Material: Recycled Glass Issue: Art glass, especially products produced at a studio (hot shop) scale by
blowing or casting molten glass, represents
one of the highest value uses for soda-lime
glass. The
retail value of these glass products is tremendous
on a per pound basis. However, these applications are also among
the most sensitive both to contaminants that
might affect the clarity, and to chemistry
that might affect the workability of the glass. This is a critical market barrier for the use
of recycled glass.
Although virgin raw material costs
may appear high compared with recycled glass,
in fact they are often a small fraction of
the finished product prices in this application,
so any risk associated with using recycled
glass obviates against its use. However, in the right kinds of businesses,
and if appropriate procedures are followed,
hot shops can be successful in using recycled
glass to create recycled content products. Best Practice: By carefully controlling batch formulas and melting
procedures, recycled glass products can be
manufactured with physical and aesthetic qualities
similar to those made with virgin glass or
batch materials.
Obtaining a quality glass for blowing
or casting from post-consumer bottle glass
hinges on four processes: sourcing, cullet
preparation, addition of batch chemicals,
and melting. The sourcing of recycled glass requires a partnership with a collection
program capable of producing a reliable stream
of quality sorted material. The cullet must be free of dirt, rocks, metals,
ceramics, and other types of glasses like
Pyrex or Visionware. A specialized collection program involving
community recycling centers or local bars
and restaurants can be developed to supply
a clean and consistent recycled glass feedstock. The quality of glass supplied by beneficiators
as “furnace ready cullet” for container plants
is probably too contaminated for hot shops,
because 1) container plants often tolerate
a higher level of rejected products due to
inclusions, and 2) the huge size of containment
vessels and sophisticated fining processes
in container plants help with contaminant
separation. Cullet preparation includes size reduction and contaminant removal. The amount of crushing
necessary to prepare recycled glass is a function
of the product being made from the remelted
glass. If
a glass with a long working range (time) is
needed for hand blowing, it is best to crush
the glass to a size that allows easy melting
with the addition of batch chemicals. In that
case, it is recommended that the glass be
crushed to a size between 3/4 and 1/16 inches,
which allows the particles to be easily coated
with batch chemicals, especially if a little
moisture is left on the glass particles after
washing.
It is best to sift the smallest powder-sized
particles (called
“fines”) out of the feedstock. Anything under 1/16 inch can cause problems
such as increased need for chemical fining
agents (usually noxious or toxic chemicals),
and increased time needed to melt the new
glass. Mold-blown objects and cast glass are less
sensitive to workability, needing little or
no fluxing, and therefore less size reduction
is necessary.
In all circumstances, it is a worthwhile
practice to give the glass final scrutiny
for contaminants before batch additions. Batch Additions help to optimize the working characteristics of the recycled glass melt. The
chemical composition of container glass used
for packaged food or drinks more or less conforms
to universal standards. It is formulated to set-up quickly and to hold its shape when mechanically
blown into molds.
However, glass for studio work, especially
for hand blown objects and complicated fabricated
products,
requires a long working range.
To obtain the working range required
for hand blowing, soda ash can be added as
a modifier to the batch.
Bottle glass typically contains 13%
to 16% soda ash, and it is recommended that
the total soda ash content remain below 18%
by weight.
Up to 2% can be expected to volatilize
during the melting process. Fining and fluxing agents also should be added
to facilitate melting and blending of various
glasses. For a more complete discussion of
chemical additives and shop practices, see
(1). Since the volume of glass needed
for studio products is relatively small, glass
is most often melted in ceramic crucibles or small refractory
lined day tanks (a tank holding enough melted
glass for one day’s work), usually near the
highest temperature the equipment can endure
(2350° to 2600° Fahrenheit).
Cullet requires about 10% less energy
to melt than straight virgin batch materials,
and is less corrosive.
At these temperatures, recycled glass
has low enough viscosity to allow bubbles
to rise readily, aiding blending of chemicals
and speeding the fining process. For information about color compatibility during
the melting and product formation processes, see the Color Modification in
Remelt Applications Best Practice. Implementation: Forging partnerships with a
reliable source of post-consumer recycled
glass is the first critical step toward using
recycled glass in hot shop operations. A good relationship with a supply source such as a community recycling
center can provide the opportunity for education
about the specific material needs of a glass
studio. Presenting
samples of ceramic stones and cords (linear
strips of higher density than the rest of
the glass) is an effective way to convince
suppliers that even small amounts of contamination
can cause big problems for the small studio
glassmaker.
Recognizing the differences in color-compatibility
of recycled cullet is an important consideration
in integrating its use with the common German
colorbars. The “pull test” is a quick method of evaluating
color compatibility (see best practice referenced
above). Benefits:
From
a cost standpoint, recycled glass may appear
to be an attractive feedstock for hot shop
operations.
However, some glass artists believe
that these savings may be small compared to
the risks associated with using recycled glass
because the raw material cost usually represents
only a small fraction of the final price of
the product. Glass art objects may sell for anywhere from
$10 to $200 per pound of glass.
By comparison, the primary supplier
of premixed batch to the Northwest art glass
community sells glass pellets for only $0.35
per pound. Thus the savings in raw materials alone are
insignificant in high-end applications, especially
hand blown products. Recycled glass may be most promising as a substitute
feedstock in production casting or pressing
operations, where the material represents
a larger percentage of the final price. Application
Sites: Glass-blowing
studios and hot shops. Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org. References: Dalbey, Randal, Recycled GlassWorks, Seattle,
WA Courtney, Norman, Courtney Design Studios, Seattle,
WA
(1)
Post-Consumer Glass Cullet Remelting
Process Assessment, Clean Washington Center, 1996.
Issue Date / Update: November 1996
|
