Glass Making Chemistry from a Social Studies Teacher

When I began this class I believed that glass was made using sand and heat. I saw in the movie Sweet Home Alabama where the actors used metal rods in the beach sand to attract lightning and make fulgurites (hollow glass tubes formed in the sand by the lightning's heat).

Fulgurite

Therefore, it made sense to me that this was how glass would be made, sand and heat. While it does appear that this is a method of making glass, it isn't that common due the the extreme temperature necessary to heat the pure silica sand (1200 degrees Celsius). The more common methods of making glass today involve adding other materials to bring the transition temperature down. The transition temperature is the point at which the silica changes states from solid to liquid.

In the Soda-Lime glass making method, sodium bicarbonate is added to the silica or glass sand to lower the transition temperature . The problem with adding sodium bicarbonate is that it is water soluble.  Other chemical compounds such as lime (calcium oxide), magnesium oxide, and aluminum oxide are added to the glass to counter the water solubility. The finished product is 70-74% silica. Soda-Lime glass accounts for about 90% of all glass produced. Another product added in glass production is cullet or recycled glass. Using cullet is environmentally friendly and uses less energy and raw materials in glass production.

Once the glass has achieved its final shape, it must be gradually cooled in a process called annealing. In the annealing process a chamber with the glass in it is heated to approximately 580 degrees and then is cooled over a specific amount of time. Cooling too quickly will cause the glass to stress.



Glass containers being formed

Questions for consideration:
1. Soda-Lime glass has a greenish color. How do you think they get the above bottles to have a brown color?
2. What do you believe will happen if you cool the glass too quickly?
3. What are sources in you school and community for cullet?
4. Research the chemical compounds used to make glass discussed above. What other uses are there for these compounds? Some may surprise you!