The float process
The float process is now used almost universally for production of glass. In this technology molten glass floats out over a bath of molten tin. The "float glass" produced in this way is sold in large sheets for processing into secondary products. In fact nearly all flat glass products are now made from float glass.
From raw materials to float glass (inside-viewoutside-view)
Charging the raw materials
The raw materials — principally sand, soda, limestone and dolomite — are kept in different silos. After being weighed separately they are mixed and then poured into the charging hopper along with the cullet (crushed scrap glass). All these operations are fully automated.
Melting in the furnace
The raw materials are melted in the furnace at a temperature of 1,550 °C produced by powerful fuel-oil and/or gas burners. As it melts, the mixture vitrifies and flows slowly down inside the furnace, undergoing a process known as “fining“. During this process, the molten glass is kept at a high temperature for several hours, enabling bubbles of air trapped within it to escape.
Floating
As it comes out of the furnace, the molten glass is poured onto a bath of liquid tin, where a sheet of glass is formed by flotation. From one end of the bath to the other, the temperature of the glass and of the tin gradually drops from 1,100 °C to 600 °C. Top rollers on either side draw out the glass mechanically to give it the required thickness and width.
Annealing
The glass emerges from the tin bath in the form of a continuous strip. This is led by a roller conveyor to an annealing tunnel, known as a “lehr”. Here, the glass is gradually cooled in a controlled way, in order to ensure perfect flatness and eliminate any internal mechanical stresses which could cause breakage. The glass comes out of the lehr at ambient temperature, ready for cutting.
Cutting
After annealing, the glass strip is inspected by an optical laser system, and then automatically cut into large sheets measuring 6x3.21m. Without any human intervention, these sheets are sorted into orders, according to the level of quality required by the customers, and are then laid on stillages by automatic stacking machines, ready for shipping.
Coating
Extremely thin, almost invisible layers of metal oxide are deposited on the glass to give it better insulation against cold and/or protection against the sun's rays. The coatings are applied either on-line on hot glass (pyrolytic coating) or by an electromagnetic process (vacuum coating).
Assembly into double or triple glazing
Two or three sheets of glass may be assembled into a surrounding metal frame which acts as a spacer, with an air-filled or gas-filled gap between them. The complete assembly is hermetically sealed. Double glazing affords twice the insulation power of single glazing, or even four time the power in the case of double glazing with super-insulating coating.
Assembly into laminated glass
Two or more sheets of glass may be assembled with one or more plastic separation layers made of PVB (polyvinylbutyral). Depending on its composition, the laminated glass affords greater protection for property (against break-in) or for persons (against gunshot, explosion or risk of falling through).
Toughening
The thermal toughening process consists of heating the glass to softening temperature (around 700°C) and then suddenly cooling it. Glass that has been toughened in this way is five times more resistant to thermal or mechanical shock. And when it does break it shatters into thousands of small non-cutting pieces. Chemical toughening consists of ionic exchange between the glass and a bath of molten salt which increases its mechanical strength.
