Material: Recycled
Glass
Issue: It is very simple to fuse recycled
container glass to make tiles and relief plaques. However, information on raw material characteristics,
temperature profiles, and operational issues has not been widely disseminated.
Best
Practice: At
temperatures considerably below melting, particles of metal, ceramics,
or glass begin to bond through a process called sintering. Sintering of soda-lime glass begins at about 1250° F. Sintering can be used to create solid glass
objects by heating glass particles to above the sintering point and
holding at that temperature until a dense body forms.
The basic steps are described below:
1)
Obtain a
mold that will withstand temperatures of at least 1600° F. For instructions on casting permanent molds,
see Permanent Molds for Fusing
Glass Best Practice. Coat
the mold with a release. Commercial
“shelf paints” are available. A mixture of 40% kaolin and 60% alumina hydrate also works well.
2)
Fill the
mold with glass pieces. Glass of any size can be used. However, the most consistent success has been
obtained with glass that has been crushed to a uniform shape and screened
to a size 16 mesh and finer. There
is a trade-off here between glass grain size and cleanliness. It is easier to obtain a glassy, translucent
product with larger grain sizes, but smaller grain sizes minimize
contamination.
3)
Fire the
kiln to fusing temperatures. For
green or amber container glass, the following schedule is a place
to start:
500°F/hour to 1100°F (organic contamination should be gone
by this point) 1000°F/hour
to 1600°F
(or go as fast as the kiln will rise) soak 5 minutes
at 1600°F (this
is the critical time & temp – experiment) Natural cooling
to 1020°F Soak 15 minutes
at 1020°F (may
need longer soak for thick or uneven pieces) 70°F/hour
to 950°F Cool 300°F/hour
to 250°F Open kiln to cool to ambient
Each
kiln acts differently. The
maximum temperature that works best in a particular kiln may be anywhere
from 1500 to 1600°F. Fusing
can also be done in cone kilns. Fire
at medium speed until organic burn-off then at maximum speed and try
cone 010, 011, 012, and 013. If
the fused tiles have bubbles or bloat, it may mean that organics need
more time to burn out, that other contamination is present.
Try soaking at 1100°F for 10 to 20 minutes.
Container glass “devitrifies” very easily. This means that the glass changes from a glass
back into a crystalline material, with two results: the glass takes
on a matte finish; and it becomes much stiffer. Slower temperature
profiles and long soaks appear to make devitrification worse.
A large amount of direct radiant heat (as from top-mounted
heating elements) appears to promote devitrification, as well.
The best strategy for starting recycled glass fusing
is to find a source of crushed glass that is reliable and consistent. Then develop a matrix for an experiment. Combine different colors and different grades,
and fire with different ramping temperatures from 1450°F to 1600°F.
Most of the early experiments will fail, but patience will
be rewarded.
Either side of the fused glass product can be chosen
as the featured side. The
side exposed to the air will have a graceful “slump” and will probably
be glossier than the other side.
The side that was in the mold can be made to take on any type
of relief.
Virtually any clean glass can be fused at the right
kiln temperatures. Although
making fused glass products as artwork or in small-scale production
has been growing over the years, most artists and product developers
have stayed away from using recycled glass because it is a more challenging
material than some of the glass designed and manufactured for fusing,
like Bullseye Glass. However, some small businesses, like this one in Seattle, are successfully
manufacturing glass tiles using this type of process.
Implementation: These instructions can be used
with the Best Practice Permanent Molds for Fusing
Glass to begin experimenting with glass fusing.
Benefits:
This
process is accessible to anyone with a ceramics kiln capable of 1600°
F.
Application
Sites: Crafts
shops, small businesses.
Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org.
References: Richerson, David W., Modern Ceramic Engineering - Properties, Processing,
and Use in Design, Mercel Dekker, Inc., 1982. For a general discussion of
glass fusing and moldmaking: For more about issues related to glass devitrification,
see CWC report Densification, Crystallization, and Sticking Behavior of Crushed Waste Glass Sintered in Refractory Molds with Release Agents, CWC, 2000.
Issue Date /
Update: April 2000
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