USG Casting Fireplace Surrounds - Manual (Page 24)

Gypsum Cements In addition to the products recommended for fireplace ornamentation, other plaster products can be used.
However, making these choices will almost always lead to a reduction in performance. Again, USG recommends
and supports only H YDROCAL FGR, E NDURACAST , or custom-formulated alpha gypsum cements manufactured by USG.
Contact your USG representative for more information.
The major difference between the various products is the amount of water used per 100 lbs. of plaster. This
is called the consistency of the product, and is a very important factor in the performance of the finished product.
In general, the higher the consistency (parts water per 100 parts gypsum cement), the lower the strength, the
more difficult and expensive the drying process, and the greater the chance for “cold flow.” Cold flow is also
referred to as “cold creep.”
Set gypsum cements are not load-bearing, meaning that if a large enough load is placed on a gypsum part,
it will slowly become misshapen. This can be prevented by using a low-consistency alpha gypsum cement and
drying it completely before putting it in service. If the consistency is high, the gypsum part will flow just from its
own weight unless it is fully supported. This is one of the reasons we support only the use of H YDROCAL FGR,
E NDURACAST , or custom-formulated alpha gypsum cements made by USG for fireplace surround manufacturing.
Some gypsum cements have an alkaline pH as a result of their formulation. If these products are used with
glass fiber for reinforcement, the fiber must be AR type (alkaline resistant), otherwise strength loss will occur
over time as the glass fiber degrades.
If you use products other than H YDROCAL FGR, E NDURACAST , or recommended custom-formulated gypsum
cements for fireplace ornamentation, consult a USG representative.
Many “invented” products comprising several raw materials are sold to manufacturers of fireplace
ornamentation. These combined materials really have no advantage over a single product in a bag, except
water-based polymers added to the liquid mix. Some of these additives work well aesthetically, but may reduce
fire ratings of the finished parts. For more information on using polymers, contact Ball Consulting, Ltd.
(www.ball-consulting-ltd.com).
Mold Materials There are several manufacturers of flexible mold materials. Eventually you will find the best source. Two sources
are Smooth-On, Inc. (www.smooth-on.com) and Polytek Development Corporation (www.polytek.com).
These companies both have a complete product line and distributors in many markets. These are very good
initial sources because they have a lot of literature and support for the new user of flexible mold materials.
Appendix
24 Casting Fireplace Surrounds