Glass Profile

Glass is one of the most common man-made materials and has been in use for more than 5,000 years. Stone Age man used natural volcanic glass—obsidian—as a cutting tool. Fulgurite is formed by lightning strikes at a beach or desert. Man-made, or artificial, glass was purportedly discovered by accident. The oldest examples of artificially produced glass—glass pearls from the graves of Egyptian kings—date back to 3500 BC. Flat glass was first produced about 200 BC. The discovery of glass-blowing around 50 BC made possible the widespread use of glass as a packaging material. The revolution in the glass container industry, however, occurred in 1903 when Michael Owens developed the first fully automatic bottle-making machine.

Physical Traits. Glass is a noncrystalline substance—that is, it’s neither a solid nor a liquid but exists in a vitreous state. As a result, glass is sometimes referred to as a super-cooled liquid. When glass cools, its atoms remain in the same random arrangement as the liquid but with sufficient cohesion to produce rigidity. When cold, glass can be carved, although it’s extremely brittle at low temperatures.

The basic ingredient of glass is silica, usually in the form of sand. A flux—potash or soda ash—is added to lower the melting point, while lime is incorporated to increase hardness. In 1676, George Ravenscroft discovered that by adding lead to the recipe, a stronger, heavier, and more brilliant glass could be produced.

Bottle Basics. Glass containers are produced from sand, soda ash, and limestone, as well as used or broken recycled glass, named cullet. The ingredients are heated to roughly 2,800oF to form molten glass that’s poured into molds, then pressed and blown into various container shapes. More than 40 billion glass containers are produced in the United States annually. Two-thirds of domestically produced bottles are clear, or “flint,” while a quarter are brown, or “amber,” and the balance are green, blue, and other colors.

Recycling Info. Glass containers are 100-percent recyclable, meaning they can be made back into glass containers with little loss of material. The U.S. glass container recycling rate, including refillables, is estimated to be around 40 percent. Other end-use markets for cullet include insulation (fiberglass) and other building materials, paving aggregate (glasphalt), abrasives, decorative glass, reflective beads, and more.