Glass and Glass Making
Glass is simply a mixture of oxides. Commercially, the main ingredient of glass is sand, or silicon oxide. This is heated with sodium carbonate at an extremely high temperature. This causes the oxides to melt.
On cooling, a frozen supercooled liquid mixture is formed. This mixture is glass.
There is an 'urban myth' that glass flows slowly. It has been suggested that very old window panes, such as those found in cathedrals, are thinner at the top than at the bottom, because the glass has flowed towards the bottom of the window pane! My thanks to Mike Woodburn for this contribution:
“Ah, the question of window glass thicker at the bottom than the top. Some people use this to support the idea that glass flows over time. There are two obvious problems with this idea: if glass flowed, there would be an open gap at the top where glass moved down and objects much older than these windows, like the hundreds of pieces of Roman Era glass that are around, would be distorted, and they aren't. What's more, you wouldn't expect thickness to be the only dimension that changes...if the "glass is a liquid" theory was true, then laying a pane flat should cause it to sag in the middle, and it doesn't.”
“Flat glass at the time the cathedrals were built was made by making a bowl shape and then spinning it out to a flat disc. The disc, after annealling is cut into relatively small pieces (even a 4 or 5 foot disc results in only 1.5-2 feet clear of the centre spot.) Further, the glass is always thicker nearer the centre and details of flaws show the curve of the stretching. When such glass is cut it is always slightly darker near the thicker edge. Glass artists quickly learned that when the thicker part is near the bottom, the resulting picture looks better to the eye. Having learned it, glass artists designed for the effect and cut the glass to match the design. Pieces cut nearer the edge tend to be flatter (less change of thickness) than those near the centre.”
I hope that clears up the fallacy!
Once it has been made, glass can be easily formed into new shapes by heating. The oldest method of preparing glass is by blowing. Glass-blowing has been practised for many centuries - the Romans were well known for their skill in producing glass ornaments.
Making glass
In the Science Club, we didn't use silicon oxide as it melts at too high a temperature. Instead we used a mixture of lead oxide, boric acid and zinc oxide - this is not ideal as lead oxide is TOXIC if inhaled! Suitable safety precautions were taken, however.Set up the apparatus as shown:
Heat the glass making mixture using a roaring flame. Ensure that the crucible lid is in place to make sure that no lead oxide dust is blown into the air.
Leave the mixture for 5 minutes, so that all the oxides have melted.
Pour the molten oxides onto the heatproof mat and allow to cool.
Click here to see Joshua Lee demonstrate the art of pouring!!!
Try adding some coloured compounds (in very small quantities e.g. cobalt chloride or copper sulphate) - this makes the glass a different colour!
Here are a few samples of our glass production line:
