Originally Published on Education.Seattlepi.com
When a dropped glass shatters or a rock chips the car’s windshield, it’s tempting to think of glass as a fragile material. Actually, it’s one of the longest-lasting man-made materials. The New Hampshire Department of Environmental Services estimates that it takes 1 million years for a glass bottle to decompose in the environment, with conditions in a landfill even more protected. Glass artifacts from glassmaking’s beginnings in Egypt, around 2000 B.C., still exist.
Naturally Occurring Glass
Examples of the long-lasting qualities of glass come from glass made in nature, an opaque material called obsidian. It results from volcanic activity melting silica rock or sand to form black, red, gray, brown or green glass. Natural obsidian deposits were mined by prehistoric peoples to make weapon points, cutting tools, mirrors and other objects. In Iraq, obsidian use dates back to Paleolithic times, around 30,000 BP. In North America, the Obsidian Cliff deposits in Yellowstone National Park were formed about 180,000 years ago and mined by native Americans for more than 10,000 years.
The basic ingredients for glass are silica sand, soda and lime. Other ingredients give glass its color, clarity or opaqueness, and strength. Different minerals give glass color, with gold giving red, manganese purple and cobalt blue. Raw ingredients need very high temperatures — ranging from 2,600 to 2,900 degrees Fahrenheit — to change into molten glass, depending on the composition. Molten glass is pressed, blown, molded, drawn or cast into glass objects. Once formed and cooled, glass doesn’t readily react with other substances to change its structure.
Glass items exist from throughout the history of glassmaking, whether they were buried in archaeological remains, sunk in sailing vessels or carefully preserved by collectors. Glass can change in appearance after it is buried, with chemical reactions between the surrounding soil and the glass often resulting in iridescent surfaces. This adds to its beauty but doesn’t detract from its strength. Glass can be brittle or strong, depending on its composition. Older glass is more brittle than modern glass, but this does not affect its decomposition rate in landfills. In landfills, glass isn’t exposed to degradation by wind or erosion.
Originally, glass was a rare and precious commodity, because it took so much fuel to melt the ingredients and because of the labor-intensive production. Modern methods allow mass production of glass containers and objects. In 2011, Americans contributed 11.5 million tons of glass to the municipal solid waste stream. Glass lends itself to indefinite recycling without loss of strength. Broken into cullet, recycled glass goes into new containers or to products such as kitchen tiles, wall insulation and abrasives.
The Value of Recycling Glass
Given the long life of glass and the ease with which it can be recycled, it makes sense to recycle glass. As of 2011, Americans recycled more than 3 million tons of glass, an increase from 750,000 tons in 1980. Most recycled glass comes from beverage and food containers. Cullet costs less than virgin glass and saves energy because it melts at a lower temperature, which in turn means lower emissions of greenhouse gases, such as nitrogen oxide and carbon dioxide. Recycling one glass bottle saves enough energy to run a computer for 25 minutes, according to British Glass.