Glass manufacture has been practised for thousands of years, and examples of Roman glass used for small items, such as bowls and jars, can be seen in museums across the world. The process of making small sheets of glass also developed over millennia, using two methods, known as crown glass and broadsheet glass. The glass from both these processes was cut into smaller pieces, and used for leaded lights1.
Examples of how these techniques have been perfected can be seen in the stained glass windows of churches, while some ancient buildings also retain their original window glass.
But how did we get from there to where we are now, with windows that we can see through, and which are sometimes double glazed?
Broadsheet glass was made by blowing a long tube-shaped bubble of glass, the ends of which were then removed and the resulting cylinder was cut open down one side and flattened on a piece of iron. This glass was at best merely transparent, due to the imprecise method of manufacture. It would let some light through, but not allow you to see through it clearly.
Crown glass was made by blowing a bubble of molten glass, opening the end opposite the blowpipe while it was still molten and spinning it out into a disk. Even though it contained air bubbles, and circular ripples, it was much better quality than broadsheet glass, but still of small size, so windows continued to be multi-paned. The central pane cut from these bullions contained the 'bulls-eye', the thickened area where the glass was attached to the blowpipe, the rod used to spin it.
The technology of glass-making has changed significantly since these techniques, but the basic principles have not. Soda-lime glass has four basic ingredients:
- Silica from sand
- Soda ash from plants, mines or the alkali industry
- Dolomite, (calcium magnesium carbonate, in the form of rock)
- A little limestone from quarries
This is formed into a batch, heated to 1,000°C or so, then spun, dragged, rolled or floated to make it flat. Earlier processes had to grind and polish the glass to remove blemishes, whereas modern processes produce so-called 'fire finish' transparent glass without this additional work.
There is a further ingredient, recycled glass, known as cullet, which accounts for 15% of the batch. This reduces the amount of energy needed to make the new glass.
The float glass method was invented in the 1950s by Ken Bickerstaff and Sir Alastair Pilkington2, who both worked for Pilkington in the UK. This method is now used worldwide to manufacture around 85% of all glass for windows. Before long all window glass will be made using this technique. In it, liquid glass is floated on a bed of molten tin - which is used because it has a high specific gravity - and drawn along at a controlled speed, which defines its thickness. Meanwhile, its top surface is polished by a layer of nitrogen gas. Glass thickness can be selected from a few tenths of a millimetre to 25 millimetres. It is cooled gradually, but not too slowly, to prevent crystallisation, and then cut into the required sizes. Computers, linked to lasers, measure and cut sheets to avoid blemishes. Other processes may also be added to introduce special properties like toughening and laminating. One of the main advantages of the float glass process is that the glass emerges, as mentioned, with a 'fire finish', with the lustre of new china-ware. The glass may subsequently be shaped after reheating it to 600°C. It then becomes malleable and can be formed into curved shapes using moulds or gravity.
The Pilkington factory was established in 1826 in St Helens in Lancashire, England, a mere 20km from the port of Liverpool, and handy for its main supplies of silica, soda ash and coal for its furnaces. Good communications via turnpike roads, canals and later railways, as well as being close to its main suppliers, including the local coalmines, all combined to keep costs low and avoid transport disruption. Initially it used soda ash from Muspratt and Gamble, a local alkali producer.
The main material cost in glass-making is soda ash (sodium carbonate, or washing soda, which reduces the melting point of silica from 2,300°C to around 1,500°C). Soda ash is 60% of the material cost, although only 16% by weight of the total batch. So a low cost and reliable source of this material was paramount. Initially it was made by burning certain plants, including seaweed (20,000 tonnes of Scottish kelp was collected in the season), or by extraction from mines, especially in North America, from where all North American requirements are derived today.
The alkali industry was founded to supply soda ash, firstly, using the Leblanc3 process (beginning in the 1820s) followed by the cleaner, and lower cost Solvay4 process (beginning in the 1870s) using salt as the main ingredient. The region of Lancashire and Cheshire where the alkali industry was centred was rich, and still is, in salt mines and brine deposits. The other main alkali manufacturers were in Glasgow and on Teeside. Not only does the industry provide the basic chemical materials required in making glass but also in many other other processes including soap, paper and textile manufacturing. The first company to use the Leblanc process was the partnership between two Dubliners, James Muspratt and Josias Gamble, in St Helens in 1823.
