cause failure due to heat, but with high-BTU
fireboxes, there may be problems associated with calcination.
Appendix
26 Casting Fireplace Surrounds