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The Engineer: The origins of windows

I have always been fascinated with the origins and evolution of our industry. How did a seemingly invisible material become such a worthy part of our everyday life? Our homes are such happy places because there exist windows.

A roof over our heads is a necessity of life. This mantra hasn’t changed since humans have been around. Shelters provide protection from predators, keep us warm and shield us from the elements. However, early shelters were rudimentary. Cooking over a fire was often done inside, therefore it was important to vent smoke to the outside through a hole. Openings were also needed to illuminate the inside of our shelters. These are the original windows. As a matter of fact, the word “window” is derived from the Old Norse word vindauga which literally translates to “wind” + “eye.”

To keep the elements out of the shelter, it was desirable to place covers over the openings and remove them when the weather was decent. A curtain made from wood or animal hides was commonly used. Animal hides were peeled into thin layers, stretched, and dried. This made them translucent and when hung across an opening, some light was allowed to pass through. The basic window was born. This was a common practice in early human history. Animal hides were also substituted with other materials such as oiled paper and slated shutters. Smaller wooden sub-frames could be constructed within a large overall wood frame. Therefore, smaller pieces of hide or paper were used to fill the spaces with less tendency to rip. These windows let a minimal amount of light in, but it was better than darkness.

It would be many millennia before a magical material was discovered. Glass was discovered likely as an accidental slag by-product during the forging and smelting process in the production of metals common at the time. Small pieces of glass were extracted from the slag and cleaned and buffed to reveal shiny, vitreous chunks. Various mineral impurities from the slag gave glass distinctive colours. Initially, these glass chunks were collected and sorted by colour and often used as jewelry. Due to the relatively low melting point (compared to the temperatures required for metal forming), it was easy to melt the chunks in a hot fire and form larger, more useful chunks.

Around the first century B.C., someone in the eastern Roman Empire dipped a long hollow iron pipe in the molten glass and blew through it (in what is known in the glassblowing world as “inflation”) making glass balloon shapes. The glassworker was now able to manipulate the glass and form it to their liking. Thus, the inception of making glass vases and glassware. During this time, it was learned that you could introduce certain additives to control the melting point, colour and forming characteristics. Adding white silica sand and natron (naturally-occurring soda ash), could form glass that was mostly transparent. Colour was controlled by adjusting the levels of other minerals. Blown glass could also be flattened out into plates. These early cast-glass panes had terrible optical properties as circular striation patterns were very noticeable. These glass castings were then fitted into small subframes and assembled into a larger frame usually made from wood. The recognizable form of the common window came to be. Windows could be made larger as their size now was limited by the wood and lead members supporting the glass. Windows existed during Roman times, but it was still not commonplace for the average person and was usually reserved for the wealthy. Many churches took advantage of this new, transparent and colourful material to brighten their prayer spaces. With the addition of mineral additives to the glass to vary the colour, these glass pieces were assembled into patterns that today we know as stained-glass windows.

Over time, the production process improved the quality and clarity of the glass. It wasn’t long before a molten blob of glass was poured down an inclined plane, and this started the era of sheet glass. In the 1950s, the Pilkington Process ushered in today’s modern float glass-making practice that made glass exceptionally clean, smooth and free of imperfections. Due to its flexibility for colour and its magical property of near-invisibility, it’s no wonder that glass is one of the most popular building materials to adorn building facades. •

Claudio Sacilotto is director of engineering and R&D for Novatech