Float glass
Encyclopedia
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
made by floating molten glass on a bed of molten metal, typically tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, although lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
and various low melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces. Modern window
Window
A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...
s are made from float glass. Most float glass is soda-lime glass
Soda-lime glass
Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes, and glass containers for beverages, food, and some commodity items...
, but relatively minor quantities of specialty borosilicate and flat panel display
Flat panel display
Flat panel displays encompass a growing number of electronic visual display technologies. They are far lighter and thinner than traditional television sets and video displays that use cathode ray tubes , and are usually less than thick...
glass are also produced using the float glass process. The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington
Pilkington
Pilkington Group Limited is a multinational glass manufacturing company headquartered in St Helens, United Kingdom. It is a subsidiary of the Japan-based NSG Group...
, which pioneered the technique (invented by Sir Alastair Pilkington
Alastair Pilkington
Lionel Alexander Bethune Pilkington, and his associate Kenneth Bickerstaff, both of Great Britain, developed the world's first commercially successful manufacture of high quality flat glass using their float glass process...
) in the 1950s.
History
Flat glass
Flat glass, sheet glass, or plate glass is a type of glass, initially produced in plane form, commonly used for windows, glass doors, transparent walls, and windshields. For modern architectural and automotive applications, the flat glass is sometimes bent after production of the plane sheet...
was generally cut from large discs (or rondels) of crown glass
Crown glass (window)
Crown glass was an early type of window glass. In this process, glass was blown into a "crown" or hollow globe. This was then transferred from the blowpipe to a pontil and then flattened by reheating and spinning out the bowl-shaped piece of glass into a flat disk by centrifugal force, up to 5 or...
. Larger sheets of glass were made by blowing
Glassblowing
Glassblowing is a glassforming technique that involves inflating molten glass into a bubble, or parison, with the aid of a blowpipe, or blow tube...
large cylinders
Cylinder blown sheet
Cylinder blown sheet is a type of hand-blown window glass. It is created with a similar process to broad sheet, but larger cylinders are produced by swinging the cylinder in a trench. The glass is then allowed to cool before the cylinder is cut. The glass is then re-heated and flattened...
which were cut open and flattened, then cut into panes. Most window glass in the early 19th century was made using the cylinder method. The 'cylinders' were 6 to 8 ft (1.8 to 2.4 m) long and 10 to 14 in (254 to 355.6 mm) in diameter, limiting the width that panes of glass could be cut, and resulting in windows divided by transoms into rectangular panels.
The first advances in automating glass manufacturing were patented in 1848 by Henry Bessemer
Henry Bessemer
Sir Henry Bessemer was an English engineer, inventor, and businessman. Bessemer's name is chiefly known in connection with the Bessemer process for the manufacture of steel.-Anthony Bessemer:...
, an English
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
engineer
Engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...
. His system produced a continuous ribbon of flat glass by forming the ribbon between rollers. This was an expensive process, as the surfaces of the glass needed polishing. If the glass could be set on a perfectly smooth body this would cut costs considerably. Attempts were made to form flat glass on a molten tin bath, notably in the US. Several patents were awarded, but this process was unworkable.
Before the development of float glass, larger sheets of plate glass were made by casting a large puddle of glass on an iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
surface, and then polishing both sides, a costly process. From the early 1920s, a continuous ribbon of plate glass was passed through a lengthy series of inline grinders and polishers, reducing glass losses and cost.
Glass of lower quality, sheet glass, was made by drawing upwards from a pool of molten glass a thin sheet, held at the edges by rollers. As it cooled the rising sheet stiffened and could then be cut. The two surfaces were not as smooth or uniform, and of lower quality than those of float glass. This process continued in use for many years after the development of float glass.
Between 1953 and 1957, Sir Alastair Pilkington
Alastair Pilkington
Lionel Alexander Bethune Pilkington, and his associate Kenneth Bickerstaff, both of Great Britain, developed the world's first commercially successful manufacture of high quality flat glass using their float glass process...
and Kenneth Bickerstaff of the UK's Pilkington Brothers developed the first successful commercial application for forming a continuous ribbon of glass using a molten tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
bath on which the molten glass flows unhindered under the influence of gravity. The success of this process lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight. Full scale profitable sales of float glass were first achieved in 1960.
