Glass

Glass is one of the most common substances seen around us. Starting from windows of buildings to household items to the ubiquitous TV in the house to the mirror on the wall, glass exists everywhere. It would be very difficult to think of a world without glass. It has always held a fascination for us to look through it and see what's on the other side.
The main uses of glass is because of its transparency. But glass has other applications as well, for example, as an insulator (glass fibre), to reinforce other material such as plastic or even concrete! Of course, over centuries, objects of art have been made out of glass.
What causes the special property of glass? What is it made of anyway?
Glass is a combination of sand and other minerals that are melted together at very high temperatures to form glass. But sand is opaque, which means you cannot see through it. Isn't it amazing that hot sand becomes glass?
Sand is the main ingredient. This is mixed with soda ash and limestone and melted in a furnace at temperatures of 1700 °C to give basic glass. Other materials can be added to produce different colours or properties. Glass can also be coated, heat-treated, engraved or decorated.
The chemical name for sand is silica (silicon dioxide, SiO2). This has a glas melting point of over 2300 °C. When pure silica is melted or fused to make glass, it has very special properties and is called quartz glass.
One common substance added to simplify the processing of glass is sodium carbonate (Na2 CO3), which lowers the melting point to about 1500 °C. However, the soda makes the glass water soluble, which is usually undesirable, so lime (calcium oxide, from limestone), some magnesium oxide and aluminium oxide are added to provide for a better chemical durability. The resulting glass contains about 70 to 74 percent silica by weight and is called a soda-lime glass. Soda-lime glasses account for about 90 percent of manufactured glass.
As well as soda and lime, most common glass has other ingredients added to change its properties. Lead glass, such as lead crystal or flint glass, is more 'brilliant'. This is because adding lead increases the refractive index. This causes noticeably more "sparkles".
Boron may be added to change the thermal and electrical properties, as in Pyrex glass which can withstand very high temepratures (Borosil). Many lab glasswares use borosilicate glass. Adding barium also increases the refractive index. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters. These are used in movie projectors, which run for a long time and can get very hot. Cerium oxide is used for glass that absorbs ultraviolet (UV) radiation.
Ordinary soda-lime glasses sometimes have a green tint because of iron oxide or chromium oxide impurities. Manganese dioxide is added to remove the green tint caused by iron oxide. Sulphur, carbon and iron salts give colours ranging from yellow to black.
Besides the chemicals mentioned, in some furnaces recycled glass called "cullet" is added. The cullet may have been left over from another batch. Cullet leads to savings not only in the raw materials, but also in the energy consumption of the glass furnace. 
At higher temperatures glass is more like a thick, viscous liquid. So when it is still hot, it can be poured, blown, pressed and moulded into a variety of shapes. The liquid has to be carefully handled to prevent bubbles which can weaken the glass or reduce its transparency.
For instance the pipes and beakers used in a science lab are shaped by blowing the hot glass into shape. This has to be done quickly before the glass cools after which it will solidify. Technically speaking even at room temperature glass is still not a solid: but it is so viscous or 'sticky' it looks and feels like a solid.
Once the desired form is obtained, glass is usually annealed (cooled in a particular way) for the removal of stresses.
Various surface treatment techniques, coatings, or lamination may follow to improve the chemical durability, strength (bulletproof glass), or optical properties (like glazing and anti-reflective coating).

How glass is made
Glass was manufactured in open pits, as long ago as 3000 B.C. It was only in about 250 B.C. that the blowpipe was invented, when people learned that glass could be blown.
Around the 17th century glass became more common because of new processes that were developed to cast it into different shapes. The furnace still used coal and wood. In the early 19th century the cylinder method of creating flat glass was first used in the United States of America. Although circular rings were visible on the glass, it was popular since it was a material that would allow light through windows while keeping the wind out and the room warm.
Glass was mass produced after the invention of the glass pressing machine in 1827. The electric furnace in 1910 allowed more mass production of glass.
Modern glass objects such as car windscreens and windows of houses are no longer hand-made. The exact composition of the glass is very important. Also the rate at which the hot glass is allowed to cool will decide on the properties of the glass. However many glass companies will still have a master blower in case of some unusual or delicate requirement. Glass blowing is an art that requires a good knowledge, patience, and good lungs!
The true revolution came in the 1950s with the invention of float glass, which is now the most commonly used glass in the world in construction.

