In this article we will look at the following glass types in detail, compare them and look at their suitability for use in glass balustrading:
- Float / Annealed Glass
- Heat Strengthened Glass
- Toughened Glass / Tempered Glass
- Laminated Glass
- Toughened & Laminated Glass
What is Glass?
As the raw materials are abundantly available and the end product widely recyclable, glass has become an essential material used in modern life, especially in the construction industry.
Glass is a solid-like and (usually) transparent material used in many applications in our daily lives. It was first discovered around 5,000 BC (some say 3,500 BC, but we are not going to split hairs!) and is made from sand, soda ash and limestone that are then often mixed with recycled glass and melted at approximately 1500°C to form molten glass.
Whilst molten, glass behaves similar to liquids. However, at ambient temperature, it is structurally like a solid. As a result, glass can be poured, blown, pressed and moulded to shape it, depending on the end result required.
The glass can then be further processed in many instances to have specific properties such as increased mechanical strength and higher resistance to breakage.
Additional materials such as iron oxide or cobalt can be added to the raw materials to give the glass a green or blue colour.
1. Float / Annealed Glass
Perhaps the most significant innovation in glass history, the float glass process, was invented by Sir Alastair Pilkington in 1952. This process is now used to manufacture glass sheets as thin as 0.4 mm to as thick as 25 mm.
Molten glass is poured continuously from a furnace onto a shallow bath of molten tin. The glass floats on the tin and forms a level surface. The thickness of the glass is controlled by the speed at which the solidifying glass ribbon is drawn off the bath.
At the end of the bath, the glass is slowly cooled down and fed into the annealing (controlled cooling) lehr (yes, that is a word, google it!) for further controlled gradual cooling. After annealing, the glass emerges with virtually parallel surfaces.
The float process has made it possible to provide the construction industry with substantial glass sheets, manufactured to near perfection, not just in flatness but with flawless transparency and lack of optical distortion.
Float glass, also known as annealed glass, is often processed further to produce tempered glass or laminated glass.
Float glass should not be used in glass balustrades without further processing first.
2. Heat Strengthened Glass
Heat Strengthened Glass is semi toughened or semi tempered glass. The process involves heating annealed glass back up to about 650 to 700 degrees Celsius and then cooling it slower than with toughened glass, which means the compression strength is lower.
This increases annealed glass’s mechanical and thermal stability, making it twice as tough as standard annealed glass.
When it breaks, the fragments are similar to annealed glass but more likely to stay together.
Heat-strengthened glass is typically specified when additional strength is needed to resist wind pressure, thermal stress or both, and the extra strength or safety break pattern of fully tempered glass is not required.
This glass is not recommended for use in balustrades or structural applications because of its limited strength compared to tempered or toughened glass. However, it is sometimes specified when there is concern about tempered glass fracturing into thousands of tiny pieces.
Heat-strengthened glass can’t be cut or drilled after heat-strengthening, and any alterations, such as edge polishing, sandblasting or acid etching, usually causes the glass to break.
Heat Strengthened glass is not considered safe to use in glass balustrades in the UK market.
3. Toughened Glass / Tempered Glass
Toughened glass, or tempered glass, has high withstanding capacity and is suitable for uses where strength and thermal resistance are required.
Toughened glass is produced when float glass is heat-treated to around 650°C, then blasted with air jets on both surfaces so that the surfaces are cooled quickly, and the inside core cools at a slower rate. This causes the surfaces of the glass to go into compression, and the core goes into tension.
Once toughened, it is nearly four to five times stronger than annealed glass and is three times as strong as heat strengthened glass.
Toughening does not alter standard glass’s basic characteristics, such as light transmission and solar radiant heat properties.
A safety glass, toughened glass, is difficult to break, but in the event of a breakage, it disintegrates into small & generally safer globules- the core of the glass releases tensile energy, which results in the formation of small, safer glass particles, rather than the rather than long, sharp, dangerous shards that float glass produces (which are far more likely to lead to injuries).
However, toughened glass is prone to very rare spontaneous breakage, which occurs when the repeated heating and cooling cause the nickel sulphide stones to increase in size. See NIS Article.
Toughened Glass is widely used in Glass balustrades, but do take into consideration the additional safety features offered by toughened & laminated glass as below.
4. Laminated Glass
Lamination is generally carried out by bonding, heating, pressing, and placing the glass panels in an Autoclave to cure.
Whilst laminated glass covers a vast spectrum of glass products, it can be easily understood when remembering the lamination bit on its own refers to when two or more sheets of glass are bonded together with one or more interlayers.
But the critical part to remember is that the glass sheet for making laminated glass can be ordinary annealed glass, float glass, tempered glass, coloured glass, heat-absorbing glass, heat-reflecting glass, etc. And then there can be anything from 2 to 9 different layers of interlayer. And to top it all, there are numerous different types of interlayers as well as different colours available!
But in general, laminated glass offers many advantages. Its impact resistance is higher than that of ordinary sheet glass. (Bullet-proof glass is, in fact, made by compounding multiple layers of regular glass or tempered glass). But the main benefit of laminated glass is that due to the adhesion of the interlayer glue, even when the glass breaks, its fragments are stuck to the film, so, in effect, it is held together by the interlayer. This reduces the hazard associated with shattered glass fragments and, to some degree, the security risks associated with easy penetration.
Alternative transparent interlayers may be used in laminated glass to obtain particular properties, such as solar control, thermal insulation, fire protection, bomb-proof etc.
Laminated glass has good transparency; however, the lamination layer is visible when looking directly at the edge of the glass panel.
Usually, if a glass panel breaks or shatters, it is improbable that both plies of laminated panels will break simultaneously, which means that the remaining ply and interlayer will support the broken glass, keeping it in place as edge protection until it is replaced or secured suitably.
Laminated Float Glass is not considered safe to use in glass balustrades in the UK market, however, Toughened and laminated glass as below definitely is.
5. Toughened & Laminated Glass
Toughened & Laminated glass panels are the most widely used laminated panels in balustrading. So, as we have covered above, toughened glass is extremely strong and laminated glass is very safe, so yes, you’ve guessed it! Combine the two for the best of the best! For the rest of this section, we will be focusing solely on TOUGHENED and Laminated glass, rather than float Laminated glass or similar.
Relatively recent improvements in manufacturing procedures have made toughened & laminated glass commercially viable in bespoke sizes and shapes. (Not so long ago, you could only choose between toughened or laminated glass but not combine them. This is still a misconception in some specifiers minds).
As toughened glass can’t be cut, drilled, or worked on after toughening (and toughening is at such a high temperature it must be done before the laminating procedure), in effect, two panels of annealed glass are cut to size and shape, drilled if required, edge work processed etc. and then toughened individually. Only then can the two identical panels be laminated together.
It’s a time-consuming process, so it does add to both the delivery leads times and costs, but the result is a very strong AND safe balustrade panel.
If a toughened & laminated panel were to break, the glass panel generally remains in place as a temporary barrier. In addition, as the toughened glass shatters, it gives a white appearance, so it is easily visible as needing replacing (as compared to just toughened glass which could break and fall away and not be immediately apparent from a distance that an area is unsafe.)
Toughened & laminated glass is the generally preferred glass to be used in glass balustrades, offering maximum strength and safety.
However, there are different lamination options to consider, read more in our separate article here:The Types of Glass Lamination and what is the best Lamination Interlayer to use in Balustrades