Manufacture
Float glass uses common glass-making raw materials, typically consisting of sandSand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, soda ash (sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
), dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, and salt cake (sodium sulfate
Sodium sulfate
Sodium sulfate is the sodium salt of sulfuric acid. When anhydrous, it is a white crystalline solid of formula Na2SO4 known as the mineral thenardite; the decahydrate Na2SO4·10H2O has been known as Glauber's salt or, historically, sal mirabilis since the 17th century. Another solid is the...
) etc. Other materials may be used as colourants, refining agents or to adjust the physical and chemical properties of the glass. The raw materials are mixed in a batch mixing process, then fed together with suitable cullet (waste glass), in a controlled ratio, into a furnace where it is heated to approximately 1500°C. Common flat glass furnaces are 9 m wide, 45 m long, and contain more than 1200 tons of glass. Once molten, the temperature of the glass is stabilised to approximately 1200°C to ensure a homogeneous specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
.
The molten glass is fed into a "tin bath", a bath of molten tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
(about 3–4 m wide, 50 m long, 6 cm deep), from a delivery canal and is poured into the tin bath by a ceramic lip known as the spout lip. The amount of glass allowed to pour onto the molten tin is controlled by a gate known as a Tweel.
Tin is suitable for the float glass process because it has a high specific gravity, is cohesive, and immiscible into the molten glass. Tin, however, oxidises
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
in a natural atmosphere to form Tin dioxide
Tin dioxide
Tin dioxide is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names , this oxide of tin is the most important raw material in tin chemistry...
(SnO2). Known in the production process as dross, the tin dioxide adheres to the glass. To prevent oxidation, the tin bath is provided with a positive pressure
Positive pressure
Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently if there is any leak from the positively pressured system it will egress into the surrounding environment....
protective atmosphere consisting of a mixture of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
.
The glass flows onto the tin surface forming a floating ribbon with perfectly smooth surfaces on both sides and an even thickness. 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. The glass ribbon is pulled off the bath by rollers at a controlled speed. Variation in the flow speed and roller speed enables glass sheets of varying thickness to be formed. Top rollers positioned above the molten tin may be used to control both the thickness and the width of the glass ribbon.
Once off the bath, the glass sheet passes through a lehr kiln
Lehr (glassmaking)
A lehr is a temperature-controlled kiln for annealing objects made of glass. The name derives from the German verb lehren meaning to teach and is cognate with the English lere also meaning to learn or acquire knowledge of ....
for approximately 100 m, where it is further cooled gradually so that it anneals
Annealing (glass)
Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change...
without strain and does not crack from the change in temperature. On exiting the "cold end" of the kiln, the glass is cut by machines.
Market
As of 2009, the world float glass market, not including China and Russia, is dominated by the four companies: Asahi Glass, NSGNippon Sheet Glass
is a Japanese glass manufacturer. In 2006 it purchased Pilkington of the United Kingdom. This makes NSG/Pilkington one of the four largest glass companies in the world alongside with another Japanese company Asahi Glass, Saint-Gobain, and Guardian Industries Corp.....
/Pilkington
Pilkington
Pilkington Group Limited is a multinational glass manufacturing company headquartered in St Helens, United Kingdom. It is a subsidiary of the Japan-based NSG Group...
, Saint-Gobain
Saint-Gobain
Saint-Gobain S.A. is a French multinational corporation, founded in 1665 in Paris and headquartered on the outskirts of Paris at La Défense and in Courbevoie. Originally a mirror manufacturer, it now also produces a variety of construction and high-performance materials.The company has its head...
, and Guardian Industries. Other companies include Sise Cam AS, PPG
PPG Industries
PPG Industries is a global supplier of paints, coatings, optical products, specialty materials, chemicals, glass and fiber glass. With headquarters in Pittsburgh, Pennsylvania, PPG operates in more than 60 countries around the globe. Sales in 2010 were $13.4 billion...
, Central Glass, Hankuk, Zeledyne, Cardinal Glass Industries.
See also
- Soda-lime glassSoda-lime glassSoda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes, and glass containers for beverages, food, and some commodity items...
, including composition and some properties of float glass - Glass productionGlass productionGlass production involves two main methods - the float glass process, which produces sheet glass, and glassblowing which produces bottles and other containers.-Glass container factories:...
- Glass batch calculationGlass batch calculationGlass batch calculation or glass batching is used to determine the correct mix of raw materials for a glass melt.-Principle:The raw materials mixture for glass melting is termed "batch"...
External links
- Float Glass Manufacture Process from Glass Association of North America