How we make glass
Raw Materials
The main constituent of flat glass is SiO2 (silica sand). This has a high melting temperature in the region of 1700°C and its state at this temperature is like syrup on a very cold day.
The basic building block of silica has a tetrahedral pyramid shape with silicon at its centre linked symmetrically to four oxygen atoms at its corners: it has the chemical formula SiO4 and is negatively charged.
On cooling molten silica quickly, a random organised network of these tetrahedra are formed, linked at their corners, to give an amorphous material known as vitreous silica.
Other raw materials include soda ash, dolomite, limestone, saltcake, and broken glass (cullet). We melt this mix at white heat to a highly viscous consistency.
This is the mix and process for many types of mass-produced glass; except that float demands highly exacting standards of quality, care and control beyond those of other everyday uses of glass.
Chemistry of glass
For practical and economic reasons, the high melting point and viscosity of silica is reduced by adding sodium oxide (a flux) in the form of a carbonate and the sodium-oxygen atoms enter the silicon-oxygen network, in accordance with their valency states. These atoms are known as Network Formers.
Other major constituents of Flat Glass: Calcium and Magnesium enter the network structure as Network Modifiers and the action of these modifiers is to make the structures more complex so that when the components are melted together, in the cooling process, it is more difficult for the atoms to arrange themselves in suitable configurations for crystallisation to occur. In the glass making process, the cooling rate is arranged such that viscosity increases and the mobility of the atoms is hindered thus preventing arrangements and crystallisation from occurring.
Thus glass is often referred to as a supercooled liquid in that it has no crystallisation or melting point and does not exhibit the phenomenon of the latent heat of crystallisation or fusion.
The Float Process
At the heart of the world's glass industry is the float process - invented by Sir Alastair Pilkington in 1952 - which manufactures clear, tinted and coated glass for buildings, and clear and tinted glass for vehicles. The process, originally able to make only 6 mm thick glass, can now makes glass as thin as 0.4 mm and as thick as 25 mm.
Molten glass, at approximately 1000°C, is poured continuously from a furnace onto a shallow bath of molten tin. It floats on the tin, spreads out and forms a level surface. Thickness is controlled by the speed at which the solidifying glass ribbon is drawn off from the tin bath. After annealing (controlled cooling) the glass emerges as a 'fire' polished product with virtually parallel surfaces.
A float plant, which operates non-stop for between 10-15 years, makes around 6000 kilometres of glass a year in thicknesses of 0.4 mm to 25 mm and in widths of up to 3 metres. The float process has been licensed to over 40 manufacturers and more than 450 float lines are in operation with a combined output of about 1.5 million tonnes of glass a week.
The NSG Group operates or has interests in 51 float lines worldwide.
The Float Process Step by Step
Stage 1: Melting and refining
Fine-grained ingredients, closely controlled for quality, are mixed to make batch, which flows as a blanket on to molten glass at 1,700°C in the melting furnace.
Float makes glass of near optical quality. Several processes – melting, refining, homogenising – take place simultaneously in the furnace. They occur in separate zones in a complex glass flow driven by high temperatures. It adds up to a continuous melting process, lasting as long as 50 hours, that delivers glass at about 1,000°C, free from inclusions and bubbles, smoothly and continuously to the float bath.
The melting process is key to glass quality; and compositions can be modified to change the properties of the finished product.
Stage 2: Float bath
Glass from the melting furnace flows gently over a refractory spout on to the mirror-like surface of molten tin, starting at about 1,000°C and leaving the float bath as a solid ribbon at 600°C.
The principle of float glass is unchanged from the 1950s. But the product has changed dramatically: from a single equilibrium thickness of 6.8 mm to a range from 0.4 mm to 25 mm; from a ribbon frequently marred by inclusions, bubbles and striations to almost optical perfection. Float delivers what is known as fire finish, the lustre of new chinaware.
Stage 3: Coating
Coatings that make profound changes in end product performance can be applied by advanced high temperature technology to the cooling ribbon of glass.
On-line chemical vapour deposition (CVD) of coatings is the most significant advance in the float process since it was invented. CVD can be used to lay down a variety of coatings, less than a micron thick, to reflect visible and infrared wavelengths, for instance. Multiple coatings can be deposited in the few seconds available as the glass ribbon flows beneath the coaters.
Stage 4: Annealing
Despite the tranquillity with which float glass is formed, considerable stresses are developed in the ribbon as it cools.
Too much stress and the glass will break beneath the cutter. To relieve these stresses, the ribbon undergoes heat-treatment in a long furnace known as a lehr. Temperatures are closely controlled both along and across the ribbon. Pilkington has developed technology which automatically feeds back stress levels in the glass to control the temperatures in the lehr.
Stage 5: Inspection
The float process is renowned for making perfectly flat, flaw-free glass. But to ensure the highest quality, inspection takes place at every stage.
Occasionally a bubble is not removed during refining, a sand grain refuses to melt, a tremor in the tin puts ripples into the glass ribbon. Automated on-line inspection does two things. It reveals process faults upstream that can be corrected. And it enables computers downstream to steer cutters round flaws. Flaws imply wastage; while customers press constantly for greater perfection. Inspection technology now allows more than 100 million measurements a second to be made across the ribbon, locating flaws the unaided eye wouldn't be able to see. The data drives ‘intelligent’ cutters, further improving product quality to the customer.
Stage 6: Cutting to order
Diamond wheels trim off selvedge - stressed edges - and cut the ribbon to size dictated by computer.
Float glass is sold by the square metre. Computers translate customers’ requirements into patterns of cuts designed to minimise wastage. Increasingly, electronic systems integrate the operation of manufacturing plants with the order book.