The history of Glass

March, 2013

From here to Mesopotamia, the history of glassIMG_0185

The first time I de-glazed a window sash and, in the process, broke the antique glass, I was heart broken.  I try to reuse every piece of material that I take out of my clients’ homes when repairing their windows and send as little as possible to the dump.  But I had just destroyed the most important part of the window.  After all, the wood sash is just a frame to hold the thin, and now I knew just how fragile, transparent material that is called a “super-cooled liquid.”  It is the glass that lets sunlight into your home and yet also keeps out the rain.  In the winter it is the glass that keeps out the snow, most of the cold air, and hopefully, keeps most of the warm heated air inside.

With that simple act of breaking the glass, I realized how curious I was about it; what is glass made of and how; how long has it been around and when was it first used in windows?  So I began to investigate.  This is what I found about the history of glass.

Glass is a simple material, a heated fusion of sand, soda and lime and it has remained almost unchanged for 5,000 years.  The Roman historian, Pliny, wrote in about 50 AD:

“There is a story that once a ship belonging to some traders in nitrum (a naturally occurring soda) put in along the coast of Palestine and that they scattered along the shore to prepare a meal.  Since, however, no stones for supporting their cauldrons were forthcoming, they rested them on lumps of nitrum from their cargo.  When these became heated and were completely mingled with the sand on the beach a strange liquid flowed in streams; and this, it is said, was the origin of glass.”

Archaeological evidence has since shown that whether accidental or not, the development of glass happened much earlier around 2500 BC and further east in Mesopotamia or modern day Iraq.  The earliest uses of glass were simple objects that could be made by pushing molten glass into forms:  beads, tiles, medallions and figurines.  A thousand years later, around 1500 BC the technique of core-forming was developed, which allowed glassmakers to form jugs, bowls, bottles, vases and drinking glasses to hold oil, perfume, wine and other precious liquids.

At about the same time, between the 14th and 12th centuries BC the recipe for glass was written down on clay tablets in Mesopotamia, the best record of its original elements.  Throughout the ancient world glass makers and artisans fired the raw materials to 1100-1500 degrees centigrade until it was molten, the consistency of taffy, then through processes such as core-forming, caning, fusing, slumping, molding, casting, clamping and simple free-blowing, created some of the most beautiful objects known to human kind.  To see a perfume bottle from ancient Rome is a thrill, realizing that in many ways it is the same as art glass today; or to find the equal of Dale Chihuly’s glass sculptures in a museum and read the date “fourth century AD,” is nearly unbelievable.

But what about my simple broken window glass?  How and when did glassmakers discover they could produce the flat transparent material that is everywhere today?  Glass has always been expensive to make so it did not appear in windows until the first century AD in the Roman Empire, and then, in only the richest households.  My peasant ancestors in Western Europe certainly did not have glass perfume bottles or beer steins in the Middle Ages and if their huts had windows at all, they did not have glass.  To keep out the weather, they closed the wood shutters, which also turned out the lights.  In the 1100’s Gothic cathedrals were built with some of the most beautiful stained glass windows in the world.  At that time Venice was the glassmaking center of the western world.  To keep the secrets of its industry safe, all of the artisans were moved onto the island of Murano.  By the Renaissance in the 1500’s, window glass was available throughout Europe and in many middle-class homes.  The hand-blown glass was produced in small panes that were joined together with lead strips to create windows.  Many wealthy families had their coat of arms put in color glass in their windows.

As glass gained popularity it also became a target of taxation.  In Britain in 1696 a window tax severely limited the number of windows people used in their homes, relegating servants to airless, lightless rooms.  In 1746 another glass tax in Britain was based on the weight of the glass, so it was made thin and therefore weak.  Bull’s eye panes also because a feature at this time.  The bull’s eye marked the place on the sheet of glass where the blower’s pontil – the blowing tool – had been attached and were therefore flawed and thus escaped the tax.  Clear window glass was a prized possession.

“One other thing people recorded with care was, somewhat surprisingly, window glass.  Other than in churches and a few wealthy homes, window glass was a rarity well into the 1600’s.  Eleanor Godfrey, in The Development of English Glassmaking, 1560-1640, notes how in 1590 an alderman in Doncaster left his house to his wife but the windows to his son.  The owners of Alnwick Castle from the same period always had their windows taken out and stored when they were away to minimize the risk of breakage.

