Glass is a mixture of metallic trioxosilicates(IV) and is prepared by heating a mixture of silicon(IV)oxide and the appropriate metallic oxide and trioxosilicates(IV) at temperatures of between 1300 and 1400 ºC. Small amounts of powdered glass and coke are normally added to improve the quality of the glass produced which depends largely on its composition.
Types of Glass
The following are the common types of glass and there chemical composition.
- Flint Glass:– Flint glass contains trioxosilicates(IV) of lead. It has a very high refractive index and appears brilliant when cut. Flint glass is used in making achromatic lenses because of its high refractive index. Flint glass is also used in making prisms.
- Lime-Soda Glass:– Ordinary glass is also called lime-soda glass. This type of glass is made from a mixture of washing soda, silica and lime stone. On heating this mixture, carbon(IV)oxide is evolved, leaving behind a mixture of sodium and calcium trioxosilicates(IV). Ordinary glass is soft and easily scratched. It also melts at low temperatures and easily cracked by sudden changes in temperature. This occurs because glass is a poor conductor of heat and does not expand or contract uniformly causing a strain to build up that makes it to break easily.
A hard glass can be obtained by using potassium trioxosilicate(IV) instead of sodium trioxosilicate(IV), or by increasing the proportion of silicon(IV)oxide.
- Colored Glass:– The different colors of glass are produced by the addition of small quantities of appropriate metallic compounds to molten glass. One peculiar property of glass is that it does not possess a sharp melting point but slowly softens before turning into a liquid when heat is applied. Glass is referred to as a super-cooled liquid because its molecules are randomly arranged similar to those in liquids and not in a definite crystalline matrix as is found in a solid.
The property of slowly softening when heated makes glass to be shaped as desired either by the application of pressure or by blowing. Articles made by this process must be annealed by heating them until they near their meting point and then allowed to cool gradually and uniformly so that no stress is formed on any part of the object or glass and prevent it from shattering easily.
- Heat-Resistant Glass:– heat-resistant glass is made by using about 80% of silicon(IV)oxide, small amounts of aluminum and sodium oxides and 12% of boron(III)oxide. This type of glass has a lower coefficient of expansion and a higher softening point than ordinary glass. It is used in making laboratory hardware and apparatus and also used in making cooking utensils. An example of heat-resistant glass is pyrex.
Aluminotrioxosilicates(IV) or zeolites are used as water softeners while sodium aluminum trioxosilicates(IV) are used as resins in ion-exchange columns for removal of calcium ions.
Clay on the other hand, is formed by weathering of certain silicate rocks that slowly breakdown to fine particles of hydrated aluminum trioxosilicates(IV) and silicon(IV)oxide. Clay is usually colored by impurities such as iron(III) oxide. However, the purest form of clay is kaolin otherwise known as china clay, which is white and soft and just like graphite, clay is made up of three layers of molecules that are loosely bound to one another, making it very slippery and plastic.
Clay is used in pottery and ceramics for the making of various household items such as tiles, cups, mugs and washing basins in bathrooms and toilets. During a manufacturing process, the clay is heated strongly to the appropriate temperature that would drive off all water of hydration. The product that comes up is a hard infusible solid which does not hydrate again on cooling. Clay is also used in making bricks, cement(made by heating limestone and clay together), as fillers of rubber, paint and paper.