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INDUSTRIAL CHEMISTRY MANUFACTURE OF GLASS Dr. G. SANGAMI Assistant Professor Department of Chemistry Sri Ramakrishna College of Arts & Science Coimbatore- 641 006 March 2024
GLASS • Glass is a hard and brittle substance that is usually transparent or translucent. • It may be comprised of a fusion of sand, soda, lime, or other materials. • The most common glass forming process heats the raw materials until they become molten liquid, then rapidly cools the material to create hardened glass.
Glass, an inorganic solid material that is usually transparent or translucent as well as hard, brittle, and impervious to the natural elements. • Glass has been made into practical and decorative objects since ancient times, • Building constructions • Housewares • Telecommunications. • It is made by cooling molten ingredients such as silica sand with sufficient rapidity to prevent the formation of visible crystals.
Glass breaks because of impurities caused by moisture or • dust in the air during glass production. • These impurities cause microscopic fractures and stress in the • glass that give way when too much pressure is applied
The agents used to colour glass are generally metallic oxides. • The same oxide may produce different colours with different glass mixtures, and different oxides of the same metal may produce different colours. • The purple-blue of cobalt, the chrome green or yellow of chromium, the canary colour of uranium and the violet of manganese are constant. • Ferrous oxide produces an olive green or a pale blue according to the glass with which it is mixed.
Ferric oxide gives a yellow colour but requires an oxidizing agent to prevent reduction to the ferrous state. Lead gives a pale yellow colour. Silver oxide gives a permanent yellow stain. Finely divided vegetable charcoal added to a soda–lime glass gives a yellow colour. Selenites and selenates give a pale pink or pinkish yellow. Tellurium appears to give a pale pink tint. Nickel with a potash–lead glass gives a violet colour, and a brown colour with a soda–lime glass. Copper gives a peacock blue, which becomes green if the proportion of the copper oxide is increased.
The Four Steps for Manufacturing of Glass 1. Collection of raw materials The raw materials such as silica (in the form of sand or quartz SiO2), soda ash (Na3CO3), limestone (CaCO3), and cullet (broken glass) are collected separately and mixed in a proper proportion. The fusion of cullet (broken glass) is added to bring down the melting point of the charge. 2. Preparation of Batch The raw materials, cullet, and decolourisers are finely powdered in grinding machines. These materials are accurately weighed in correct proportions before they are mixed. The mixing of these materials is carried out in mixing machines until a uniform mixture is obtained. Such a uniform mixture is known as batch or frit. It is taken for further process of melting in a furnace.
3. Melting or heating of the charge The glass batch is melted either in a pot furnace or in a tank furnace. It is made of fireclay or platinum. The heating is continued until the evolution of carbon dioxide, oxygen, sulfur dioxide, and other gases stops. (a) Pot furnaces Pot furnaces are used mainly in the manufacture of optical glass, art glass, and plate glass and in small scale units. The pots are crucibles made of selected clay, high alumina fire clay as mullite (3Al2O3.2SiO2), or platinum. (b) Tank Furnace In a tank furnace, both materials are charged into one end of a large 'tank' built of refractory blocks. The tank has a capacity of 1400 tons. The glass forms a pool in the heart of the furnace across which the flames play alternately from one side to the other. Heating is done by burning producer gas mixed with air over the charge. The cullet (broken glass) melts first and helps in the fusion of the rest of the charge. A high temperature of 1500-1800 C is maintained to reduce the viscosity of glass melt and to obtain a homogenous liquid.
The chemical reaction of the formation of glass in a furnace Heating is continued till the glass melt is free from gas bubbles like CO2, SO2, etc. Undecomposed raw materials and impurities form a scum called glass gall which is to be skimmed off. The clear liquid is now allowed to cool after the necessary decolorizers or coloring agents. It is cooled to 700-1200˚ C so that it will have the proper viscosity for shaping.
Processing of Glass (a) Shaping The molten glass is run into molds and automatic machines turn them into desired shapes such as sheets, tubes, rods, wires, etc. The molten glass is given proper shape according to our need. It can either be done by hand or by machine. Hand fabrication is adopted for small scale production and machine fabrication is adopted for large scale production. (b) Annealing After shaping, the glass articles need to be cooled down gradually and slowly. Rapid cooling may cause fracture and crack. Annealing is carried out in special chambers, where the temperature is brought down slowly. The entire process of annealing may require a few days. This whole process of slow and homogenous cooling of glass articles is known as the annealing of glass.
Why Annealing is important? The annealing of glass is a very important process. If the glass is allowed to cool down rapidly then the superficial layer of the glass cools down first and interior portions remain comparatively hot and therefore remain in the state of strain. Hence, if annealing is not done then glass articles may break into pieces under very slight shocks or disturbances. (c) Finishing After annealing the glass articles are subjected to finishing such as cleaning, grinding, polishing, cutting, etc. Certain decolorizing agents such as cerium oxide, neodymium oxide, etc. are added to neutralize an undesired color present in the glass.
TYPES OF GLASS • OPTICAL GLASS • COLOURED GLASS • LEAD GLASS
OPTICAL GLASS • The optical industry encompasses a wide range of applications that relate to the properties and motion of light. • From lenses for eyeglasses and cameras to fiber optics, optical solutions are a constant part of daily life. • Optical glass is a fundamental material used in many optical applications, including prisms, lasers, beam splitters, and other optical components.
Optical Glass Properties • Optical glass may contain a variety of additives, such as boric oxide, lead, zinc oxide, fluorite, and barium oxide to enhance its ability to transmit light within certain wavelengths. • Depending on the chemical composition of the glass, different wavelengths of light, both visible and invisible, can be absorbed, transmitted, or refracted to achieve the desirable optical effect for a given application. • The two most common types of optical glass are flint glass and crown glass. Flint glass is produced using lead, while crown glass contains a higher level of potassium oxide.
Applications of Optical Flat Glasses • Due to its exceptional level of clarity and durability, optical glass is the most commonly used material for a wide variety of optical applications, including • Lenses for analytical and medical equipment • Photographic lenses • Windows for optical systems and instruments • Glass substrates • Lead radiation glasses • Laser systems • Beam splitters
COLOURED GLASS • The glass which has some colours in it is called coloured glass. • Coloured glass is made by adding a small quantity of metallic oxides to the mixture of limestone or Calcium carbonate (CaCO3), soda ash or Sodium carbonate (Na2CO3), and sand (SiO2) • The colour of the coloured glass is decided by the type of metallic oxide added to the mixture. • Example: The addition of Nickel oxide (NiO) gives violet colour to the glass whereas the addition of Chromic oxide (Cr2O3) imparts emerald green colour to the glass. • Coloured glass is used to make decorative items, sunglasses, and to make lights for signals in automobiles.
