Tempering

Tempering is a thermal or chemical treatment whose purpose is, unlike that for annealing, to compress the surface layer of the glass, thus improving its resistance to breaking.

The thermal tempering process entails heating the glass object (for example, a sheet which has been shaped and possibly even curved) up to 600° (temperature at which glass is in a plastic state) and then to quickly cool it. In the first moments of this operation, the surface cools quicker than the inside and, in a few seconds, given the low thermal conductivity, the temperature difference between the surface and the heart of the object reaches its maximum level. Then the inside part cools quicker than the external one, which means that the temperature difference is progressively diminished, until it reaches room temperature.
The overall result of this process is the introduction of permanent tensions in the glass: the surface is compressed, the inside is in traction. The formation of this tension state causes the absorption of elastic energy in the glass. When tempered glass breaks, the stored energy is freed in the form of surface energy; for this reason, when it breaks, small blunt fragments are created, unlike what happens when ordinary glass breaks (this is the reason for the importance of tempered glass as safety glass).
Consequently, any attempt to cut, puncture or model the tempered glass leads irremediably to its breakage. All these operations must be carried out before tempering.

Features
- Mechanical resistance.
An 8 mm-thick tempered sheet, resists breakages from a drop of a 1 kg marble free falling from a height of 2 metres. The same marble, which falls from a height of 30 cm, breaks an annealed sheet of the same thickness. As it is compressed, the surface of the sheet prevents the spread of micro-breaks towards the inside, thereby improving resistance to damage.
- Tempered glass is insensitive to changes in thermal temperatures (from 100 to 200°C depending on the thickness), while an annealed sheet breaks under sudden changes in temperature between 50 and 100°C.
- The flexion resistance of tempered sheets is three times higher than that of an annealed sheet.

To obtain a layer of higher surface compression, chemical tempering can be used. The advantage of this technique, compared to thermal tempering, is that high temperatures are not needed, with the consequent danger of distortion of the object and it can also be used for complex shaped objects.
This technique entails substituting, at a temperature of about 450°C (less than for annealing), some of the sodium ions in the surface layers with potassium ions, which are bigger.
This swapping of ions occurs by immersing the glass object in a bath of molten potassic salts (KNO3). The substitution of sodium with potassium brings about the expansion of the surface glass lattice in relation to the internal layers: the result of this is that the external part is in compression and the internal one in traction.
The thickness of the layer placed in compression is very thin (about 50 micro-metres) and this constitutes serious limits to the use of chemically tempered glass.
A feature of this glass is that it does not present any increase in optical distortion compared to annealed glass, unlike thermally tempered glass.
The applications of tempered glass is very broad. Its characteristics often make it indispensable, if not obligatory.
The automobile industry was the first to use tempered glass for side windows, rear windows and openable sun-roofs; the household appliance industry uses it for oven doors.
The building industry uses it for transparent walls, doors, windows and parapets.
In interior decorating it is used for shop windows, shelves, partitions, steps, sanitary fixtures, automatic doors. In urban design it is used for bus shelters, telephone booths and bill-boards.