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The History of Glass
 
 
The special human relationship with glass has enjoyed an unbroken history for millennia. Ever since our earliest ancestors figured out how to manipulate objects, enabling them to live and work better, human progress has depended on our relationship with tools, and the various materials that make them. Glass is one of the most versatile and useful materials to be forged by nature and improved by human ingenuity. We've used glass to hunt, cook, construct buildings, and beautify our bodies and homes. From eyeglasses to microscopes to telescopes-and windows of course!-glass has given us a better perspective on the world. Along the way, a few myths have evolved. For example, many people believe that a glass window pane will very gradually grow thicker at the bottom as the glass flows downward. Thanks to cartoons and comedies, others think that a certain pitch of the human voice can break windows. Neither is true. Glass is a complete solid that will flow only when liquefied at temperatures of hundreds of degrees centigrade. An opera singer can warble all she likes, but it takes the sound of a huge explosion to shatter glass. It's time to shatter a few myths about glass, with a brief history of this wonderful substance which is an essential part of daily life.

Arrowheads and Aztec Sacrifices

Glass erupted on the scene millions of years ago: literally. The very first glass known to prehistoric peoples, which was used to craft utensils, weapons and decorative objects, was obsidian, or black glass. Obsidian was formed naturally from volcanic lava which cooled too quickly to revert to stone. It comprised the core elements found in glass today; silica (quartz sand), soda and lime. Archaeologists and anthropologists have found ancient obsidian artifacts all around the world. Our ancestors discovered deposits of shattered obsidian which they crafted into a range of items. In the Western hemisphere this included Apache arrowheads for battle and hunting and implements used by the Aztecs in their human sacrifices.

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 Obsidian, which is formed naturally from lava, was used to create the earliest glass objects - arrowheads and other sharp tools.

Spinning Glass from a Roman Yarn

In his Natural History (77 AD), the Roman historian Pliny the Elder wrote that thousands of years earlier, "A trading ship carrying nitrum (soda) anchored (off the coast of Asia Minor); its crew went ashore to prepare their dinner. Finding no stones on the beach with which to prop their cooking pots over the fire the sailors used lumps of nitrum from their ship to support their cookware. When these became heated, they combined with the sand from the beach to form a strange liquid that flowed in streams; and this, it is said, was the origin of glass."

A nice tale, but not true. Cuneiform tablets containing glass-making recipes indicate that glass was probably first manufactured by the people living in Syria, Babylonia (Iraq) and Mesopotamia (Iran), sometime around 3,000 BC. They likely discovered the process by accident.

Druids and Pharaohs:
The Glass Connection

In its earliest form, glass was made from impure ingredients, resulting in a green tint. It was primarily decorative. A few thousand years would pass before glass possessed the clarity and transparency for window panes. However, as time progressed, several compounds were introduced to create a range of colors mimicking precious and semi-precious stones. Some of these colorants were cobalt, copper, silver, chrome, iron, gold, manganese, nickel, selenium and cadmium. Glass was "core formed" or "rod formed". Core formed objects were made by molding molten glass around a removable core or center, usually a combination of dung and clay mixed with water. In the rod forming technique, beads and other small items were made by manipulating a glob of molten glass on a long rod which was thrust into a kiln until the glass was malleable.

In the third millennium BC, Phoenician trading vessels carried glass and glass-making to Egypt where it was used as decoration by the aristocracy, and as far away as the Celtic cultures of Britain. The Egyptians used glass beads as trading collateral in their dealings with other African peoples, who didn't have access to the secrets of glass-making. The Romans, and other Europeans after them, continued using glass beads and other objects to trade in exchange for African valuables.

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 Mirrors may have originated with the Egyptians, and were referred to in the Bible.

Rome's Windows Weren't Built in a Day

The glass-makers of Ancient Rome produced an early type of flat glass without a greenish tint, or any other color for that matter. It was suitable for windows, but they weren't at all like the windows we have today. They were small, very thick, and opaque rather than transparent. A certain amount of natural light filtered in but you couldn't see through the glass. A notable feature of many Roman windows, and windows for centuries to come, was a bull's-eye pattern in the center. This was a consequence of the method used to manufacture the first flat glass. The glass-maker would blow a large bubble of glass, spinning it round rapidly while the glass was still soft. The result was a glass disc attached to the blow pipe. The blow pipe was removed, and the disc was annealed (the process of cooling glass), then cut into small panes. There was a bull's-eye on the pane where the blowpipe had been attached.

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 Crude flat glass was used by the Romans, ideal for keeping in
warmth in the colder climates.

