Humans did not invent glass making: long before humans learned the secret, nature was making glass. When lightning struck sand it melted it into long, thin tubes of glass. Erupting volcanoes melted rocks and sand into glass. Humans found this naturally made glass and improved the process. The earliest glass made by humans was probably a glaze on ceramic pottery made somewhere around 3,000 B.C.
Today sand, soda ash, lime, and sometimes gypsum or dolomite are melted together in large furnaces to over 2,000 degrees Fahrenheit until the mixture becomes a syrupy mass. While malleable it can be shaped by blowing to make hollow vessels, drawing into sheets or tubes, pressing into a mold, or sculpting things with globs of hot glass.
Colored glass is produced by adding small amounts of natural elements to the molten glass. For instance, brown glass is made by adding iron, sulfur and carbon to the mix.
Producing virgin glass takes less energy than does producing metal or plastic and glass recycles endlessly without losing any of its strength. Also, glass containers are far more stable than plastic or metal containers so they do not leech or out-gas anything into the food they contain and glass containers can be safely reused over and over.
Crushed recycled glass is called ‘cullet’. The proportion of cullet in new glass can be as high as 90%. Cullet melts at a lower temperature so for every 10% of cullet in the glass mix, the factory can use 2% less energy to produce the same quality of glass.
While glass is made from all naturally occurring materials, nature cannot recover glass through decomposition as it will with some other products. If dumped into a landfill, glass will remain there, taking up space, forever – OK, a million years or so: pretty much forever. Because it does not contain or release any toxins, it is safe to dispose of glass this way, but trashing glass removes a valuable resource from the materials chain.