Ceramics Glass

Of the various glass families of commercial interest, most are based on silica, or silicon dioxide (SiO 2 ), a mineral that is found in great abundance in nature - particularly in quartz and beach sands. Glass made exclusively of silica is known as silica glass. Silica glass is used where high service temperature, very high thermal shock resistance, high chemical durability, very low electrical conductivity, and good ultraviolet transparency are desired. However, for most glass products, such as containers, windows, and light bulbs, the primary criteria are low cost and good durability, and the glasses that best meet these criteria are based on the soda-lime-silica system.

Common Glasses After silica, the many “soda-lime” glasses have as their primary constituents soda, or sodium oxide (Na 2 O; usually derived from sodium carbonate, or soda ash), and lime, or calcium oxide (CaO; commonly derived from roasted limestone). To this basic formula other ingredients may be added in order to obtain varying properties.

Common Glasses For instance, by adding sodium fluoride or calcium fluoride, a translucent but not transparent product known as opal glass can be obtained. Another silica-based variation is borosilicate glass (Pyrex), which is used where high thermal shock resistance and high chemical durability are desired—as in chemical glassware and automobile headlamps.

Lead Glasses In the past, leaded “crystal” tableware was made of glass containing high amounts of lead oxide (PbO), which imparted to the product a high refractive index (hence the brilliance), a high elastic modulus (hence the sonority, or “ring”), and a long working range of temperatures. Lead oxide is also a major component in glass solders or in sealing glasses with low firing temperatures.

Industrial Glassmaking Glasses of commercial importance are composed of a variety of chemical compounds. For glass manufacture on an industrial scale, these chemical compounds must be obtained from properly sized, cleaned, and treated minerals that have been pre- analysed for impurity. Silica (SiO 2 ) is obtained from clean sand. Appropriate mineral sources for soda (Na 2 O) are soda ash (sodium carbonate (Na 2 CO 3 )) and sodium hydroxide. Lime (CaO) is obtained from limestone (calcium carbonate (CaCO 3 )) or from dolomite (calcium magnesium carbonate) when magnesium oxide is also needed.

Additional Raw Material Because of the ready availability of the raw materials, glass is a most abundant material. In addition to the mineral ingredients, a glass batch traditionally consists of 25 to 60 percent cullet. Cullet is crushed rejected glass, generally of the same composition as the mineral mixture, that is included because its early melting in the furnace brings the mineral particles together, resulting in accelerated reactions. SiO 2 has a melting point of 2000 o K. The addition of Cullet (powdered scrap glass) lowers the melting point therefore reducing the energy required and the time taken to process the raw materials.

Characteristics of Glass Generally, glasses can be said to be; Brittle, Transparent (dependent on raw materials and additives) Hard, Chemically unreactive Electrically Insulating

Designing the Characteristics Most glasses consist of Silica (SiO 2 ) mixed with metal oxides. High volume products (bottles, windows etc.) are made from soda-lime silica glass. This glass has very poor thermal shock resistance. This is as a result of its high thermal expansion creating internal stresses when subjected to large temperature differentials. Glass is a brittle material so it is unable to resist the internal stresses resulting in the glass fracturing.

Pyrex Pyrex is a type of glass and glassware that is resistant to heat, chemicals, and electricity. It is used to make chemical apparatus, industrial equipment, including piping and thermometers, and ovenware. Chemically, Pyrex contains borosilicate and expands only about one-third as much as common glass (silicate) when heated. As a result, it is less apt to break when subjected to rapid temperature changes. It is resistant to many chemicals and is an electrical insulator. Fibres and fabrics made of it possess excellent heat insulation and fire-resistant qualities.

Treated Glasses Glass can be processed after manufacture to introduce specifically desired properties. Two very common forms of treated glass are –Toughened Glass –Laminated Glass

Toughened Glass Toughened Glass is made by heating the glass almost to melting point. The surfaces are then cooled, quickly, by blowing air over them, while the centre remains hot and plastic. This results in a compressive force being produced on the outer surfaces and a tensile force in the centre. When subjected to impact, it will shatter into tiny fragments (eg. Car windscreens)

Laminated Glass In lamination, the mechanical energy associated with applied stress is absorbed by successive layers of glass and laminate, leaving less energy for crack development. Most glass products are laminated by bonding sheets of tough polymers such as polyvinyl butyral, polyurethane, ethylene terpolymer, and polytetrafluoroethane (Teflon) to glass surfaces, generally by heat- shrinking.

Watch the video on laminating glass. Paired sheets of glass, 3 to 6 mm thick, with a fine coating of talc to keep them from fusing, are placed over a metal support frame. The two plies are heated almost to softening, at which point bonding occurs basically by gravity action. After cooling, the plies are separated and a polymer interlayer introduced. The entire laminated assembly is gently heated in an electric furnace and either squeezed through a pair of rollers or pressed between moulds. Not only does the interlayer help to absorb the energy of an impacting object, but the adhesion of glass to the polymer minimizes the risk of flying shards upon fracture.

For aircraft, windshields may have several laminates, sometimes as many as three glass plies and two plastic interlayers. Bulletproof glass is often laminated, although a single ply of dead-annealed glass as thick as 20 to 25 millimetres is used in some applications. The reason for having dead-annealed glass is the absence of tension in the interior; internal tension would cause the glass to shatter upon impact of the first bullet, thereby rendering the person behind the glass vulnerable to the second bullet.

Glass as a structural material Glass is increasingly being used as a structural material. Plate glass and glass bricks can be used as wall and flooring materials

Structural properties of glass Glass bricks and plate glass are ideal structural materials as they are resistant to tensile and compressive forces. They are also transparent, allowing light to pass through the walls. This can have aesthetic and psychological benefits as it allows natural light into buildings and can visually link spaces, creating more interesting interiors.