Glass

The supremacy of glass can be explained by looking back to the origins of its discovery and understanding its many advantages. As glass was discovered about 4000 years BC, its production has been studied, developed and improved over the centuries, giving glass a leading role in many fields. And yet, between legends and clear facts, the history of glass remains mysterious. In his Natural History, Pliny the Elder tells us about the discovery of glass:

Several millennia BC,the journey between Egypt and Phoenicia was long. Phoenician merchants stopped for the night on the comfortable sand on the banks of the Belus river, near Sidon. The campfire was lit on the sand between natron bricks, used at the time in wool dyeing. After their meal, and after the usual endless discussions and travellers' tales, they fell asleep…

At dawn, the first to wake went to the dying embers to heat a little water. His cries woke his companions. What was happening? Trembling, he pointed to the natron bricks on the sand, transformed into clear, sparkling, hard jewels. This was no doubt the handiwork of the gods. But the leader of the caravan, calming them down, noticed that the new material encompassed both the natron and the sand. Relighting the fire beneath both the sand and the natron, he discovered that the new material resulted from the fusion of the sand and the natron. Taking hold of the still-soft material, he could mould it, making it into a container or turning it around a stick. An almost magical new material had been born. (SOURCE)

This romanticised story contains certain key points. The discovery of glass did indeed take place in the Middle Eastern regions that used to be known as Phoenicia and Mesopotamia, located in the modern countries of Lebanon and Iraq. From this point on, glass continued to intrigue and fascinate, and people began to explore its properties. The first glass objects were beads, necklaces and statuettes that seem to date back to 3000 BC. The first documents about trading in glass that have been found are from about 1400 BC. It appears that the Egyptians were already blowing glass at this time, and the Phoenicians were trading in Egyptian glassware. Glass appeared and had a certain success in cities such as Babylon and Assur. Here bricks have been found covered entirely in glass, to make them more solid. The Egyptians and Phoenicians' reputation for glass quickly grew and considerable glassworks were founded in the cities of Alexandria and Sidon. The market continued to spread and by about 1200 BC glass had reached Greece and Italy. Of course when these territories fell under the grip of the Roman Empire, most of the glassworks that had developed there had to work for Rome. It was thus under Roman stewardship, in the first century AD, that "millefiori" vases and the first flat glass appeared. Thanks to Rome's legendary organisation and administration, the merchants of the period circulated along Roman roads and spread the art of glass making throughout the empire. Belgium, for example, known as Gaul at the time, was introduced to glass in the third and fourth centuries. But when the empire fell in 476, Europe entered a dark period. All the glassworks were lost, and the only ones that remained during the Middle Ages were located in prosperous places and important cities such as Byzantium.

In the thirteenth century, Byzantium (which had become Constantinople) was sacked during a crusade and many master glass makers fled to Venice, giving the city something of a glass monopoly. To protect the secrets of glass production, and also because of the resulting fumes and odours, all the city's glassworks were transferred to the island of Murano to the north of Venice. The secrets could not be kept for long, however, and by the sixteenth century all of Europe held the keys to glass production. A little later, in the seventeenth century, France's finance minister, Jean-Baptiste Colbert, founded the Manufacture Royale des Glaces. The company was created to compete with Murano and became the supplier to the château of Versailles. It still exists, but now bears the name of Saint-Gobain, of which we will hear more later.

In the eighteenth century, the glass industry underwent a revolution. More and more of the objects of daily life could be produced thanks to improvements in furnaces. However, these furnaces needed fuel to run, which is why glassworks moved near to coal mines. In the nineteenth and twentieth centuries, new production and mechanisation processes (which we will examine later) were discovered and industrialisation intensified. The early twentieth century saw some important mergers in the Belgian glass industry, creating the UVMB (Union des Verriers Mécaniques Belges) et Glaver, which themselves later merged to become Glaverbel. Despite the crises of war and foreign competition, Belgium managed to become the world's leading glass producer in 1945. In 1960, the technique of float glass appeared, once again revolutionising the glass industry by surpassing all other techniques. These days, the glass industry is still going strong and the material is now used in a huge variety of fields, as explained above.

