Glass
Chemistry of Glass - Thousands of different chemical compositions can be made into glass. Different formulas affect the mechanical, electrical, chemical, optical and thermal properties of the glasses that are produced. There is no single chemical composition which characterizes all glass.
Glass is composed primarily of silicon dioxide, together with some sodium oxide and calcium oxide and small quantities of other chemicals. Sand and limestone are used to make calcium oxide and sodium carbonate is used to make sodium oxide.
Typical glass contains formers, fluxes, and stabilizers.
Formers make up the largest percentage of the mixture to be melted. In typical soda-lime-silica glass the former is silica (Silicon dioxide – SiO2) in the form of sand.
Flux lowers the temperature at which the formerwill melt. Soda (Sodium carbonate – Na2CO3) and Potash (Potassium carbonate), both alkalis, are common fluxes. Potash glass is slightly more dense than soda glass.
Stabilizers make the glass strong and water resistant. Calcium carbonate often called calcined limestone is a stabilizer. Without a stabilizer, water and humidity attack and dissolve glass. Glass lacking lime is often celled "waterglass."
- Silicon dioxide (silica or sand) SiO2
- Sodium oxide Na2O
- Sodium carbonate Na2CO3
- Calcium oxide CaO
- Limestone CaCO3
- Boric oxide B2O3
- Potassium oxide K2O
- Lead oxide PbO
- Aluminium (impurity) Al2O3
Large quantities of energy are required to manufacture glass. Scrap glass is added to raw materials to make the process economic. Silicon dioxide has a melting temperature of 2000 K. Between 15% and 30% of powdered scrap material is added to lower the temperature needed, this also reduces the electrical energy and the time to produce the glass.
Sola-lime silica glass is used for high volume products such as bottles and windows. This glass has poor thermal shock resistance. Bosilicate glasses (pyrex) are available to overcome this problem, they contain 60-80% silicon dioxide, 10-25% boric oxide, 2-10% sodium oxide and 1-4% aluminium (impurity)
Toughened glass is made by heating glass almost to the melting point. The surfaces are then cooled while the centre still remains hot and plastic. Once totally cooled the glass is tough but will shatter into tiny pieces if subjected to a strong impact (windscreen glass).
Laminated glass has a thin layer of material, usually plastic, between the layers. This prevents cracks from growing. This is used as a security glass as it is difficult to break through. It can even be made to be bullet-proof.
Glasses begin as mixtures of oxides. Their compositions can be represented by listing the weight percentages of their components. Compare the percentages for:
- a typical, modern soda-lime-silica glass (used to make bottles and windows);
- laboratory and some baking ware;
- optical, high lead crystal;
- 96% silica glass (can withstand very high temperatures);
- a typical, ancient Roman soda-lime-silica glass.
WHY DOES...
...number (3) formula cause concern for many people?
...number (5) composition have so many components?
WHAT IS...
...the oxide common to all five?
...the necessary oxide used to produce quality optical lenses?
...the most likely formula that you would use to make a space vehicle window?
...the most common glass manufactured for thousands of years?
HOW HAS...
...glass made space research possible?
...culture been transformed and become dependent upon glass?
Silica / SiO2 / 73.6% / 80.0% / 35.0% / 96.5% / 67.0%
Soda / Na2o / 16.0 / 4.0 / -- / -- / 18.0
Lime / Cao / 5.2 / -- / -- / -- / 8.0
Potash / K2o / 0.6 / 0.4 / 7.2 / -- / 1.0
Magnesia / MgO / 3.6 / -- / -- / -- / 1.0
Aluminia / Al2O3 / 1.0 / 2.0 / -- / 0.5 / 2.5
Iron oxide / Fe2O3 / -- / -- / -- / -- / 0.5
Boric oxide / B2O3 / -- / 13.0 / -- / 3.0 / --
Lead oxide / PbO / -- / -- / 58.0 / -- / 0.01