I have chosen this small piece of window glass, bearing the rough mark where the pontil rod was broken off, to represent the huge difference glass has made to our lives. It was discovered about 5,000 years ago, is made from basic ingredients and we take it completely for granted - but imagine life without it. Two techniques for making window glass were brought to Britain by French Huguenot refugees in the 16th century. Crown glass was blown and spun into a large disc, which was then cut into panes - this piece is the centre of such a disc. The green colour comes from naturally occurring iron in the sand used to make it. Broad glass was cheaper. It was blown into a large cylinder, which was cut and flattened. Glass was once owned only by the rich - now it is everywhere. It allows light into our rooms, and keeps out the cold. Lighthouses lenses magnify the glow from a small bulb so it can be seen for miles. Think glass-fronted buildings, bottles, jars, chemical apparatus, screens -including your tinted windscreen - barometers, mirrors, marbles, stained glass, studio glass, chandeliers, drinking glasses, traffic light lenses, telescopes, microscopes..... Think glass.