As we saw above, glass seems to have been discovered completely by chance. However, the many years that have passed since then have provided plenty of time to study and improve production techniques and processes. Surprisingly, the main constituents of glass have not changed much since its discovery. Instead, the evolutions that have taken place relate to the many ways of working the material.

Obsidian (source : geology.com)Firstly, it is interesting to note that we can find glass of natural origin, i.e. not made by humans. When the right conditions occur together, natural phenomena can produce glass. Two examples of this are obsidian and fulgurite. Obsidian is in fact a silicate rock (containing silica salt, one of the main components of sand) which vitrifies when it cools quickly. Obsidian is thus often found near volcanoes, and in prehistoric times it was often used to make arrowheads, blades and jewellery. This glass has also inspired many authors in the literary world and in video games, where it is often attributed legendary properties. Fulgurite, meanwhile, is formed when lightning strikes sand. The temperature of sand struck by lightning (and thus the silica it contains) can reach several thousand degrees, transforming it into natural glass in a cylindrical form. However, this glass is full of impurities from the sand and is thus neither transparent nor vitreous.

As we have already seen, the main constituent of glass is silica, also known as silicon dioxide (SiO2), which is found in large quantities in sand. The purest sands can contain up to 99%. Silica generally represents 70% of glass, but if its quantity is increased in the production process this also increases the hardness of the glass, as it will have a lower expansion coefficient. Nevertheless, a temperature of 1730°C is required to melt the silica, and a huge expenditure of energy is needed to achieve this.

Another element, known as a flux, can be added to silica to reduce this melting temperature to 1400°C.  The most commonly used flux is soda, or sodium oxide (Na2O). In the story of how glass was discovered, soda is found in the natron that formed the bricks placed in the fire by the travellers. Natron is actually a natural variety of sodium carbonate, which thus contains soda.

The third essential element for producing glass is a stabiliser. Adding a flux to the silica helps to lower the melting temperature, but it also increases its solubility in water. Another ingredient thus needs to be added to strengthen and consolidate the material. The most common stabiliser used in glass production is lime, or calcium oxide (CaO), because this increases the glass's chemical resistance and reduces its solubility.

Fulgurites (source : Oxford Museum of Natural History)Of course many other fluxes and stabilisers can also be used, including magnesium oxide (MgO) as a flux to produce window glass, or iron oxide (Fe2O3) as a stabiliser, which gives the glass a particular tint. In the case of crystal, the lime is replaced by lead oxide (PbO), which makes the glass more brilliant and easier to work with. To colour the glass, very small quantities of metal oxides are added to the composition. The different metals give the glass different colours – copper tints the glass green, for example. There are several different families of glass, which can be distinguished according to their basic components. The glass we have described above is called a soda-lime-silica glass, because it is made of soda, lime and silica. This is the most widespread type of glass, and is used to make windows and furnishings. By varying the oxide constituents, we can create borosilicate glass, which contains boric anhydride (B2O3) and is more resistant to heat. This type of glass is widely used to make laboratory and kitchen containers. A third family of glass is the glass-ceramic family, which consists of silica, alumina, titanium salt and lithium oxide. This type of glass is used for stove tops and telescopes. Finally, there is lead glass, which includes crystal, as we saw above, and glass that protects against X-rays (when the lead content is 60%). 

This brings us to the question of glass transformation techniques. Obviously the oldest and best known is glass blowing. This technique involves "gathering" molten glass from the furnace with a hollow tube or blowpipe and blowing into the pipe, firstly in short puffs and then more steadily to increase the volume of the glass blob, known at this stage as a "parison". This blowing technique was used for both flat glass (such as windows) and hollow glass (such as bottles and containers).

