Since 1994, the Glass Floor has been one of the CN Tower’s must-see attractions. It takes advantage of the strength and transparency of glass to create a thrilling experience, at 342m. There are many buildings around the world that have installed their own “look-down” attractions, but our Glass Floor was the first. These attractions rely on glass’ unique properties, but what is it that makes glass and, thus, the Glass Floor, so strong?
All glass is made, at least in part, of Silicon dioxide or Silica. Quartz is a naturally occurring crystal of Silica, with its molecules arranged in ordered, repeating structure.
There are even plants and animals that have skeletons and support structures made of Silica. However, Glass can refer to a whole range of materials, all with slightly different properties. Crystal glass is Silica with about 25% lead oxide, which makes it both very clear and highly elastic, a property that allows for the “ring” of a crystal wine glass. Aluminasilicate glass is primarily used for fiberglass, which you see in canoes and covering hockey stick blades. Window glass or Soda-lime glass is made of Silica, sodium oxide, lime and magnesia, and also used for soda bottles, vases, mason jars and other household items. It is also what our Glass Floor is made of.
What makes glass so strong?
Glass is a non-crystalline, amorphous solid…what does that mean? It means that the pattern of its molecules is does not repeat in a symmetrical way. Think of a spider-web, but with missing strands. By contrast, Quartz, pure Silica, is a crystal, with all of its component molecules align in a repeating, symmetrical pattern. When a quartz crystal is heated, those highly organized molecules start to get energetic and the bonds between them relax. When it cools, the Silica bonds reform, but the crystal structure is lost, at least over long stretches.
Window glass has many other substances mixed in with Silica, which is a huge advantage in manufacturing. If you made a window out of pure Silica it would be very strong but very difficult to make, specifically when it comes to melting temperature. In order to shape glass we need to melt it. The melting point of Silica is 1600 degrees Celsius, which is really hot! The additional compounds in Window glass, like lime, magnesia and soda, disrupt the structure of Silica molecules, lowering its melting point to a much more manageable 1000 degrees C, but malleable at just above 560 degrees C.
The bonds between Silica molecules are very strong and the internal structure, although it looks hap-hazard, is a massive network of those strong bonds. This is what gives Glass its strength.
If these bonds are so strong and so numerous, why is glass transparent? Why does light pass through glass, but not your hand or the concrete of the rest of the CN Tower? It turns out, that visible light has the wrong amount of energy to interact with Silicon atoms. Most of an atom is empty space, with electrons orbiting around the atom’s core at very specific distances. If light does manage to hit an electron whizzing around, it needs to have just the right amount of energy to make it move. As it happens, visible light has either too much or too little energy to move or “excite” any of the silicon atom’s electrons. Silicon atoms are great at absorbing UV light, but not plain old visible light, so it passes right through.
The next time you’re at the Tower, enjoy the view and test the strength of the Glass Floor. It’s five times stronger than the required standard; strong enough to hold 35 moose or 3 ½ Orcas!
Fact or Fiction: Have you ever heard that glass can droop or flow over long periods of time? It’s been said that very old window panes in Renaissance England have slowly deformed, with more glass at the bottom than at the top. But there are artefacts made of glass from the Roman Empire, even older, that show no signs of deformation. It could be that they are different kinds of glass, or it could come down to the way Renaissance glass was made. Crown Glass was a method of making large sheets of glass by first blowing a sphere of glass, allowing it to flatten and cool, then reheating and spinning it on a table. The centrifugal force of the spin would cause the glass to spread out in a circular sheet. The centre would often be thicker and if a square was cut from that portion, the thicker part would be placed at the bottom of the window. It would need to be over 500 degrees Celsius for glass to even start to “creep” or deform in the proposed way!
Want to learn more? Check out these source links:
How Stuff Works – What makes glass transparent?
TED Ed – Why is glass transparent?
Study.com – Transparent and Opaque Materials in Electromagnetic Waves
University of California, Riverside - Is glass liquid or solid?
Encyclopedia Britannica – Crown Glass