There is more to glass than meets the eye
Science Feature by Professor Glenn Patrick
THE recent reports about problems with the newly-installed glass balustrades on the seafront roof area above Marine Parade in Lyme Regis have caused some heated discussion.
Regular readers will know that Lyme Regis Town Council arranged for the roof area to be re-surfaced and a new balustrade to be installed in 2020, but concerns have recently been raised after one of the glass panels was broken by a flying stone.
It got me thinking about what we know about this amazing material that we take for granted in our everyday lives.
Glass is useful for storage – what else would we make our wine bottles and glasses out of (and please do not say plastic)?
We are literally surrounded by the stuff sitting behind double-glazed windows at home and windscreens in our cars, giving us protection as well as light.
For those of us who wear spectacles, it even improves our view of the world. We also have glass oven doors which must withstand heat, but still enable us to see the dinner roasting, smartphone screens which respond to our touch, and thin strands of glass bringing us the internet over optical fibres.
I was a member of a team at CERN that even used 12,000 blocks of lead glass – as used in fine crystal glassware – to build a particle detector!
Despite all these applications and the fact that glass has been around for about 5,000 years, we still do not fully understand the material.
I remember being told by a science teacher that glass is a liquid, and this is often repeated. Proof of this is supposed to be found in the stained-glass windows of churches and cathedrals that are thicker at the bottom than at the top.
We are expected to believe that over the centuries the glass has flowed to the bottom, but this cannot be true because the atoms of glass move far too slowly.
The thick end of stained-glass windows is usually due to the medieval manufacture of glass panes, which in those days were not particularly flat or uniform.
The panes of glass were often made by a process which involved spinning it into a disk before it was cut. This made the glass thicker at the edge of the disk and the craftsmen naturally put this thickest end at the bottom of the window.
So, what is glass? Well, it is neither a solid nor a liquid, but is somewhere between the two and can be regarded as a different state of matter. This is because solids are made up of highly organised structures of atoms (think of crystals like sugar or salt) whereas liquids are not organised.
Glass is what is called an amorphous solid, which means that it is more organised than a liquid, but it does not have the rigid structure of solids.
It was the mass production of motor cars that created a demand for so-called ‘safety glass’, which was less likely to break and less likely to cause injury if it did.
Henry Ford was the first car manufacturer to use laminated glass in 1919. When it shatters, laminated glass is held together by a plastic inter-layer bonding the layers of glass.
It was discovered accidentally in 1903 by the French chemist Edouard Benedictus when he dropped a beaker and noticed that the glass held together because it had previously contained a clear liquid plastic (cellulose nitrate) which had dried.
In England, John Crewe Wood also patented laminated glass using tree resin as the protective layer. In 1937, it was made compulsory for all motor vehicles to have laminated glass and we have come to rely on it for protection.
Toughened – or tempered – glass is about four times stronger than ordinary (or annealed) glass. It also has the benefit that it shatters into small pieces, which are less harmful than the jagged fragments of normal glass.
It is produced by heating it to over 600 degrees Celsius and then going through a quenching process, which rapidly cools the glass using pressurised air. This induces a state of compression on the surface of the glass whilst the interior is in a state of tension.
It is these balanced stresses which gives the glass strength and makes it break into relatively harmless, granular pieces.
Toughened glass has routinely been used in the rear and side windows of vehicles because it is cheaper than laminated glass, but still has some of the safety requirements.
Toughened laminated glass should combine the best of both worlds and is increasingly used where safety is paramount.
Both laminated and toughened glass were recently deployed in a test panel installed on Sidmouth seafront. This was part of a trial to see whether glass could be used as part of a scheme to raise the sea wall, but still retain some attractive views.
According to East Devon District Council, the panel consisted of two 19mm thick toughened glass panes laminated together with a 0.89mm SentryGlas interlayer.
It survived the battering of three major storms, but was shattered by vandals! At least the lamination did its job and held all the pieces together.
What about bullet-proof glass? This is constructed from multiple layers of laminated glass and thermoplastic, which are sandwiched together. The resistance to projectiles depends on the thickness and structure of these layers.
Such glass is routinely used in armoured vehicles and ‘The Beast’ famously has five-inch-thick glass to protect the President of the United States.
The thicker the glass and the more layers, the greater the protection, but the challenge is to keep it transparent.
In the 1950s, the Mona Lisa was attacked with acid by a visitor and sadly it had to be put behind safety glass, but this obscured the visibility to the chagrin of art-lovers.
However, in 2019, the masterpiece was placed behind a special bullet-proof glass that, although constructed from multiple layers of glass and PVCB sheets, is especially transparent meaning that the enigmatic smile can still shine out.
Astronauts on the International Space Station have a unique view thanks to the Cupola observation dome, which consists of seven windows. This glass not only has to withstand the extreme temperature fluctuations of space, but also the impacts of space debris.
Even though it is made from multiple layers of thick fused silica and borosilicate glass it can still get damaged as demonstrated by Tim Peake who took a photograph in 2016 showing a 7mm chip from a tiny fleck of debris hitting it.
As the town council has discovered, all glass has its limits and a seaside location several feet above a busy walkway needs special consideration.
I am reminded of the phrase “people in glass houses shouldn’t throw stones”, but maybe that is a glass half-empty point of view.
Glenn Patrick is a particle physicist and science communicator who explores the quantum world of sub-atomic particles (including at the Large Hadron Collider) and now lives in Lyme Regis.