We often look right past it. Or, at least, through it. Glass is one of the oldest materials that humans have manufactured and one of the more interesting.
The origins of glass are lost in antiquity. No one knows exactly where the first glass was made. Indeed, as glass can occur naturally in the form of obsidian from volcanoes, it could be argued that glass pre-dates human history.
But the first deliberate production of glass has been postulated to have occurred around 3000 BCE in the Middle East, possibly in Egypt or Mesopotamia or even along the coast of the Mediterranean. The requirement for soda and for sand has led archeologists to a few likely sites and excavations have discovered very old and serviceable kilns.
Pottery has an even older history and glass making could be considered an off-shoot of this more fundamental industry. There are hints that the glazing of pottery began around 8000 BCE. Glazing is a form of glass formation using much the same basic compounds to seal the fired clay. The skills developed for making glazes are transferable to making glass, suggesting that glass making could have originated in a number of locations.
But Roman historians, such as Pliny, tell us that the invention of glass was actually an accident. Phoenician traders noticed a clear liquid developed when the blocks of stone that they used for cooking interacted with the sand beneath them. While it is possible that this is the original discovery of glass, the intensity of the fire required to generate glass would have meant that it was unsuitable for cooking. However, some ancient fire pits that were routinely used over and over again did develop a form of glazing on their inner surfaces.
In any case, what we do know is that Egyptian glass makers were producing glass vessels by 1500 BCE. The first manual that we know about on glass making was written on stone tablets and appeared around 650 BCE. The techniques for glass making were spread through out the Mediterranean by Phoenician traders.
Early Egyptian craftsman used a method called "core-forming" for making vessels. A core of compacted sand, clay, and dung was dipped into a pot of molten glass and then tipped upside down so that the glass flowed over its surface. The surface could be made smooth by rolling the vessel on a flat surface.
The glass was allowed to harden and the core broken up and scrubbed out of the inside, resulting in a semi-clear vessel. This technique is still used today albeit in a slightly different form and not just for making glass objects. The disadvantage to the method is that the core must be re-built or sculpted for each individual piece and removing the core sometimes resulted in debris trapped on the surface.
A major advance in glass making took place early in the first century when glass blowing was discovered. The glass "gather" at the end of the tube is literally blown out by the glass maker in much the same way that we now blow bubbles out of gum. The resulting bulb could be fashioned by rolling or cut and re-attached to generate quite intricate works. Modern glass blowers still employ essentially the same techniques.
Glass blowing also allowed for the mass production of uniform items. Instead of free blowing the glass, the gather could be placed inside a mold and the glass blown into shape such as bottle or a cup. Modern machinery is now used to blow glass into molds to make a wide variety of glass bottles and jars. The distinctive ridge down the side of a jar is a product of this method as it forms where the two halves of the mold meet.
Glass blowing was also used to generate sheet glass. A large bubble was blown and allowed to sag in on itself, generating a flat sheet of glass. A square sheet could then be cut but the distinctive nipple where the rod was attached remained. As such, the glass was transparent to light but not particularly good as a window for looking out of.
Sheet glass without distortions was made by rolling the blown glass into a cylinder and then cutting off the ends. A slice down the length of the cylinder, and some careful manipulation, results in a large flat sheet. With the invention of better methods for making flat glass, windows really came into their own as architectural features.
Modern sheet glass is made by floating molten glass on a large pool of liquid tin. It produces a much flatter glass with few defects.
In the end, though, all glasses are really just a flux of silica and a few other components melted to generate a hard, brittle, transparent, amorphous solid -- something that we learned to do thousands of years ago. Still, it is difficult to imagine what our lives would be like without it. Just ask someone who wears glasses.