For hollow glass, a punty (a solid rod) is used to take hold of the glass when it reaches the desired volume and remove it from the hollow blowpipe. The glass is then shaped horizontally with a block (a wet wooden ladle) and vertically to lengthen it. The glassblower can add other sections, such as handles, and modify the form of the neck, created by detaching the blowpipe. The piece is then detached from the punty and placed in an annealing furnace, which distributes the heat uniformly through the piece to avoid any thermal shock.

There are two techniques for blowing flat glass. The first is the broad sheet technique, also known as the Lorraine technique, which was used until 1630. It involves taking a gob from the furnace, blowing it and swinging it so that it extends under its own weight. When a cylindrical form has been created, the blowpipe is removed and the ends are cut off the cylinder, keeping only a roll of glass. This is then split down its length and flattened in a spreading furnace. After finishing in the annealing furnace, we are left with a flat sheet of glass.

Glass Blowing Crown glass blowing (source: Van Ruysdael)

The second technique, which replaced broad sheet glass in 1630, produces crown glass, and was known as the Normandy technique. This involves blowing a globe of glass and then attaching it to a punty. The blowpipe is then removed, leaving an orifice in the glass globe, and the globe is then squashed. The flattened globe is transformed into a disc using centrifugal force. The globe is spun very quickly using the punty, widening the opening in the globe until it forms a glass disc. The disc is then separated from the punty, leaving a trace known as a bull's eye. The disc is then placed in the annealing furnace to harden.

Normandy Blowing Technique

Of course, techniques have evolved over time and the processes for producing hollow glass and flat glass have taken opposite directions While hollow glass is still produced by blowing, flat glass has moved away from this technique via many others over the last century. Let us first look at the evolution of hollow glass production techniques.

HOLLOW GLASS

The evolution of glass manufacturing processes has mostly involved mechanising blowing practices to increase productivity. The process itself has changed little: a gob of glass is gathered and then blown to the required volume before being placed in a mould and inflated to its final form. Today the process is varied depending on the object to be produced. For example, the blow and blow process is used for bottles and perfume containers, while the press and blow process is used for jars to contain dry or pasty foods and for fruit juice containers. agFor objects such as glasses, tumblers, plates and salad bowls, a pressing technique or a centrifugal technique (such as the crown glass technique) is used for round objects. If the product has to present a surface free of defects, such as a stem glass, the rotational press and blow technique predominates. In this process, the object is first pressed and then rotated in the mould while being blown. Finally, for decorative glass and art objects, manual processes have kept their monopoly. In all these operations, the glass is formed in two stages: first in the blank mould and then in the finishing mould, where it is cooled by the mould before being reheated to achieve a uniform temperature and solidify the piece. The transfers from one mould to the other are a very important element of the final object's quality, because of their effects on its temperature. The production of hollow glass is thus influenced by many temperature and time parameters, such as the starting temperature of the parison, the duration of each phase of the production cycle, the temperatures of the moulds etc.

FLAT GLASS

In the case of flat glass, many processes succeeded each other through the first half of the twentieth century until the milestone year of 1960, when the float glass process was invented. This process revolutionised the flat glass industry and is still used today to produce the vast majority of flat glass. Let us first look at the processes that existed before the 1960 turning point.

Fourcault process

fourcault-pittsburgh Fourcault process/Pittsburgh (source: New Glass Technology)

The Belgian engineer Émile Fourcault patented a flat glass production process at the beginning of the twentieth century. But due to lack of funds it was not until 1914 that his machine was built, replacing the flat glass blowing processes we saw above (the broad sheet and crown glass techniques). The Fourcault process operates quite differently. The glass is not blown at all – it is drawn out vertically. The drawing machine is positioned vertically above the molten glass, contained in a basin.

The molten glass at the surface flows through an essential part known as the debiteuse into the drawing machine. It passes between several rollers, which flatten and cool the glass. The sheets of glass can reach up to 15 metres in height. The process was improved in 1925 by the Pittsburgh Plate Glass Company, which had the idea of immersing the debiteuse completely in the basin of molten glass instead of floating it on the surface. This enabled the glass to cool better during the drawing process, limiting surface defects. The edges of the glass were cooled first, giving the sheet of glass a stable structure more quickly.

