Through thick and thin

Today’s Friday science feature looks into... glass! Its properties, strengths and weaknesses are explored by Dr Roger Jewsbury, head of department of chemical and biological sciences, School of Applied Sciences, University of Huddersfield — Oldham’s degree partners
They are the cause of racing hearts on city sight-seeing trips — the glass elevator, the sheer glass wall, ceiling or floor on the viewing platform on the 100th floor of a skyscraper.

This month, America’s tallest building, Chicago’s Willis (formerly Sears) Tower, opened an all-glass viewing chamber — a trend that will undoubtedly be repeated here in the future.

This leads us to ask the question — how is it that glass is safe?

Our first experience with glass as a child is probably when it breaks. Glass is hard and brittle and breaks into sharp pieces.

And yet glass is increasingly being used in structures in which it has to be unbreakable for safety reasons — aeroplane windscreens, glass-bottomed boats and viewing platforms.

So what is glass and how has it been transformed from the material we first came across in bottles and jars?

Glass may have only become commonplace in the last few hundred years, but the means to produce it have been known for thousands.

It is made by melting silica, the main component of sand. Sodium carbonate (also known as washing soda) is added to reduce the temperature needed for melting, along with lime.

This glass is known in the trade as soda glass. Glass is unlike crystalline solids which have ordered regular structures, its structure is more like that of a liquid, and this is why it is transparent.

This unusual behaviour is achieved by cooling very quickly from the liquid, so that an ordered structure does not have time to form.

The colours of glass found in stained glass windows and wine bottles are created by the addition of small amounts of other metal compounds.

It is to keep these added metal compounds apart that glass is separated by colour in recycling bins.

Recycling not only reduces landfill, but it also takes one third less energy to melt recycled glass than the raw materials.

A problem that many of us will have experienced with common soda glass is that sudden changes of temperature, for example by pouring boiling water into a bottle, will cause it to break.

This is because the glass expands rapidly in the hot region but not in the surrounding cooler region and is why thick glass is more likely to break in this way than thin glass.

This problem was solved over 100 years ago by making a glass that expands much less on heating — by adding the element boron to form borosilicate glass which may be more familiar by the trade name, Pyrex.

Glass breaks start at tiny cracks in the surface. If these could be avoided it would be very strong, but they form through contact with other surfaces and even the gases in air.

The manufacturing process is designed to protect the surfaces from contact with air or surfaces and further protection can be achieved by special coatings.

Glass in shelves and some windows will have been toughened or tempered. This is a process of heating and cooling to give glass which will break less easily and when it does it breaks into small pieces.

This process works because the density of glass is greater when it cools slowly so rapid cooling of the surface makes the surface less dense and puts it under compression, which reduces the likelihood of the tiny cracks growing.

The same effect can be achieved by chemically replacing some of the sodium atoms in the surface layer with similar but larger potassium atoms.

But tempered glass will still break, so it is not enough for the most demanding applications.

For these, lamination must be used. Glass sheets are interspersed with thin polymer layers.

The polymer layers add strength and, should one of the glass layers break, keep the structure intact.

Lamination is used in car windscreens, so that even if the screen breaks the fragments remain in the layer rather than damaging the driver and passengers.

And how safe is a viewing platform going more than a metre out from a building?

Well it is specified to hold five tons — considerably more than a group of even the most overweight tourists.