In 1851 the window tax (some say the phrase 'daylight robbery' stems from this tax), was repealed in the UK, which led to rapid growth in Pilkington's output of window glass from 50 tonnes each week to 150 tonnes by 1854. The supply of soda ash became so important to the business in this period that the company decided to build their own factory for its production in nearby Widnes, which at that time was the centre of the Leblanc alkali industry. The industrialist John Hutchison, the so-called father of Widnes, moved from St Helens in 1847 to build the first chemical business in the town. This employed many of the chemists and entrepreneurs of the day who went on to set up their own businesses in the chemical industry, one being Henry Deacon who moved from Pilkington's to set up his own business in Widnes, after a spell with Hutchinson's. He was also a friend of Michael Faraday. Henry Deacon married Emma Wade, after whom the present Wade Deacon High School is named. Widnes was closer to the Cheshire salt mines, and connected by rail to the Lancashire coalfields, as well as lying on the north bank of the river Mersey, which subsequently became heavily polluted. It also had a ready supply of labour from Irish immigrants escaping the potato famine.
Brunner Mond used the cleaner Solvay5 process exclusively in the UK. John Brunner6 and Ludwig Mond from Germany set up this company, which later became one of the core companies that formed ICI7. Its major customer for soda, in the form of caustic soda, was Lever Brothers8, which became the dominant soap manufacturer in the UK after the takeover of its main competitors including Pears, Gossages of Widnes and Crosfields of Warrington. Gossages moved into making soap in 1855, after dabbling with soda ash manufacture, before being bought by Brunner Mond and then Lever Brothers. Crosfields began making soap in 1813, and also dabbled in making soda ash. After several changes of ownership, including Brunner Mond, Lever Brothers and ICI, the company is now owned by PQ Corporation.
Chemical and glass-making industries have several things in common. They are process industries, where raw materials are converted into products, either different chemicals or types of glass, in a continuously controlled process, using complex chemical and mechanical engineering involving furnaces, belching chimneys, many pipes, valves and smells that need significant capital investment, all of which have a life span of 10 to 15 years.
They are also global industries, and have grown by cross-licensing their technologies and trade secrets, setting up joint ventures, merging, acquiring and continually reorganising their centres of production and products, as new processes make existing ones obsolete.
Today, more than 40 manufacturers in 30 countries, with almost 400 float lines in service are using the float glass process. Around 1m tonnes of float glass is made every week.
Glass is very heavy9, and transporting it is uneconomic above around 600km by road, making local manufacturing necessary in many markets. Transport by sea does not have the same restrictions.
In 2009 some 53m tonnes of flat glass were manufactured, around six billion square metres, valued at 23 billion euros. Some of this is processed further to create additional qualities, giving a market value of around 50 billion euros. Seventy percent is used for windows in buildings, 20% for internal use, and 10% for vehicles.
Four companies - NSG Group, (the owners of Pilkington since 2006), Saint-Gobain, Asahi and Guardian - produce over 60% of the world's float glass.
Self-cleaning glass, in combination with solar radiation 'control' coatings, low-emissivity energy-saving glass, laminated safety glass and noise-reduction laminates have been developed for building and automotive use. An innovation is the use of flat glass in photovoltaic applications to generate electricity from solar energy.
Changes to building regulations now demand that low-emissivity glass is used in Germany, which has seen the market increase from under two million square metres in the 1990s to over 25 million now. This trend is being repeated in the UK, France and other countries, as legislation comes into force, dramatically increasing the requirement for this type of coated glass.
A way to reduce air conditioning costs is to use solar radiation control glass. This has a special coating applied to its surface, which reflects up to 75% of the solar heat whilst transmitting the majority of the visible light.
The demand for energy efficiency is also increasing the market for photovoltaic and solar thermal energy panels. Spain has recently introduced building regulations that require photovoltaic and solar thermal panels be used in new buildings. Many other countries have introduced grants and other incentives to encourage its use. In all of these systems, glass is an integral component.
Over the last 35 years, motorcar glazing has increased by around 50%. Average windshield glazed area has increased by over 60%. New styling requirements for windscreens that extend into the roof, or wrap around into the side of the vehicle, have needed improvements in both gravity sag and press bending technologies used to form flat glass into shapes.
Glass has become an important component in buildings and vehicles, not just for styling and light transmission, but also for energy conservation, privacy, solar power generation and energy use control. It seems that this versatile material, which was invented many thousands of years ago, has many more years ahead of it in serving humanity in many different ways. Will we see intelligent window glass that can allow light through in either or both directions, and do the same for heat energy depending on temperature differences? Will it also, at the same time, be generating electricity from solar energy, as well as being self-cleaning? We shall see.