Libbey-Owens process

Libbey Owens Libbey-Owens process (source: New Glass Technology)

In 1918, the American engineer Irving Colbourn designed a new process for the Libbey-Owens company. This was also a glass drawing system, but it differed from the Fourcault and Pittsburgh processes. The molten glass was drawn up vertically from the basin for 1.50 metres and then curved horizontally by a bending roller. The horizontal glass cooled more uniformly, and the process was able to produce sheets of glass of any size.

In about 1940, a new twin grinding process appeared. Molten glass flowed naturally from the basin between two laminating rollers before entering the annealing furnace. It then entered the twin grinding and polishing machine. The word "twin" refers to the fact that the glass was ground on both sides simultaneously. This process delivered glass of very good quality, but unfortunately the production costs were high.

Float process

Sir Alastair Pilkington's float process appeared in 1960, and has dominated flat glass production ever since. In this process, molten glass is fed on to the surface of a bath of molten tin inside a closed furnace. To prevent the tin from oxidising, the process takes place in a reducing atmosphere, i.e. an atmosphere from which oxygen has been removed. Given that the density of molten glass (2.5) is lower than that of tin (7), the glass literally floats on the surface of the tin, like oil poured on to water. This means that the sheet of glass has a uniform thickness and is perfectly smooth, so that no additional polishing is required. The glass is then transferred to the annealing furnace to lower its temperature gradually and uniformly.

Float Process Float Process

All these processes produce a flat glass known as annealed glass. This means that the glass has been gradually cooled in an annealing oven, as we have seen in the processes described above. All the potential tensions have been eliminated from the glass and the glass is more solid. This type of flat glass can then be worked and transformed in a number of ways. We will discuss these different sorts of glass in the next section.

As we have seen, flat glass can be produced in a number of different ways, though the float process predominates today. Once flat glass has been produced, there are almost as many possible transformation processes as production processes. Flat glass is used in areas that concern us every day. Its three primary areas of use are construction (windows, furnishings), the automotive industry and solar energy (photovoltaic panels). Flat glass thus needs to comply with certain standards and essential characteristics associated with its uses, such as safety, insulation etc.  When it leaves the annealing furnace, the flat glass is called annealed glass, and this constitutes the basis for all other types of flat and window glass: single glazing, double glazing, triple glazing, tempered glass, laminated glass etc. This section is dedicated to all these different types of glass and their properties.

TEMPERED GLASS

The first type of glass, which is regularly mentioned but perhaps not well understood, is tempered or toughened glass. The tempering process, known since the eighteenth century, was industrialised about seventy years ago. Tempered glass began to be marketed in the late 1940s. Since then, it has been used in many fields, because tempering gives glass a specific advantage: when it breaks, it shatters into tiny blunt-edged pieces, minimising the risk of injury. This is made possible because the tempering process modifies and improves the glass's mechanical strength. It is five to six times stronger than normal glass.

Thermal tempering

The glass is heated to a temperature of 620°C, softening it, before being rapidly cooled. This sudden cooling is known as tempering. During this process, the exterior layers of the glass are cooled but the interior takes longer to harden. The interior tends to contract during the cooling phase that follows, while the exterior layers have already returned to their rigid state. This tension between the different layers of glass gives the material the new properties explained above. The tempering process can be applied to glass with thicknesses of 4 to 19 mm. It can also be carried out on glass that is a little thicker or thinner, but special precautions must be taken to avoid damaging the furnace. Several types of furnaces are used for this type of tempering, known as thermal tempering. A vertical tempering furnace is used for large, thick sheets of glass, but horizontal furnaces also exist and are more convenient for thinner sheets.

