Constituents of Glass and their main functions

The various constituents and their functions are explained below:

1. SILICA: 

• It is the main constituent of all kinds of glass.
• If silica only used in the manufacture of glass, it could be fused at a very high temperature but it will give a good glass on cooling. Since it fuses at very high temperatures some alkaline admixture like sodium carbonate or potassium carbonate is added to make it fuse at lower temperature.
• The admixture also make the liquid silica more viscous and better workable.

2. POTASSIUM CARBONATE

• It is an essential component of glass, it is an alkaline.
• This renders glass in-fusible and give fire resisting properties to it.

3. SODIUM CARBONATE:    

• It is an alkaline.
• It is added in suitable proportion to reduce the melting point of silica and to impart viscosity to the molten metal.
• It quickens the fusion of glass and as such excess of it is harmful.

 4. LIME:

• It is added in the form of chalk.
• It imparts durability and toughness to the glass.

5. LEAD OXIDE:

• It makes glass bright and shining.
• It gives color to the glass and therefore its presence in glass is not much desirable.

6. MANGANESE DIOXIDE:

• It is also called as "glass makers soap", because it is used either remove color from glass or add color to it.
• It is added in suitable proportions to correct the color of glass due to the presence of iron in raw materials of glass.

7. CULLET:

• It is broken glass of the type desired to be manufacture that is added as a raw material to bring down the cost of production.
• It is added in small quantity to provide body to the glass.

8. COLOURING SUBSTANCES:

          From many years ago glassmaker was adding substances to glass's raw material or batch to produce coloured glass. Many of the substances used in the making of coloured glass by our forefathers can no longer be used due to their properties, which contravene all aspects of health and safety in the work place.

Metals Used to Impart Color to Glass
Cadmium Sulfide	Yellow
Gold Chloride	Red
Cobalt Oxide	Blue-Violet
Manganese Dioxide	Purple
Nickel Oxide	Violet
Sulfur	Yellow-Amber
Chromic Oxide	Emerald Green
Uranium Oxide	Fluorescent Yellow, Green
Iron Oxide	Greens and Browns
Selenium Oxide	Reds
Carbon Oxides	Amber Brown
Antimony Oxides	White
Copper Compounds	Blue, Green, Red
Tin Compounds	White
Lead Compounds	Yellow
Manganese Dioxide	A "decoloring" agent
Sodium Nitrate	A "decoloring" agent

There are also other factors, which have to be taken into consideration when producing coloured glass:-

1. The temperature of the melt/bath.
2. Temperature of reheat during the working of glass.
3. The temperature of the 'Lehr' (Annealing oven)
4. Duration of the melt/batch.
5. Time and temperature relationship at different stages in production.
6. The type of colorant being used.
7. Concentration of the colorant.
8. Atmosphere of the furnace.
9. The composition of the colorant within the glass composition as is the case when iron is added to the glass. The type of iron oxide formed decides if the glass will be blue or yellow.
10. The number of times the same glass is melted. (Repeated melting of the cullet will usually give a darker tone to the finished piece.).