But windows didn’t always have glass in them, they were just gaps in a wall to let in light, open to the elements until someone hung some sacking or animal skins up to block out the wind, rain or snow.

The sand commonly used to make glass is comprised of small grains of quartz crystals, known as silica, soda ash and limestone. In the beginning, it was difficult to manufacture glass, as glass melting furnaces were small and the heat they produced was hardly enough to melt glass.

Turns out the Romans had cracked it (so to speak) all those years ago, but before them, the earliest known man-made glass dates back to around 3500 BC, with finds in Egypt and Eastern Mesopotamia. The discovery of glassblowing around the 1st century BC was a major breakthrough in glassmaking, with Syrian craftsmen being responsible for inventing glass-blowing, and this revolutionary discovery made glass production easier, faster and cheaper.

Glass production flourished and spread. But early glass panes were small and often had air bubbles, distortions and curved ripples, some of which can still be seen today. Early window glass started with a long balloon of blown glass, the ends being cut off and the resulting cylinder being split into two. The half cylinder would be placed on an iron plate and flattened, the ‘bottle ends’ were the cheaper leftover panes, and can still be seen today in some old windows.

While ancient China, Korea and Japan widely used paper windows, the Romans were the first known to use glass for windows around 100 AD. In England, animal horn was used before glass took over in the early 17th century. (I didn’t know this! Less costly than glass, cow horns had been widely used in the middle ages to make windows - they were soaked in water to soften them, heated and then cut and rolled into strips. I doubt you could see through them though!)
Unfortunately, the introduction of real glass in England wasn’t so welcome, as, in 1696, William III introduced a ‘window tax’, and to avoid paying the charge, many people just bricked up their windows. (William’s window tax is where the term ‘daylight robbery’ originates from!), and the tax remained in place for an incredible 156 years.

Henry Bessemer introduced an early form of ‘Float Glass’ in 1843, which involved pouring glass onto liquid tin, which was improved on by Pilkington who further developed the revolutionary float glass process in the mid-20th century. This made modern-style floor-to-ceiling windows possible. With this process, the molten glass is poured onto a bed of molten tin, and floating on the tin, the molten glass spreads out to form a level surface, with this method still being the industry standard today.

Originally, Pilkington’s process enabled glass only to be made at 6.8 mm thick, but today it can be as thin as 0.4 mm or as thick as 25 mm. While the principles of the process remain unchanged, the surface quality of the glass has greatly improved, providing an end product devoid of distortions or flaws. Nowadays, the glass industry is flourishing with many different types for many different uses, which include laminated glass, heat-strengthened glass and toughened glass.

One major breakthrough was the revolutionary invention of double-glazing, originally called ‘ thermopane’, where two panes of glass sandwiched a barrier of a safe and non-reactive gas called Argon gas, which became an efficient method of retaining heat. It is believed that modern day double-glazing as we know it was invented in America by C.D Haven in the 1930’s, though the Romans invented a similar idea of heat retention over 2000 years ago.

Now we even have self-cleaning windows – how fantastic is that? This is not just a simple pane of glass. It has a really thin coating on the outside (we’re talking a layer a mere 10–25 nanometers deep) of titanium oxide that acts as a ‘photocatalist’, which basically prevents the water droplets forming and spreads water like a great big cleaning cloth over the surface of the glass.

Author

Marilyn writes regularly for The Portugal News, and has lived in the Algarve for some years. A dog-lover, she has lived in Ireland, UK, Bermuda and the Isle of Man. 

Marilyn Sheridan