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Technology Brief POST-CONSUMER CONTAINER GLASS |
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The small scale art glass product
manufacturing industry has been identified by the Clean Washington Center
as a potential high-value market for recycled glass. A study was conducted to establish effective
and repeatable methods for incorporating post-consumer soda-lime bottle
glass into the melting practices of small glassmaking studios. The economic feasibility of converting to recycled
glass on a small scale was also examined.
Market Potential From a cost standpoint, recycled glass may appear to
be an attractive alternate feedstock.
Recycled glass generally requires less energy to process and
costs less than materials typically used in glassmaking shops. However, some glass artists believe that these savings are small
compared to the risks associated with using recycled glass, and the
additional time required to prepare recycled glass for melting. For high value products, especially hand blown art objects, the
savings in raw materials alone are usually insignificant.
In addition, some crystal clear products require a
clarity of glass that can never be obtained with post-consumer bottle
glass. Colored or iridescent
products, however, can be manufactured using post-consumer glass without
diminishing the aesthetic integrity of the piece.
The market potential for recycled glass, therefore, may be most
promising in production casting or pressing operations where the material
cost represents a larger percentage of the final price of the product.
Process Assessment Although some paper (i.e. remaining bottle labels)
is acceptable, and shouldn’t diminish the quality of the remelted glass.
Successfully producing quality blown or cast products using recycled
glass hinges on four critical procedures:
sourcing, cullet preparation, batch chemical proportioning, and
melting. A series of trial melts were conducted in three
different small scale art glass studios to test various methods of preparing
and melting recycled glass for use in blown and cast products. Based on these trials, a recommended protocol
for processing recycled glass was developed.
Sourcing. The first critical step is connecting with a reliable
source of clean cullet that is free of dirt, rocks, metals, ceramics,
and other glasses like PyrexÒ. This will help to ensure that the final product is free of stones,
cords, or other defects.
Cullet Preparation. Before the cullet is melted, it must be cleaned to
remove any remaining debris or dirt and coarsely crushed. Excessive amounts of paper should also be removed,
It is recommended that, whenever possible, the glass be crushed and
screened to between 1/16 and 3/4 inches.
Fluxing and fining agents did not combine well with larger sized
cullet in test trials. Grains
under 1/16 inch should be sifted or washed out of the feedstock, as
they tend to trap small air bubbles in the batch, making the glass more
difficult to melt, and increasing the need for additional chemicals. |
(1) to insure compatibility with common colorbars, (2) to increase the working range of the remelted glass, and (3) to facilitate the melting and blending of recycled glasses
from various sources.
Trial melts suggested that the following chemical additions
produce a glass with a relatively quick set time and a linear expansion
coefficient (LEC) that matches that of German colorbars. To each 100 pounds of glass, the following
chemicals were mixed and added: 2 lbs. soda ash 33 g. borax 175 g. niter 150 g. fluorspar 1/2 lb. lithium carbonate 50 g. antimony oxide 25 g. manganese dioxide For some cast products, where color compatibility is not an issue and a longer working range is unnecessary,
it may be possible to either cut these amounts in half or eliminate
them altogether. To obtain the
working range required for freehand processes, additional fluxing agents
will probably be required, especially if the glass is hand blown.
Melting. During trial melts, furnaces were charged at regular
intervals for three hours at about 2350 degrees Fahrenheit. The glass was then allowed to melt at the same
temperature for another four hours, and finally left to idle for several
hours. This melt cycle was found
to be capable of producing a very clean glass, free of fines, small
bubbles, or other defects.
It is important to note that there are many variables
that influence the physical characteristics of remelted glass. Different shop conditions and base glasses will
result in remelted glasses with different physical characteristics. Each individual studio is encouraged to experiment
with the formula above, making changes as necessary to arrive at the
chemical proportions that work best with its own individual processes
and applications.
Report
Issue Date: December
1997 Fact Sheet Update: November 1997 |
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Experimental Procedure Bottle
cullet was pre-crushed with a hand crusher to a size of two inches minus
and washed. Ceramics, metals,
lead wrappers, and some plastics were also removed, but paper was left
in the cullet.
Colorants
and fluorine-based opacifiers were sourced from local suppliers. Fluxing agents were added to the formulations
both to aid in melting the base glasses and to keep the colorants in
solution.
Gobs
of each glass were pressed in a hand-press using a patterned four-inch
square tile mold.
Test
trials indicated that the formulas above yield relatively consistent
and satisfactory transparent colors.
Additional research is required to identify methods of further
decolorizing green bottle cullet to produce lighter colors with this
base glass. |
Test
trials failed to produce consistent opal glasses. Opalizing agents (fluorine-based) were difficult to blend, and separated
in the melt. This problem was
exacerbated by the fact that the fluorine tended to volitilize during
the melting process. Several
attempts were made to improve the melt conditions and arrive at appropriate
fluorine concentrations. The
best results were obtained using a 150 to 200 mesh pulverized glass
sand as a feedstock, and a shortened melt time.
If these adjustments are made, it should be possible to obtain
a whole set of opal colors from recycled glass.
Note: The protocol summarized in this Technology Brief assumes working knowledge of glass chemistry.
Report
Issue Date: December 1996 Fact
Sheet Update: November 1997 |