Even in the largest houses generally only the windows in the most important rooms had glass in them.  All the others were covered with shutters.  Lower down the economic scale, windows remained rare until quite late.  Even glaziers rarely had glass windows in their own homes at the time William Shakespeare was born, in 1564; by the time of his death half a century later, that had changed somewhat, though not completely.  Most middle-class homes had glass in about half the rooms by then.

The first glass factory in America was built in 1608, and glass was carried in the first cargo exported to England. Although other glasshouses were operated in the colonies, especially in New Amsterdam, the first successful and enduring large-scale glasshouse was set up in New Jersey in 1739.

But the manufacture of glass, among many other things, started to change in the early 19th century with the advent of the Industrial Revolution.  The French had invented plate glass in 1688, so called because the molten glass was spread across tables known as plates and made principally for use in mirrors.  This allowed the creation of large panes of glass, which made shop windows possible.  However plate glass had to be cooled for ten days after being rolled out and required a lot of grinding and polishing.  When Friedrich Siemens invented the tank furnace in 1838, sheet glass could be produced in far greater quantities than molten glass.  Although it did not have the strength of plate glass, sheet glass cooled faster and needed less polishing, therefore could be made more cheaply and in large quantities. Both British glass taxes were abolished by 1851 and suddenly the cost of glass in England was cut in half.

The invention of a glass-pressing machine (c.1827), used by the American manufacturer Deming Jarves in his Boston and Sandwich Glass Company (1825-88), permitted the manufacturing of inexpensive and mass-produced glass articles.

Glass as we know it today was finally available in the early 20th century when a Belgian named Fourcault managed to vertically draw a continuous sheet of glass of a consistent width from the tank furnace and large-scale commercial production of sheet glass got under way by 1914.

Remember that first piece of antique glass I broke?  I recycled it by cutting it to fit smaller panes in other window sash, so it lives on and I am my heart is healed.

Finally, an explanation of the types of window glass we use today:

BLOWN glass is often called antique or wavy glass because it has waves and bubbles and other imperfections as a result of the manufacturing process.  It is found in many historic buildings and is highly prized.  To make it, glass bulbs are blown, just as for vessels, but instead of becoming gigantic vases, the hot glass is cut and laid on tables (also called plates above) to cool, then cut to window size.  Replacement blown glass can be salvaged from older homes, and re-cut.  My broken piece of blown glass was re-cut to fit a multi-pane sash on another job.  There are also manufacturers of new blown glass, but it is very expensive and not quite the same.

FLOAT glass is the most common window glass today. The float process developed after the Second World War by Britain’s Pilkington Brothers Ltd., and introduced in 1959, combined the brilliant finish of sheet glass with the optical qualities of plate glass. Molten glass, when poured across the surface of a bath of molten tin, spreads and flattens before being drawn horizontally in a continuous ribbon into the annealing lehr. Float glass comes in different thicknesses and we either single (1/16”) or double (1/8t”) strength, depending upon the size of the sash.

SAFETY or TEMPERED glass is heat-strengthened, four times stronger than float glass.  It is required in windows or doors within 18” of the floor.

LAMINATED glass is two pieces of glass permanently bonded together with an invisible plastic interlayer.  The resulting piece is 7/32” thick.  If broken, the glass fragments adhere to the plastic.  Laminated glass stops 99% of UV rays, protecting interior furnishings.  It also reduces heat loss and helps reduce noise infiltration.  The process was invented and developed by the French scientist Edouard Benedictus, who patented his new safety glass under the name “Triplex” in 1910.

INSULATING glass is also called “double glazing” or “thermal glass.”  It is 2 or 3 layers of glass with an air space between.  It has a spacer all around the perimeter, which unfortunately conducts heat and cold.  As a result it is only effective in reducing heat loss if the piece is larger than 24” x 24.” Low-emittance (Low-E) coating can be added to multilayer or insulating glass to further reduce heat loss, thereby improving the energy savings of windows. It has a minimum thickness of 3/8.”