Chemical tempering

There is also a chemical form of the tempering process. This method involves tempering the glass in a bath of molten salts, usually potassium nitrate (KNO) for soda-lime-silica glass. The potassium nitrate replaces the soda in the surface of the glass, compressing the external layers of the glass. This makes the glass better able to resist mechanical tension and temperature differences. It is even stronger than thermally tempered glass. The process can also be carried out with glass less than 4 mm thick, unlike thermal tempering, which requires more precautions to be taken. However, chemical tempering is more costly and only affects the surface of the glass, which means that its mechanical strength is considerably reduced if the glass is scratched. Whether it is tempered thermally or chemically, this type of glass is considered to be a safety glass and its use is essential in many fields: in cars, household appliances (oven windows), interior furnishings, industrial equipment (automatic doors, lifts) and street furniture (bus shelters).

SEMI-TEMPERED GLASS

There is also a type of glass known as semi-tempered glass. As its name suggests, this glass has also undergone a thermal treatment as explained above, but it is not considered to be a safety glass. This is because the fragments are larger when it breaks and its mechanical strength is lower than fully tempered glass. However, semi-tempered glass is stronger than annealed glass and it generally remains in place when broken. It is thus used in areas where the glass needs to be reinforced but safety glass is not essential – in aquariums, for example. Like tempered glass, it cannot be reworked after tempering because it would lose its new characteristics. Sandblasting, cutting and engraving thus need to take place before tempering, but only glass up to 10 mm thick can be treated. The tempering process is the same as above, but the heating and cooling parameters vary for semi-tempered glass.

LAMINATED GLASS

PVB

Another type of safety glass that is often used is laminated glass. To produce this type of glass, a film of PVB (polyvinyl butyral) is generally inserted between two sheets of washed glass. The assembly of the PVB and the glass takes place in a closed atmosphere to avoid dust particles between the layers. Air is then eliminated from between the layers of glass and PVB and the assembly is sealed before the next step, known as autoclaving. This is the step in which the final bonding between the glass and the PVB takes place. The layers are subjected to 12 bar of pressure and a temperature of about 140°C to bond them together. The PVB interlayer, originally opaque, then becomes completely transparent. However, it is possible to produce PVB interlayers that remain opaque or are coloured. It should also be noted that PVB laminated glass is not moisture resistant. The use of laminated glass dates back a century. Legend has it that French chemist Édouard Benedictus discovered it quite by accident when working with liquid plastic in a beaker in 1903. When the beaker was dropped on the floor, he noticed that it cracked rather than breaking into pieces. He looked more closely and saw that the plastic he was studying had formed a thin film on the surface of the glass, preventing it from shattering. Édouard Benedictus studied the phenomenon in more detail, and in 1910 he submitted a patent for laminated glass, which he called Triplex. Almost immediately, laminated glass was used in cars, where it replaced annealed glass for the windows. When broken, laminated glass does not shatter because the fragments of glass remain bonded to the PVB interlayer. This limits the injuries caused by broken glass in car accidents.

EVA

In modern laminated glass, other materials can replace PVB. The most common alternative is EVA (ethylene-vinyl acetate), which has the advantage of 100% moisture resistance. The laminating process is the same as with PVB, but only 10 bar of pressure is required with EVA. In addition, the humidity and temperature do not need to be controlled during the laminating operation when using EVA. The use of EVA thus appears less restrictive than PVB for producing laminated glass.

WIRED GLASS

Wired glass is another type of safety glass, but it is not as strong as laminated or tempered glass. To produce wired glass, a metal mesh is inserted between two sheets of annealed glass. Wired glass thus has the same strength as ordinary glass, but when it breaks the pieces of glass remain in place and the metal grid prevents ingress. The mesh does not contribute to the glass's mechanical or thermal resistance. It should be noted that this type of glass is used less and less, and it is now being replaced by laminated glass.

INSULATING GLASS

All these types of glass can be used to produce glazing units with various advantages and qualities. Let us look at the different possibilities.

Single glazing

Single glazing used to be used in all situations, and could not offer all the possibilities available today. Each situation requires specific glazing, and more transformations of the glass are required. Single glazing is just a simple annealed glass as explained above. These days, single glazing has been more or less abandoned, as it does not provide adequate thermal insulation. Its use results in energy costs that are much too high. Single glazing is not too bad in terms of acoustic insulation, however, and it also makes an excellent firebreak when treated appropriately. And as we saw in the section on annealed glass, this type of glazing is still used decoratively – for glass cases or mirrors, for example.

