Glass for Dummies
By Frank Fulton
By Frank Fulton
That being the case, there is also a good chance that at some point in
your glass industry career one of your friends, family, customers, or
colleagues will ask you a question about glass.
If you’re reading this, you are probably involved in the glass business.
That being the case, there is also a good chance that at some point in your glass industry career one of your friends, family, customers, or colleagues will ask you a question about glass. The objective today is to make you look like a genius when asked some basic questions by somebody who will never know if you’re making up the answer or giving them the straight, informed goods. You may also find this useful for boring people you meet for the first time at a party.
What every good Glassman (or woman) ought to know . . .
Glass is roughly composed of 70 per cent silica sand, 13 per cent lime, and 12 per cent soda. The remaining five per cent is made up of about 50 other chemical elements to affect colour, viscosity, or durability. When heated to about 1,500 degrees C this mixture turns into a thick molten mass like molasses on a cold day.
It is a very unique material in that its physical structure does not conform to the liquid, solid, or gas. It is actually more of a liquid than a solid, and The American Society for Testing and Materials (ASTM) in its definition of glass states that it is “an unusual material which has the random atomic arrangement of liquid but which somehow has been frozen in place so that it is a solid and permanent substance”. Other definitions refer to glass as an “amorphous solid.”
Like many great inventions glass was first produced by accident in about 5000 BC, when a crew of Phoenician sailors transporting blocks of soda landed ashore on a beach near Belus in Asia Minor. When it came time to eat, they couldn’t find any rocks to put their cooking pots on, so they improvised and used blocks of soda to support the pots over the fire. Once the fire got blazing, the sand and soda created a pool of molten glass.
Around 1500 BC Egyptians discovered how to produce goblets and small bottles by repeatedly dipping a silica paste core on the end of a metal rod into molten glass, then later removing the core. At some point around 250 BC, Babylonians began using hollow metal rods for this procedure and stumbled upon glass blowing. As a result, producing bowls, bottles, and cups became relatively much easier and less expensive, and the Roman’s demand for glass products as status symbols skyrocketed. Demand throughout the Roman Empire in many ways led to the birth and growth of the glass industry. There is even evidence that crude cast glass was tried in some important Roman buildings and villas, but mostly the rich used thin, translucent sheets of alabaster to enclose wall openings. Unfortunately, with the demise of the Roman Empire, the fledgling glass industry stagnated for almost a thousand years and almost disappeared.
The 11th century saw the birth of the flat glass industry. German glass craftsmen are reported to have developed a technique, further developed by Venetians in the 13th century followed by the French, where molten glass was blown into an elongated balloon shape. The ends of the balloon were cut off, the remaining cylindrically shaped glass cut in half lengthwise with shears, then flattened on a hot iron plate. This produced what was known as “broad sheet” glass. The quality wasn’t all that good and later on they began to produce “blown plate” glass, which was essentially hand polished broad sheet glass. It was a bit better and easier to see through but labour-intensive and expensive.
For the next 600 years, most flat glass was produced from flattening out blown glass in one manner or another. “Crown glass” was first produced in France around AD 1320 by blowing a globe of molten glass and spinning it on a table using centrifugal force to create a glass disc about 1.6 metres in diameter. Small pieces were cut from the disc and soldered together with lead strips to create window glass. “Cylinder glass” was produced by blowing a cylindrical shape hanging vertically and could create a “pod” measuring up to three metres long and 45 centimetres wide, that was then cut and flattened in a similar manner to broad sheet glass. Both methods required laborious hand grinding and polishing until the industrial revolution brought about steam-powered machines to improve on these functions.
It’s interesting to note that Pilkington Glass originated in 1826 as the St. Helens Crown Glass Company.
As the production of glass lites became possible, some of the rich began to replace the shutters on the “wind-eyes” in their homes with windows made of glass.
In 1688 the crafty Frenchman, Louis Lucas de Nehou perfected a technique for the production of “cast polished plate” glass by pouring molten glass onto a large heated iron casting table and rolling the material into sheets up to three metres by 1.8 metres. The product was then ground and polished. You can just imagine how the architects ate this up even though it was very expensive. Finally, large vision areas were possible instead of a bunch of small pieces of glass held together with lead.
By 1773, the British Cast Plate Glass Company was producing cast polished plate. PPG started production in the U.S. in 1883.
By the mid 1800s the use of glass to enclose wall openings became commonplace.
The first drawn glass, as opposed to blown or cast, was produced in the U.S. in 1903, made possible by the technical advances in power and machinery available at the time. “Machine drawn cylinder” sheet glass was similar in nature to its blown predecessor, which used gravity but the cylinder was mechanically drawn vertically in sizes up to 12 metres long that were annealed, cut, then reheated and flattened into large sheets.
A huge advance was developed in Belgium by Emile Fourcault, and the commercial production of vertically “drawn flat sheet” glass began in 1914. For the first time it was possible to create a continuous ribbon of reasonably good quality and economical glass by mechanically drawing it from a tank of molten glass through a ceramic die, known as a Debiteuse, followed by a controlled cooling process. The ribbon was then cut to length at the top of the cooling shaft. Sheet glass could be produced from one to six millimetres thick depending on the speed the glass was drawn from the tank. The downside to this process was that difficulties in controlling the initial cooling of the molten glass would result in visual waves in the glass.
The most significant strides in the manufacture of glass were made by Alastair Pilkington in the 1950s when he conceived the idea of forming a horizontal ribbon of glass by floating the melted raw materials at high temperature over a bath of molten tin. Interestingly, Sir Alastair was not even related to the founders of the company of the same surname. The float glass process superseded both the plate and sheet glass processes to become the universal process for the manufacture of the high-quality flat glass we are accustomed to today.
With this modicum of glass knowledge you should now be capable of impressing your friends and maybe even the occasional architect.
*Frank Fulton is president of Fultech Fenestration Consulting, offering tech-nical and improvement project assistance to the glass and metal industry. You can reach him at firstname.lastname@example.org.