Float Glass (BOX)
In earlier centuries, window glass or flat glass was made by blowing large cylinders or large disks. These were more than 2 metres long and about 30 cm in diameter. After the cylinder was blown, it was cut open and flattened. The sheet was then cut into window panes. Many old panes still show the circular ring-marks of the original cylinder. Also the panes could not be made larger (which is why many old buildings have small windows, or else many small window panes set in a large space).
In 1848 Henry Bessemer, an English engineer, desgined a new system to produce a long ribbon of flat glass. The molten (hot liquid) glass was pressed between rollers to form the ribbon. However this ribbon was not smooth since the surface of the rollers was rough. So the surfaces of the glass needed polishing before use and this was expensive.
People tried to find a smooth body to set the glass on, so that it would cut costs. One solution was to lift up a thin sheet from a vessel of melted glass and wait for it to cool and solidify. But the quality of this glass was not so good.
The final solution was found by Sir Alastair Pilkington and Kenneth Bickerstaff in the UK. They used the fact that tin has a low melting point. They heated and melted the tin in a big bath and then poured the molten glass over it. The glass, being liquid, flows smoothly over the tin under its own weight (due to gravity) and forms a smooth, continuous layer or ribbon. The success of this method depends on carefully balancing the molten glass to get uniform thickness. It was only in 1960 that the method was fully achieved.
Nowadays, flat glass furnaces are used that are 9 m wide, 45 m long, and contain more than 1200 tons of glass. The raw materials, referred to as batch, are mixed together and heated to form a large pool of molten glass. The molten glass is fed into a bath of molten tin (about 3-4 m wide, 50 m long, 6 cm deep) through a delivery canal.
The amount of glass poured onto the molten tin is controlled automatically. The tin bath is in a protective atmosphere of nitrogen and hydrogen. That is, natural oxygen present in the air is removed to prevent oxidation of the tin. The glass flows out onto the tin surface forming a floating ribbon with perfectly smooth glossy surface on both sides with an even thickness of approximately 7 mm. Thinner glass is made by stretching the glass ribbon to achieve the proper thickness. Thicker glass is made by not allowing the glass pool to flatten to 7 mm. Machines called attenuators are used in the tin bath to control both the thickness and the width of the glass ribbon.
As the glass flows along the tin bath, the temperature is gradually reduced from 1100 °C until the sheet can be lifted from the tin onto rollers at approximately 600 °C. It then passes through the lehr where it is further cooled gradually so that it anneals (cools and sets) without strain and does not crack from the change in temperature. The glass travels down the rollers in the lehr for about 100 meters and comes out at the "cold end" where it is cut by machines.
Some tin is absorbed into the glass, and with a proper ultraviolet light a sheen can be seen which differentiates the tin from the non-tin side.

END OF BOX

Some glasses that do not include silica as a major constituent may have physical and chemical properties useful for their application in fibre optics and other specialized technical applications. These include fluorozirconate, fluoroaluminate, aluminosilicate, phosphate and chalcogenide glasses. A flat-panel computer screen uses thin sheets of special alkali-free glass.

