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Planning Retirement Online

Shedding light on windows.

April 2011 

cleaning windowsNow it is spring many of us will be bustling around with a bucket of hot water and a sponge or newspaper cleaning our windows.

Windows are such an essential part of our everyday life that we tend to forget what a brilliant invention they are.

In days gone past windows of course were simply holes in a wall, sometimes covered at night with animal hide or wood. Paper windows were widely used in ancient China and Japan and I can’t think they did a lot to keep out the bitter snow storms of the region. Shutters came next but it wasn’t until glass became available that windows as we know them started to appear. The Romans were the first to use glass for windows although the quality of glass was very poor and let in minimal levels of sometimes severely distorted light. Even so, I am still amazed at the brilliance of these people who had the vision and capability to even consider that something solid could be made that would also let light pass through.

It was really only in the early 17th century that glass became fairly common in the windows of affluent homes. At this time a type of glass called crown glass was the most common glass for windows. Sheets of glass were made by blowing large cylinders of glass which were then cut open, flattened and cut into panes before the glass cooled. The cylinder method meant there was a limit to the size of glass that could be cut resulting in windows being divided into small panels, a common feature of older windows.

An English engineer, Henry Bessemer, made a big breakthrough in 1848 when he devised a new system that produced a continuous ribbon of flat glass by pressing it between two rollers but this was a process that needed exact control and was also expensive because the glass surfaces needed careful polishing after manufacture.

Surprisingly it was only in the 1950s that the modern manufacturing process of “float” glass was developed. The new glass making process was the brainchild of Sir Alastair Pilkington and Kenneth Bickerstaff and produced a perfectly smooth, flat and clear glass. It is still the key glass manufacturing process today and enables glass panes to be made of enormous length and width, but it is quite a complex procedure.

Basically the ingredients for glass (a combination of several ingredients such as sand, soda ash, dolomite, limestone, salt cake and cullet) are mixed and put into a furnace heated to around 1500 degrees C. Then a thin layer from this molten glass is poured onto a tank of molten tin in an atmosphere of 100% nitrogen. Because the glass is less dense than the tin, the glass floats at the top and forms a smooth, glossy surface on both sides. Even better, if maintains an even thickness throughout. It is then put onto tin rollers and cooled very slowly – if it cooled too quickly into a solid, cracks and imperfections could appear. The cooling is done in a nitrogen atmosphere to prevent the glass from oxidizing and becoming opaque. When you learn more about glass making, it makes you wish you had listened more carefully in those chemistry and physics lessons at school!

Technical developments in glass have continued in recent decades. Today you can buy the widest range of speciality glass from safety, coloured and patterned glass to tempered glass for extra strength and low-e glass which has been treated with a thin layer of oxidized metal on one side to help its insulating properties.

When you get your sponge and bucket out to clean your windows this spring, give a thought to the brilliance of so many people throughout the ages who have worked so hard to throw some light into our lives!

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