Double glazing

The most commonly used type of insulating glass is double glazing. This consists of two panes separated by a dehydrated air gap providing thermal insulation. The glass panes are usually 4 mm thick and are held apart by a steel or aluminium spacer about 15 mm wide. This is described as a 4/15/4 double glazing unit, and it represents a large proportion of the glazing used today. The spacer is hollow and is generally filled with tiny desiccant beads to absorb any moisture. The unit is then coated with butyl or silicone to seal it. However, many other techniques can be used for the spacers. Some spacers are welded to the glass with metal, and some are organic. It is even possible not to use spacers, and to bond the two panes directly together, but this technique presents a number of production constraints. Spacers containing desiccant beads seem to be the favoured choice, because they offer very effective insulation. In addition, other elements than air can be used between the glass panes to provide better thermal insulation. Other gases are also good insulators. Dehydrated air can be replaced with argon or, for a slightly higher price, krypton. In the 1970s and 80s, a solution offering even better insulation was to create a vacuum between the panes. This technique had to be abandoned, however, because the lack of pressure inside the unit compromised the surrounding seals, causing condensation inside the unit. In any case, whether the interior is filled with air or another gas, double glazing removes the sensation of cold when touching the window, eliminates interior condensation and considerably reduces heat losses, unlike single glazing. It is easy to see why double glazing is preferable to single. With regard to the acoustic insulation or soundproofing of double glazing, several options are available. First, it should be noted that laminated glass insulates much better against noise pollution than simple annealed glass. Laminated double glazing will thus provide better acoustic insulation than ordinary double glazing. In addition, lamination with resin rather than traditional PVB improves the acoustic insulation still further. The thickness of the glass also plays an important role. For more effective insulation, standard practice is to use double glazing described as asymmetric. This means that the two glass panes are of different thicknesses. In general, the thickness of one pane is increased so that each pane can hide the weaknesses of the other when it reaches its critical frequency. Another process involves replacing the argon or krypton between the panes with another gas, sulphur hexafluoride (SF6), which is very effective at masking the noise of motorway traffic, for example. However, SF6 reduces the thermal insulation of the window and has a negative impact on the environment. In view of these two major disadvantages, this type of acoustic insulation is likely to disappear.

Triple glazing

Another option is triple glazing. As its name suggests, this consists of three panes of glass (instead of two), separated by steel or aluminium spacers as found in double glazing. The whole unit is also sealed with butyl or silicone. Although in thermal terms triple glazing offers much better insulation than double glazing, it cannot be used in all situations. Adding a third pane of glass to the unit considerably increases both its weight and its thickness. Checks must be carried out to ensure that these characteristics correspond to the required values. Triple glazing is thus only used in certain circumstances. In addition, due to its greater thickness, this type of glazing has lower solar and light transmittance than double glazing.

COATINGS

The insulation characteristics of all these types of glazing, whether double or triple, can be increased by adding coatings to one or more surfaces of the glass. Low-emissivity (also known as energy-efficient) and/or solar control glazing benefits from increased thermal insulation because of a coating of noble metals (generally silver). This layer, applied by sputter deposition, prevents the glass from transmitting too much heat to the exterior while allowing light and heat to enter. The coating is applied to the spacer side of the glass. More unusual is smart glass, another type of glazing with a coating of metal oxides. This double glazing contains a film coated with a metallic layer that conducts electricity. Inside the film are liquid crystals. When no electrical voltage is applied to the film, the crystals are mobile and randomly arranged. When light passes through, it is diffused in all directions, creating a frosted effect. Conversely, when a voltage is applied, the crystals line up, light can pass through easily and the glass appears transparent. It is easy to see that the possible transformations of flat glass are many, even countless. After this overview of the different transformation techniques and possibilities, let's look at the ones we offer at Sprimoglass. 

 
 

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