Shedding Light On the Dark Ages

Despite the murky reputation of the bleak centuries following the decline of the Roman Empire, glass got lighter. During the Dark Ages, better raw materials were used and more efficient furnaces were built. Ash from plants and trees created a superior flux (material that enables glass to melt at a lower temperature). This ash contained large amounts of potassium oxide, rather than sodium oxide which was previously used. Wood fuel for furnaces meant that glassworks were usually situated in forest areas away from towns. Meanwhile, across Asia in the Far East, the Chinese started to make glass objects in the 5th century AD.

The Glass Enlightenment

In 1000 AD, the Egyptian city of Alexandria was considered the preeminent center for glass-making, but northward in Europe, the miraculous art of stained glass was evolving. In the 12th century, gorgeous, multi-colored windows started gracing churches and cathedrals across the continent, with the finest-Chartres and Canterbury cathedrals, for example-produced in the 13th and 14th centuries. Most of the flat glass for religious stained windows was made in France.

The 11th century AD also saw the advent of the first magnifying glass, which was used by medieval monks as they pored over their calligraphic manuscripts. The first eyeglasses further reduced squinting in the late 13th century. In 1291, on the Italian island of Murano near Venice, a transparent glass called cristallo was developed, and crystal glassware soon became popular throughout Europe. From the 1400s to the 1700s, the Venetians dominated ornamental glass production.

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 The first magnifying glass in the 11th century eventually led to development of the microsocope and telescope.

Revolutionary Glass

In England, deforestation was a problem as early as the 15th century. After 1615, glass-makers were required to use coal instead of wood in their furnaces. In the late 17th century, the English discovered that adding lead oxide to the glass process resulted in a substance which was solid, heavy and durable. Also at this time, the French perfected grinding and polishing techniques and produced the first plate glass, but only the rich could afford it.

Then, in the 1700s, there was a political revolution in France and the start of an industrial revolution in England: and a revolution in glass production. Compressed air technology created flatter, better glass panes. Cooling air was blown into a large glass cylinder in controlled amounts. This cylinder was then slit lengthwise. It was reheated and allowed to flatten under its own weight. Large, relatively inexpensive lites (panes) of glass became available and by 1860 flat glass prices had dropped, making glass affordable in all building construction.

In the 1820s, a hand-operated machine ended the age of blowing individual bottles, glasses and flasks. In the 1870s, the first semi-automatic bottle machines appeared. Plate glass production expanded as water power, then steam, and then electricity made grinding and polishing faster and easier. By the 1860s, stores and office buildings were outfitted with plate glass. The latter part of the 19th century saw the center of plate glass production move from France and Belgium to the US. At this time, machinery that rolled glass speeded up the manufacturing process. The glass was pushed through two rollers and emerged as a flat sheet onto a steel table. The glass sheets annealed (cooled) slowly on layers of shelves, then were cut. The first wired glass was made in the 1890s.

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 With the French Revolution came a revolution in glass, with better flat panels possible due to compressed air technology.
 

A Bright New Century of
Glass Technology

Transportation, communication and architecture all benefited from breakthroughs in glass production throughout the 20th century. Machines were developed that produced endless sheets of flat glass for windows (window) glass. New processes strengthened glass by thermal and chemical tempering. Tints were applied to glass to reduce heat transmission and glare, and glass coated with metal oxide films reflected heat or conducted electricity.

But it is probably the car that motivated the most important changes. Before 1919, windshields were made of ordinary plate glass and were therefore highly dangerous when broken. The auto magnate Henry Ford created the new process of glass lamination, and laminated windshields became mandatory. The cellulose has since been replaced with polyvinyl butyral (PVB). In the 1950s, side and rear windows were replaced by tempered glass, which breaks into small pebble-like pieces when broken. Fibre-optics and the first photo-sensitive glass came into being in the 1940s. In 1959, float glass replaced flat glass as a preferred material for residential and commercial windows. The most exciting recent development is Low-E glass, with a low-emission coating that improves the energy efficiency of windows.

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 Mirrored coatings on today's glass office towers offer much more than just looks. They help save energy by keeping the climate outside.

A Window On the Future of Glass

About 75% of the injuries incurred at the 1995 Oklahoma City bombing were caused by flying glass. Glass manufacturers are united in their efforts to research and blast-test advanced, heat-hardened and chemically laminated glass. In Japan, engineers are hard at work on a personal digital assistant that will be a small sheet of glass you can hold in your hand. It is expected that soon new anchor systems, cheaper, thinner laminates, and novel blast-resistant curtain walls will be available on the commercial market. As with our ancestors, glass continues to benefit the quality of human life.