Chemistry of 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 that characterizes all glass.

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) in the form of sand.
  • Fluxes lower the temperature at which the formers will %%melt%%. Soda (Sodium carbonate) 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.

Glasses begin as mixtures of oxides. Their compositions can be represented by listing the weight percentages of their components. Compare the percentages for 1, a typical, modern soda-lime-silica glass (used to make bottles and windows); 2, laboratory and some baking ware; 3, optical, high lead crystal; 4, 96% silica glass (can withstand very high temperatures); 5, a typical, ancient Roman soda-lime-silica glass.

 

 

1

2

3

4

5

Silica

SiO2

73.6%

80.0%

35.0%

96.5%

67.0%

Soda

Na2O

16.0

4.

--

--

18.0

Lime

CaO

5.2

--

--

--

8.0

Potash

K2O

0.6

0.4

7.2

--

1.0

Magnesia

MgO

3.6

--

--

--

1.0

Alumina

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

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?

 

Names, historical sources, and symbols

Silica (sand, quartz pebbles)

silicon dioxide

SiO2

Soda (soda ash (Na2CO3): natron, marine plant ashes

sodium oxide

Na2O

Lime (chalk, limestone (CaCO3))

calcium oxide

CaO

Potash (ashes of inland plants (K2O))

potassium oxide

K2O

Lead (oxidized lead metal)

lead oxide

PbO

Boron (modern mineral)

boric oxide

B2O3

Magnesium (impurity)

--

MgO

Aluminum (impurity)

--

Al2O3

Iron (impurity)

--

Fe2O3

 

Published on December 1, 2011