The first traces of glass objects date back to nearly 3,000 BC. Since then, the major stages of the manufacture of this material, which can be as hard as fragile, have not changed much.
The basic component of glass production is sand or, more precisely, the silica (SiO2) it contains.
Melting sand (silica)
The trouble is that silica has the unfortunate tendency to melt only at very high temperatures, above 1,700 ° C. Then, to facilitate the process, we add some fluxes such as soda, potash or lime. Water and recycled glass debris called “cullet” are also added. All in very precise proportions.
It is at this stage as well that some additives are inserted which are function of the use which will be made of the glass:
- Magnesium oxide, for example, to make the glass more resistant; iron oxide to give the glass a greenish tinge.
- The mixture is then baked and brought to a temperature of about 1,500 ° C. This is the temperature at which the sandy mixture turns into liquid glass.
Glass shaping (hot transformation)
Once melted, and before its cooling, the glass can be shaped according to different techniques. It can, for example, be blown, poured into a mold or floated on a tin bath.
Thermal treatment of glass
The glass then generally needs to be reinforced through the application of heat treatment. To eliminate the stress points that occur during cooling, annealing may be used at temperatures up to 600 ° C.
To further improve the strength of the glass, it can be quenched. In this case, after being heated to some 600 ° C, the glass is rapidly cooled under the effect of a high-pressure blower.
Glass is one of the most easily recyclable materials. In fact, recycled glass melts at much lower temperatures than silica and the addition of fluxes becomes superfluous. However, production from recycled glass requires at least 20% new raw material.
And the US’s roughly 33% glass-recycling rate, which pales compared with the 90% recycling rate in Switzerland, Germany, and other European countries, is not the result of a lack of technical know-how.
“Glass is 100% recyclable,” says Robert Weisenburger Lipetz, executive director of the Glass Manufacturing Industry Council (GMIC), a nonprofit trade association. “It has an unlimited life and can be melted and recycled endlessly to make new glass products with no loss in quality,” he adds.