With the growth of "single stream" glass collection systems, an increasing number of communities are burdened with piles of valueless mixed color glass. High-value markets can be created to use this glass, but the challenge is to develop processing systems and manufacturing processes using post-consumer recycled glass that create products with enough value to support recycling collections.

Glass recycling is one of the oldest types of recycling programs. Turning used bottles into clean, crushed glass cullet that can be added to a molten glass mix and molded into new bottles is a proven process with good value. Every community used to boast a glass blower, but bottle manufacturing (like many industries) has consolidated, resulting in fewer and larger manufacturing facilities. The cost of shipping glass to the bottle manufacturers has therefore increased. In addition, recycling programs in many communities are producing more mixed broken glass, which requires significant, costly processing to make it acceptable for re-melt into new glass products.

The recycling industry is searching for good value markets that involve cost-effective processing, as well as local markets for the recycled glass. Over the last 20 years, many communities have been processing, or simply breaking, their mixed color glass locally and using it as construction fill or daily landfill cover. While these applications can be effective alternatives to disposal, and can help to keep collection programs going, they generate no positive cash flow and are not long-term solutions to glass markets.

A few companies have been processing glass as granular products like abrasives and filtration media, but progress is slow. With economic development and job creation being the mandate for all local agencies from departments of trade to ecology, garnering the highest value per pound for the glass is critical.

Tile Opportunities

With U.S. consumption of ceramic tile exceeding 3 billion square feet per year, assuming an average 4 lbs per square foot, that equals 12 billion lbs, or 6 million tons of tile per year. If a competitive tile manufacturing process using recycled glass as a raw material could be developed, the potential for recycled glass markets would be tremendous.

Even the least expensive tile, retailing at $2 per square foot, translates into $800 per ton. Higher-end designer tile, at $20 per square foot or more, can assign values of $8000 per ton to the finished product. An increasing number of tile products are being manufactured using recycled glass. Current products separate themselves into five categories: poured glass; glass/ceramic; glass terrazzo; sintered (or fused) glass; and cut, enameled and slumped tile.

In poured glass products, glass is melted and poured into molds in an energy-intensive process. The tiles are beautiful, but labor and energy costs are high; the only substantial manufacturer involved with this process has moved its main production outside of the U.S. Poured glass requires clarity, so the glass needs to be clean and color sorted.

Glass/ceramic products combine glass with clay in conventional ceramic tile manufacturing processes to make ceramic-looking tiles. The glass acts primarily as a body flux. The largest manufacturer of this kind of tile uses only finely crushed and graded plate glass that has been specially processed. The glass needs to be fine (less than 100 mesh) to work with conventional clay equipment like presses and extruders. Since clay is the binder in this tile, a drying step is required to gain the green strength needed for firing. Uniformly processing glass into fines is a considerable expense.

Glass terrazzo products use glass rather than the traditional stone chips in a cement-derived base. After curing, the surfaces of the terrazzo tile are ground to expose the glass pieces. The grinding is a labor- and materials-intensive process, but one advantage of using cement as a binder is that it cures in the air without drying equipment. A disadvantage is that-as with all cement products-sealers are required. If owners don't maintain the sealer, terrazzo products can absorb stains.

In sintered (or fused) glass products, clean granular glass is fired in a mold. The glass needs to be clean because any contaminants will show up as flaws in the tile. Since the mold weighs as much as the glass, this process also consumes more energy than is desirable. A number of small studios are selling tiles like these fired in batch kilns.

The process for making cut, enameled and slumped tile begins with pieces of plate glass large enough for cutting, so this method is only possible starting with pieces of post-industrial plate glass. A number of products sold as "recycled" through this method are only recycled in the sense that the glass was already made when the business that did the slumping purchased it.

Unfortunately, "greenwashing" is becoming increasingly popular in marketing some of these products. Greenwashing is the colloquial term for presenting products as having recycled content when they actually use specially prepared glass or glass from art glass manufacturers. The U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) Green Building Rating System^TM, which is the primary certification system for green buildings, requires that products be manufactured within 500 miles of their use. However, the system cannot track the fact that, in many instances, specially prepared glass is shipped across the country before entering the manufacturing process. Ideally, post-consumer glass should be collected and processed in or near the same community where it is manufactured into end products.

Developing a New Process

A variety of equipment and systems is available that can be located in a community to accept and process all types of post-consumer and post-industrial waste glass. Options include a small bottle crusher used in a restaurant or bottle return center, a glass crusher or windshield recycling system for industrial waste glass, or a system that pulverizes and screens the mixed glass that has been separated at a material recovery facility after consumers put it on the curb for recycling. Finally, a glass processing plant would receive all types of glass for final cleaning, milling, mixing and packaging.

This processing is a necessary step in making a local supply of recycled glass available. A tile manufacturing process combining the best aspects of current products would:

• Use post-consumer glass with minimal processing
• Have low energy input for manufacturing
• Have the appearance of glass and a sealed surface
• Be air-curing without needing drying
• Fire very quickly
• Not require grinding or sealers to attain a beautiful finished surface
• Not use molds in the kiln or furnace

A hybrid glass/ceramic process combining these characteristics is under development. Results of recent tests using granular (8 mesh and finer), crushed, post-consumer glass in brick manufacturing at rates of up to 67% by dry weight illustrated some of the unexpected advantages of using glass as a grog rather than as a fine flux.¹ The advantages included reduced water requirements for forming, and energy savings as high as 40%. In essence, the clay was the cold binder, and heating the brick to fusing temperatures resulted in the final strength. This turns on its head the concept of using fine glass in small percentages as a flux to stimulate the ceramic materials to react at lower temperatures.

The new process under development takes a similar approach, but uses inorganic air-curing binders to achieve green strength. The tile can be glazed, or surface fluxes can be added, to achieve a sealed, glossy surface (see Figure 1). Since the surfaces can be modified, low-grade post-consumer glass can make up the majority of the body. Using an inorganic binder instead of clay keeps the pores open during most of the firing, enabling faster firing cycles without bloating or black coring.

Building Bridges

Bridges must be built between different industries so that partnerships between local collection and recycling companies, and tile, brick or paver manufacturers, can form for the benefit of the companies and the community as a whole. Innovations in recycling practices, processing and product manufacturing all come together when manufacturing tile with recycled glass content.

Today, the average consumer-along with the corporate purchaser and LEED architect-has a new-found or renewed interest in buying products with genuine recycled content. The product must provide good value, quality and function. Tile produced with recycled glass offers a genuine "green" product that is pleasing to the eye and the budget.

For additional information regarding glass recycling, contact Andela Products, Ltd., 493 State Route 28, Richfield Springs, NY 13439; (315) 858-0055; fax (315) 858-2669; e-mail candela@candelaproducts.com; or visit www.candelaproducts.com. Robert Kirby can be reached at kirbgood@earthlink.net.