Material: Recycled Glass Issue: Most molds used for fusing
glass are either expensive to make or are waste molds, only useable
one time. In order for small-scale
glass fusing businesses to develop, permanent, inexpensive, castable
molds from readily available materials are needed.
For a description of recycled glass fusing, see Fusing Recycled Glass Best Practice. Best Practice: The requirements for a permanent mold for fusing glass
include: the ability to withstand repeated kiln cycles up to 1600°F;
that it be easily formed or cast; and that its surface should freely
release fused glass on its own or with a coating.
In addition, it should be inexpensive enough that a product developer
can freely cast test molds without incurring unreasonable costs. Calcium aluminate cement is
a high-alumina cement used in refractories.
One type of calcium aluminate, which is high in both alumina
and iron, is called fondu. When
combined with suitable aggregates, fondu can be used as the cement for
making permanent molds that will repeatedly withstand the temperatures
used to fuse glass. Suitable aggregates are granular materials
that are stable at kiln temperatures and have coefficients of thermal
expansion compatible with fondu. Among
suitable aggregate are fireclay grogs, olivine sand, chamotte, flint,
and other temperature-stable pre-fired aggregates that will not go through
a silica conversion when heated to 1700°F. To make permanent molds:
1.
Find a source
for calcium aluminate cement. It
can often be purchased in 100-pound bags at cement supply stores.
One manufacturer is LaFarge. Also find sources for one coarse and one fine compatible aggregate.
In general, coarse means particle sizes larger than 50 mesh and fine
means particle sizes smaller than 200 mesh.
2.
Find the most
efficient particle packing relationship using 25% cement by weight combined
with the two aggregates (for a description of a particle packing technique,
see Simple Particle Packing Best Practice). One mixture that has worked well is: 100
grams fondu cement 50 grams kaolin clay (improves the mold surface) 100
grams fine aggregate 150 grams coarse aggregate
3.
Prepare a master
and a moldbox for casting. The
master is the original form, which is to be reproduced. Keep
in mind that calcium aluminate shrinks while curing, so the master must
be made with a non-rigid material.
Foam sheathing, clay, and urethane pourable molds all work well.
4.
Blend the dry
ingredients
5.
Put the water
in the mixing bowl. Then add
the dry ingredients. The instructions
for fondu cement recommend a maximum water:cement ratio of 2:5 by weight. In casting fusing molds, especially by hand,
it may be necessary to use a water:cement ratio as high as 1:1. Experience will tell. The water requirement is highly dependent upon
the gradation and character of the aggregates. For example, the mix above could require up to 100 grams of water.
The most important characteristic is that the mixture flow into
the mold with vibration and not result in an inordinate amount of pooling
water.
6.
Mix. Mixing can be done by hand in a large plastic
mixing bowl while wearing a heavy duty glove (nitrile gloves work well)
or with a cement or grout mixer. All
materials must be thoroughly wetted.
7.
Scoop or pour
the wet mixture into the moldbox and place on a vibratory compactor
(for instructions on how to make a vibratory compactor, see A Simple Vibratory Compaction Table Best Practice).
Vibrate until the top of the mixture is level and smooth. Excess water may be poured off.
8.
Cover the mold
with plastic to keep moisture in and leave the mold overnight. Adequate drying is a matter of experience.
Too warm a drying temperature and/or exposure to dry air may
cause the mold to shrink too fast and crack.
9.
Remove the
mold from the moldbox.
10.
Fire the mold
empty in a kiln and soak for one hour at a temperature at or above expected
glass fusing temperatures. 1700°
F has been found to work well. Expect
to smell sulfur as the fondu fires.. Coat the mold with a light
coat of release before each firing.
Commercial glass fusing “shelf paints” are available. Alternatively, a combination of 40% kaolin
60% alumina hydrate works well. Implementation: These instructions for mold
making can be used with the Best Practice Fusing Recycled Glass to begin experimenting with
glass fusing. Benefits:
Current
prices and sources, f.o.b. Seattle, for the materials described above
are as follows:
Material Source
Price LaFarge
Fondu Lone Star
Cement $32.00/100# Kaolin
clay Seattle Pottery Supply $20.00/100# Ione
grog 65 mesh Seattle Pottery Supply $19.50/100# Ione
grog 20x48 mesh Seattle Pottery Supply $19.50/100#
Therefore, 400 pounds of mixture
can be made for $90.50, under 23˘ per pound. If properly made, these molds have been used more than 20 times
without cracking or spalling. Application Sites: Crafts shops, small businesses. Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org. References: For a general discussion of
glass fusing and moldmaking: Lundstrum, Boyce, Glass Casting and Moldmaking, Vitreous Press, 1989 Issue
Date / Update: November, 1998 |
