CWC Technology Brief

Post-Consumer Container Glass Remelting Process Assessment

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.

Batch Chemical Additions. Because bottle glass is formulated to set quickly, and multiple re-heatings tend to drive off dissolved gases and volatile alkalis in the glass, additional batch chemicals need to be added prior to remelting. These chemicals serve three primary functions:

(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.

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Fact Sheet Update: April 1997