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 sodalime-silica glass the former is silica (Silicon dioxide – SiO2) in the form of sand.
Flux lowers the temperature at which the former will 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."
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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:
1.
2.
3.
4.
5.
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
Soda
Lime
Potash
Magnesia
Aluminia
Iron oxide
Boric oxide
Lead oxide
SiO2
Na2o
Cao
K2o
MgO
Al2O3
Fe2O3
B2O3
PbO
1
73.6%
16.0
5.2
0.6
3.6
1.0
----
2
80.0%
4.0
-0.4
-2.0
-13.0
--
3
35.0%
--7.2
----58.0
4
96.5%
----0.5
-3.0
--
5
67.0%
18.0
8.0
1.0
1.0
2.5
0.5
-0.01