The chemistry of glass
Matter can be solid liquid, or gas. You may have heard of crystalline solids. In such solids, there is what is called long-range order. This means that the atoms which make up the crystal are neatly ordered in the entire crystal. For instance, in a salt crystal made of sodium chloride, the sodium and chlorine atoms are uniformly placed to form a square lattice (CHECK!). It is this uniformity that gives a crystal its properties. An amorphous solid, on the other hand, has no long range order, and its atoms will not be neatly ordered.
So, what about glass? Glass is an amorphous solid, since it has no long range order. However, there is still a technical difficulty. Glass is found to have no proper melting point. For instance, at 0 °C, water freezes into ice. This is the transition temperature of water into ice. However, glass is formed by rapid cooling of a hot liquid (quenching). This liquid passes through a glass transition temperature when the supercooled atoms get frozen into the solid state. It does not have a transition similar to that of water-ice.
However, in practical terms, the glass containing the water in front of you or the glass of your TV screen is sufficiently solid that it is fine to think of it as a solid.

Natural glass
Sometimes, glass is created naturally when lava is produced from a volcano. The naturally high temperatures cause the sand to fuse into glass. This glass is usually black in colour because of the impurities and is called Obsidian. It has been known since the stone ages, when it was used to make sharp knives. The heat generated by lightning strikes also creates glass from sand.
When meteorites from outer space hit the earth, the heat of the impact also causes glass to form. These glasses were very controversial and for a long time geologists thought that they were a form of obsidian.
These glasses formed from meteor impacts are called Moldavite. They are usually an olive-green or dull greenish colour and mostly found in Central Europe, including Bohemia, Moravia in the Czech Republic occasionally in Austria and Germany. It is believed that they were formed after 15 million years after a single massive meteor impact in central Bohemia.
Glass has also been seen to form where nuclear explosives are tested by humans. When the explosion occurs underground, the sand fuses due to the high temperatures.

Man-made glass

Humans are thought to have made glass first in Syria, Mesopotamia or old Egypt. While small beads were made initially, soon it became a major trading industry between different powerful nations and their rulers. Even in India, the early manufacture of reddish brown glass beads can be traced back to the Indus Valley civilisation of 1730 B.C. Glass has also been mentioned in many old Indian texts.
Glass making soon moved from beads to ornaments and dishes to use in palaces and places of worship. Stained glass was first produced by architects in Southwest Asia using coloured glass rather than stone. By the 11th century, clear glass mirrors were being produced in Spain. The first glass factories were built by Muslim master craftsmen in the Islamic world. The first glass factories in Christian Europe were later built in the 11th century by Muslim Egyptian craftsmen in Corinth, Greece.
Beginning in the late 20th century, glass started to become highly collectible as art. Works of art in glass are housed in a variety of museums, including the Corning Museum of Glass in Corning, New York, which has the world's largest collection of glass art and history, with more than 45,000 objects in its collection.
Objects made out of glass include not only traditional objects such as vessels, vases, bottles, paperweights, marbles, beads, but an endless range of sculpture and installation art as well.

Why glass is transparent
Ordinary glass is commonly used because of its transparency to visible light. When light falls on an object it is absorbed, reflected, or simply passes through. A black object absorbs all the light falling on it and none is reflected back into the viewer's eye. So the absence of any colour is sensed as black. A mirror simply reflects back most of the light that falls on it.
A glass reflects a very small amount of the light and also absorbs hardly any. As a result most of the light passes through it, making it look transparent. This is a property of the atoms that make up the glass.
Another important reason for the transparency is the rather smooth and ordered glass surface. If it was not smooth, light would scatter unevenly off it. If natural light falls on such a rough object, all colours in the light are scattered and so the object ends up looking white rather than transparent. You may have seen that when glass cracks, the glass looks whitish at the rough edges of the crack.
There is another component of ordinary sunlight. This is the ultraviolet or UV light which is not visible to us. It turns out that ordinary glass actually absorbs UV light. This leads to the glass-house effect where glass-windows are used to heat rooms in a cold place.
If glass is made with pure silica (pure sand) it is called quartz glass and does not absorb UV light. These very expensive glasses are used in specialised appplications, for instance in an optical fibre core. An optical fibre carries light along its length. Such optical fibres are widely used in fiber-optic communication, which permits transmission over longer distances and at higher data rates than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss. Here, the signal is kept inside the core of the fibre by total internal reflection.