CWC Technology Brief

Workshops on Recycled Glass as an Alternative Ceramics Raw Material

Container glass is a vitreous silicate, the primary ingredients of which are silica sand (SiO2), calcium carbonate (CaCO3), and soda ash (Na2O). Although silica has a melting point of over 3000°F, in the presence of the other fluxing and fining agents, the sand forms a glass at temperatures below 2500°F. With proper cooling, the silicate remains a glass at room temperatures. Most clays are silicates, as well.

The Clean Washington Center (CWC) has performed research on a number of ways in which the properties and techniques of traditional ceramics can be combined to help create markets for glass and to enhance the properties of ceramics.

Several businesses in the state of Washington have been created based on these techniques. They represent an opportunity to perform small business development manufacturing unique products from recycled commodities using readily available raw materials and appropriate technologies.

Workshops

The CWC has developed a hands-on workshop to demonstrate ways in which glass can be used in ceramics processes. The target audience for the workshops is the existing ceramics community that exists in every part of the country, and throughout most of the world.

Some of the applications demonstrated in the workshop use skills, like moldmaking, that are already familiar to ceramics craftspeople. Other applications, like clay body flux, use glass as an alternative raw material in the same processes ceramicists use every day.

"Starter kits" of raw materials can be shipped to the workshop from Seattle to make it easy for local craftspeople to begin with proven materials. Among the applications covered are the following:

Glass Fusing

At temperatures considerably below melting, particles of 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.

Because there are no binders, fusing must be done in molds. A number of commercial "castable" moldmaking products are available, which mix and pour and set up like cement, but can withstand repeated firings to over 1500°F. The CWC has also developed formulas for making inexpensive molds from local materials.

Glass fusing can be used to make tiles, plaques, simulated stones, bricks, and pavers.

Clay Body Flux

Ceramicists have used vitreous silicates to enhance the properties of clay bodies for generations. In the 1970's, following the energy crisis, the United States Bureau of Mines became interested the possibility for energy savings from incorporating recycled glass into clay bodies for making bricks.

In clay bodies, finely ground glass acts as a flux, causing the creation of a denser, fully fired clay product at kiln temperatures up to 400°F cooler than normally used. In addition, the glass can help create a denser, more freeze-resistant clay body.

At lower firing temperatures, ceramicists should expect different mature clay body colors. The color changes, however, can be offset by the addition of oxide colorants. The fluxing effect of glass fines can be used to enhance the properties of any clay body.

The CWC has assembled a body of references on this application and is sponsoring tests for further development.

Powder Processes

High-tech ceramics manufacturers have used powder processes for many decades to make precision parts from finely ground ceramic and metal powders. Some powder techniques have been proven for use with recycled glass.

As opposed to the fusing processes described above, with powder processes, well-defined gradations of finely-ground raw materials are combined with water and binders to form products at room temperatures. The binders cause the products to get hard, or "set-up," at room temperatures.

After the products have set up, they can fired in kilns without molds. During the firing, the binders burn out and the glass particles fuse. The advantages of these processes include not needing a high temperature mold and the ability to make three-dimensional products like bowls.

Summary

The Clean Washington Center and the Recycling Technology Assistance Partnership (ReTAP) have developed hands-on workshops to demonstrate some new techniques for using recycled glass as an alternative ceramics raw material. For more information on the workshops, call Bob Kirby, industrial engineer with ReTAP, at (206)389-2808.

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 Issue Date: April 1997