Glass is not only one single compound. It is a highly complex mixture, which cannot be assigned any particular chemical formula. The approximate formula may be given as:
x R2 O.y MO. 6SiO2
Where R = Alkali metal
M = Bivalent metal
x and y are number of molecules.
The main ingredients for the manufacture of glass are silica, sodium carbonate (soda ash), borax, alumina and waste glass pieces or cutlet. The specification of the raw follows: –
(a) SILICA OR SAND (SiO2)
The sand used in the manufacture of glass should be pure quartz. The iron contents of the sand must be as low as possible (0.5% for table glass and 0.0753% for optical glass). The presence of iron imparts green or brown color to the glass.
(a) BORIC OXIDE (B2O3) This oxide is added in the form of
(b) Orthoboric acid (H3 BO3)
(c) Borax (Na2B4 07.10 H2O).
The function of boric oxide is to impart the property of high refractive index and low dispersion power to glass. It also lowers the co-efficient of expansion and acts as a flux..
(c) PHOSPHORUS PENTAOXIDE (P2 O5)
This oxide is added in the form occalcium phosphate. It produces translucent glass.
(d) ARSENIC OXIDE (As2 O3)
This oxide is used as such. It acts as a decolorizer and removes the air bubbles encaged in the molten glass.
2. BASIC OXIDE
Common basic oxides used are
a. SODA ASH
Sodium Oxide present in the glass may be added in the form of Soda ash (Na2CO3), Salt cake (Na2S04) or sodium nitrate (NaNO3). The nitrate is useful in oxidizing iron. Sodium carbonate is replaced by potassium carbonate while making Jena and hard glass. Such glasses have a high melting points and resistant to chemicals
LIME OR CALCIUM OXIDE
This oxide is added in the form of lime (CaO) or lime ston (CaCO3). Calcium oxide is an essential part of commercial soft window glass. It increases the stability of glass.
This oxide is added in the form of an ore called Feldspar (K2O.Al2 03.6Si02). The function of the aluminum oxide is to lower the melting point range of glass. It also decreases the co-efficient of linear expansion.
(d) LEAD OXIDE
This oxide is added as such and lead glasses are of very great importance in optical work because of their high index of refraction and high dispersion. Large quantities are used for the construction of electric light bulbs, neon-sign tubing and radio tones because of the high electric resistance e of the glass.
These are the broken pieces of the waste glass and facilitate melting. The quantity of cutlet added varies from 10 to 50 percent of the total. charge.
Various substances used for the production of different colours in glass are: