Float Glass Production Process

History of float glass

In the earlier days, window glass was made by blowing glass bottles or large glass disks. The bottles were cut into pieces, flattened together and then window panes were cut from the large surface. Most glass for windows up to the early 19th century was made from rondels, while during the 19th century it was done using the bottle method.

Alastair Pilkington has been identified by many sources as the inventor of the float glass process, even though it was first patented in 1848 by Henry Bessemer, an English engineer. Before the development of float glass, larger sheets of plate glass were made by casting a large puddle of glass on an iron surface, and then grinding and polishing both sides for smoothness and clarity - a very expensive process.

Float glass

Float glass is sheet glass made by floating molten glass on a bed of molten tin. This method gives the glass uniform thickness and a very flat surface. Float glass is more commonly known as window glass. Because it is inexpensive and sometimes free, it is often used in the glass fusing process. The molten glass spreads onto the surface of the metal and produces a high quality, consistently level sheet of glass that is later heat polished. The glass has no wave or distortion and is now the standard method for glass production; over 90% of the world production of flat glass is float glass.

Basic float glass process



The phrase “to float” means “to be buoyant”. And this is basically the principle on which the float glass manufacturing process is based. In the float glass process, molten glass is fed onto a float bath of molten tin. This tin bath is 4-8 meters wide and up to 60 meters long. To prevent the tin surface from oxidizing with the atmospheric oxygen, the tin bath is placed under a protective gas atmosphere. This atmosphere must be carefully controlled since its composition is instrumental for the properties of the contact surface between the glass and the tin which, in turn, influence the thickness of the glass sheet.

The glass floats like an endless ribbon on the tin. At the entrance where the glass first makes contact with the tin surface, the temperature of the liquid metal is about 600oC. Tin is the only metal that remains in a liquid state at 600oC.

Immediately after the exit from the float chamber, special rollers take up the glass and feed it into the annealing lehr from which it exits at about 200oC. After cooling to room temperature on an open roller track, it is cut, packed, and stored either for shipment or for further processing into products such as safety glass, reflective glass, self-cleaning glass, mirrors or double glazed or multi-glazed units.

Float glass can be made in thickness between 1.5 to 20mm. There are two techniques to accomplish this. To produce thin float glass, rollers control the width and speed of the glass ribbon. For thick float glass, the glass floats against graphite barriers, so that the ribbon flows out thicker. Thus the desired widths and thicknesses can be achieved.

While each glass plant is different from the other, the float glass production process can be divided into five universal steps:

1. Batching of raw materials:

The main components, namely, soda lime glass, silica sand (73%), calcium oxide (9%), soda (13%) and magnesium (4%), are weighed and mixed into batches to which recycled glass (cullet) is added. The use of ‘cullet’ reduces the consumption of natural gas. The materials are tested and stored for later mixing under computerised control.

2. Melting of raw materials in the furnace:

The batched raw materials pass from a mixing silo to a five-chambered furnace where they become molten at a temperature of approximately 1500°C.

3. Drawing the molten glass onto the tin bath:

The molten glass is "floated" onto a bath of molten tin at a temperature of about 1000°C. It forms a ribbon with a working width of 3210mm which is normally between 3 and 25mm thick. The glass which is highly viscous and the tin which is very fluid do not mix and the contact surface between these two materials is perfectly flat.

4. Cooling of the molten glass in the annealing lehr:

On leaving the bath of molten tin, the glass - now at a temperature of 600°C - has cooled down sufficiently to pass to an annealing chamber called a lehr. The glass is now hard enough to pass over rollers and is annealed, which modifies the internal stresses enabling it to be cut and worked in a predictable way and ensuring flatness of the glass. As both surfaces are fire finished, they need no grinding or polishing.

5. Quality checks, automatic cutting, and storage:

After cooling, the glass undergoes rigorous quality checks and is washed. It is then cut into sheets of sizes of up to 6000mm x 3210mm which are in turn stacked, stored and ready for transport.

Applications

1. Float glass is used for glazing wherever full transparency is required in buildings.
2. It is used as a base material for safety glass, reflective glass and self-cleaning glass, among others.
3. It can be used in precision mechanics, especially where extreme surface flatness is required. Eg., for visual displays.
   

History of Glass Making

What is Glass?

Glass is a type of solid material which is typically brittle and transparent. Glass is commonly used for bottles, glasses, furniture, windows, building facades, and even eyewear. Glass is defined as an inorganic product of fusion which has been cooled through its transition into the solid state without crystallizing. Most glass contains silica as its main component. The term glass was coined in the Roman Empire several centuries ago.

Spark of Glass

Before the human race started to manufacture glass, they had found natural glass in two different forms. When lightning strikes sand, the heat makes sand to fuse into long, slender glass tubes called fulgurites. This kind of glass is commonly called petrified lightning. The tremendous heat from a volcanic eruption also sometimes fuses rocks and sand into a type of glass called obsidian.

Obsidian or Volcanic Glass



In early times, people shaped obsidian into knives, arrowheads, jewellery, and even money. Obsidian was highly prized in prehistory wherever it was found. The glassy material came in a range of colours – right from black and green to bright orange, and was found wherever rhyolite-rich volcanic deposits were found. The shiny beauty, fine texture, and the sharpness of its flaked edges made obsidian a very popular trade item.

It is generally believed that the first manufactured glass was in the form of a glaze on ceramic vessels, around 3000 B.C. The first glass vessels were produced in 1500 B.C. in Egypt and Mesopotamia. The glass industry was extremely successful for the next 300 years, and then saw a decline. It was revived in Mesopotamia in 700 B.C. and in Egypt in 500 B.C. For the next 500 years, Egypt, Syria, and the other countries along the eastern shore of the Mediterranean Sea became glassmaking centers.

At the early stages, glassmaking was a slow and expensive process, and required hard work. Glass blowing and glass pressing were unknown, furnaces were small, clay pots were of poor quality, and the heat was hardly sufficient for melting. But glassmakers eventually learned how to make coloured glass jewellery, cosmetics’ cases, and tiny jugs and jars. People who could afford them—the priests and the ruling classes—considered glass objects as valuable as jewels. Soon merchants learned that wines, honey, and oils could be carried and preserved far better in glass bottles than in wood or clay containers.

Turning point with blowpipes

The blowpipe was invented in 30 B.C., probably along the eastern Mediterranean coast. This invention made glass production easier, faster, and cheaper. As a result, glass became available to the common people for the first time. The long thin metal tube used in the glass blowing process has changed very little since then. In the last century BC, the ancient Romans then began blowing glass inside moulds, greatly increasing the variety of shapes possible for hollow glass items.

Glassblowing

Glassblowing is a glass forming technique that involves inflating the molten glass into a bubble, or parson, with the aid of the blowpipe, or blow tube. A person who blows glass is called a glassblower, glass smith, or gaffer. Free-blowing is a kind of glass blowing technique.

Free-blowing

This glass making technique was used until the late nineteenth century and is still widely used. The process of free-blowing involves the blowing of short puffs of air into a molten portion of glass which is gathered at one end of a blowpipe. This has the effect of forming an elastic skin on the interior of the glass blob that matches the exterior, formed by the removal of heat from the furnace. The glassworker can then quickly inflate the molten glass to a coherent blob and work it into a desired shape.

First Golden Age of Glass: Roman Empire

Glassblowing was greatly encouraged under the Roman rule. Glass manufacture became important in all countries under Roman rule. In fact, the first four centuries of the Christian era can justly be called the First Golden Age of Glass. The glassmakers of this time knew how to make transparent glass, and knew offhand glass blowing, painting, and gilding (application of gold leaf). They knew how to build up layers of glass of different colours and then cut out designs with high precision.

It was the Romans who began to use glass for architectural purposes, with the discovery of clear glass (through the introduction of manganese oxide) in Alexandria around AD 100. Cast glass windows, albeit with poor optical qualities, thus began to appear in the most important buildings in Rome and the most luxurious villas of Herculaneum and Pompeii.

The decline of the Roman Empire and culture slowed progress in the field of glassmaking techniques, particularly through the 5th century. Germanic glassware became less ornate, with craftsmen abandoning or not developing the decorating skills they had acquired.

Early Middle Age

Towards the year AD 1000, a significant change in European glassmaking techniques took place. Given the difficulties in importing raw materials, soda glass was gradually replaced by glass made using the potash obtained from the burning of trees. At this point, glass made in the north of the Alps began to differ from glass made in the Mediterranean area, with Italy, for example, sticking to soda ash as its dominant raw material.

Second Golden Age of Glass

Glass manufacture had developed in Venice by the time of the Crusades (A.D. 1096-1270), and by the 1290's, an elaborate guild system of glassworkers had been set up. Equipment was transferred to the Venetian island of Murano, and the Second Golden Age of Glass began. Venetian glass blowers created some of the most delicate and graceful glass the world had ever seen. They perfected Cristallo glass, a nearly colourless, transparent glass, which could be blown to extreme thinness in almost any shape.

Sheet Glass

The 11th century also saw the development by German glass craftsmen of a technique - then further developed by Venetian craftsmen in the 13th century - for the production of glass sheets.

By blowing a hollow glass sphere and swinging it vertically, gravity would pull the glass into a cylindrical "pod" measuring as much as 3 meters long, with a width of up to 45 cm. While still hot, the ends of the pod were cut off and the resulting cylinder cut lengthways and laid flat.

Glazing remained, however, a great luxury up to the late Middle Ages, with only buildings like royal palaces and churches adorned with glass windows. Stained glass windows reached their peak as the Middle Ages drew to a close.

By the late 1400's and early 1500's, glassmaking had become important in Germany and other northern European countries. It became important in England during the 1500's.

Lead Glass



By 1575, English glassmakers were producing Venetian-style glass. In 1674, an English glassmaker named George Ravenscroft patented a new type of glass in which he had changed the usual ingredients. This glass, called lead glass, contained a large amount of lead oxide. This brilliant glass with a high refractive index was very well suited for deep cutting and engraving.

Plate Pouring Process

In 1688, in France, a new process was developed for the production of plate glass, principally for use in mirrors, whose optical qualities had, until then, left much to be desired. The molten glass was poured onto a special table and rolled out flat. After cooling, the plate glass was ground on large round tables by means of rotating cast iron discs and increasingly fine abrasive sands, and then polished using felt disks. The result of this "plate pouring" process was flat glass with good optical transmission qualities. When coated on one side with a reflective, low melting metal, high-quality mirrors could be produced.

Glass in America

Sandwich Glass, an early American glass was made by the Boston and Sandwich Glass Company, founded by Deming Jarves in 1825. In the early 1800's, the type of glass in greatest demand was window glass. At that time, window glass was called crown glass.

Crown Glass

Other types of sheet glass included crown glass (also known as "bullions"), relatively common across Western Europe. With this technique, a glass ball was blown and then opened outwards on the opposite side to the pipe. Spinning the semi-molten ball then caused it to flatten and increase in size, but only up to a limited diameter. The panes thus created would then be joined with lead strips and pieced together to create windows.

Cylinder Process

By 1825, the cylinder process had replaced the crown method. In this process, molten glass was blown into the shape of a cylinder. After the cylinder cooled, it was sliced down one side. When reheated, it opened up to form a large sheet of thin, clear window glass.

In the 1850's, plate glass was developed for mirrors and other products requiring a high quality of flat glass. This glass was made by casting a large quantity of molten glass onto a round or square plate. After the glass was cooled, it was polished on both sides.

Modern Flat Glass Technology



In the production of flat glass, the first real innovation came in 1905 when a Belgian named Fourcault managed to vertically draw a continuous sheet of glass of a consistent width from the tank. Commercial production of sheet glass using the Fourcault process eventually got under way in 1914.

Around the end of the First World War, another Belgian engineer Emil Bicheroux developed a process whereby the molten glass was poured from a pot directly through two rollers. Like the Fourcault method, this resulted in glass with a more even thickness, and made grinding and polishing easier and more economical.

An off-shoot of evolution in flat glass production was the strengthening of glass by means of lamination (inserting a celluloid material layer between two sheets of glass). The process was invented and developed by the French scientist Edouard Benedictus, who patented his new safety glass under the name "Triplex" in 1910. In America, Colburn developed another method for drawing sheet glass.

The float process developed after the Second World War by Britain's Pilkington Brothers Ltd., and introduced in 1959, combined the brilliant finish of sheet glass with the optical qualities of plate glass. Molten glass, when poured across the surface of a bath of molten tin, spreads and flattens before being drawn horizontally in a continuous ribbon into the annealing lehr. Till today, 90 percent of flat glass is manufactured by this process.