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54.2 RAW MATERIALS FOR MAKING GLASS

Glass is completely a vitrified product which has no definite composition. The major ingredients form over 90% of all types of glasses are Lime (CaCO3), Sand (SiO2) and soda ash (Na2CO3). The raw materials used in the manufacture of glass can be broadly divided. into the following categories:

  1. Acidic oxides
  2. Basic oxides
  3. Cullets and
  4. Colouring matter

1. ACIDIC OXIDES

A large variety of acidic oxides can be used. The choice depends upon the quality of glass to be made. The various acidic oxides that can be used are as follows:

  • Sand (SiO2). It forms silicates on fusion with the other ingredients. The sand used in glass manufacture should be pure and free from iron as it imparts colour to the glass. The sand is washed before its use in order to make it free from impurities. The sand used should not be too fine as it makes the reaction violent. It should not be too coarse either because it makes the reaction slow.
  • Borax (Na2B4O7.10H2O). It is a minor ingredient which supplies B2O3. Besides its high fluxing power, borax lowers the coefficient of expansion of glass and adds to its chemical durability.
  • Phosphorus(V) Oxide, P2O5. It is used as Na2HP04 or Ca3(PO4)2 and is added in tableware glass for imparting bright appearance.
  • Arsenious oxide, As2O3. It is generally used to remove air bubbles from the glass.

2. BASIC OXIDES

Some of the basic oxides used are as follows: ‘”

  • Limestone (CaCO3). It provides CaO. Sometimes burnt dolomite containing CaO and MgO is employed as a substitute of lime.
  • Feldspar. It is a naturally occurring mineral with a formula Na2O. A12O3. 6SiO2 (soda feldspar) or K2O.Al2O3.6SiO2 (Potash feldspar). It is cheap, pure and easily fusible. Feldspar is used as a flux, i.e., to lower the melting point, and to retard denitrification
    of glass.
  • Lead oxide. Litharge (PbO) or red lead (Pb3O4) is used for the production of heavy and high quality flint glass, which is used in optical instruments, tableware and decorative articles.

3. CULLET

It is the crushed glass from imperfect or defective articles or their trimmings. It makes the melting easy and also utilization of waste.

4. COLOURING MATTER

Coloured glasses are obtained by adding certain metallic oxide or salts in the fused mass. Various colouring agents and the corresponding shades are grouped as follows:

Red : CdO (1.4%), Se (1 %)

Ruby red : (i) Colloidal gold or ruby gold,         (ii) Cu20

Light yellow : Cerium oxide (2%), Titanium oxide (TiO2)

Deep blue : Cobalt oxide (CoO) (0.1%)

Greenish blue : Copper (II) oxide, CuO

Emerald green :Cr2O3 (0.15%), CoO (0.001%)

Amber : FeS (0.3%)

Black : MnO2 + Fe2O3

Yellow : CdS