“All New” Glass Fun Facts: Part 2

The WGS Blog returns to provide More.  Glass.  Trivia!

As far as the early history of glass making in Britain goes, the Romans brought the technology with them. This led to the European-wide spread of glass manufacture. British history records glass “Manufacture” dating back to the 13th century when “Broad Sheet” glass can be located to the areas around Sussex and Surrey. In the meantime the Venetians had thrived as glassmakers, as their glass became popular due to its brilliance and creative form. By 1330 the French had also developed “Crown Glass”. This took until the 17th century to be produced in England, in London.

 In England in 1676 George Ravenscroft invented “Lead crystal” by introducing lead oxide to the glass which took on a more brilliant appearance.

 The 17th century brought a new glass process from France, “Plate glass”, a term still used today. This was a process of pouring molten glass onto a table and then rolled. Once cold, the glass was ground under large grinding disks until optically smooth, making it perfect for mirrors. The French had legislated heavy duties on imported glass products which made it impossible for the Venetians to Export, and also offered generous incentives to any Venetian willing to work for them. By the 18th century this technology was being used in England at Ravenhead, producing the first English Polished Plate.

Crystal-Palace-general-view1

The Crystal Palace was a cast-iron and plate-glass building originally erected in Hyde Park, London, England, to house the Great Exhibition of 1851

1834, Robert Lucas Chance introduced “Improved Cylinder Sheet” glass which was produced using a process invented in Germany. This produced even finer and larger glasses. This was the glass used to glaze the “Crystal Palace” in London. Until a change in legislation in 1845 when the “Excise Act” was repealed, glass manufacture was under developed in Britain. Once the heavy tax burdens previously placed on glass manufacture were removed, production grew.

By the end of the 19th century glass bottles were being made by machine, increasing production threefold. The now “Chance Bros.” invented “Machine Rolled Patterned Glass”. By the start of the 20th century, “Owens Glass” in America had further developed bottle manufacture which increased output by 10-fold to some 2,500 bottles per hour.

bottle.make

By 1910 the first “Laminates” had been produced by Edouard Benedictus, a Frenchman, who named the process “TripleX.” 1914 saw the start of producing glass by the “Drawn” method. Invented by a Belgian man named Fourcault, glass was drawn vertically from a tank. A further development by Richeroux, another Belgian, was to pour the molted glass from a pot between 2 rollers to give a more even thickness and evenness for polishing.

In 1917 “Sheet Glass” was invented by Colburn in America and developed by “Libby-Owens”, a partnership of Michael Owens of Owens glass and his backer E. D. L. Libby. Further improvements were made by “Pittsburg Plate Glass” or PPG.

By 1923 came the first UK production of continuous polished plate glass using the single grinding system. Closely followed in 1938 by the twin grinding system. And then the float process was launched in the marketplace; invented by Pilkington Bros, and introduced in 1959. The significance of this process is that it produced glass with a brilliant finish and without the need to grind or polish the surface, making mass-produced glass with the qualities of polished glass. This was achieved by floating the molten glass on a bath of molten tin, creating a “glass ribbon”, even in width and thickness. This is still the process used today for the production of what is now termed “Float Glass”. 

Glass is commonly used for windows, bottles, modern hard drives and eyewear, and examples of glassy materials include soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, and aluminium oxynitride.

Much of the functional architectural glass – like that used in shower doors, table tops, car windows, skylights, etc. – goes through a process called tempering.  Glass is pretty wonderful stuff, but it does have some bad habits.  First, it is brittle and has a tendency to crack when struck or heated unevenly.  Second, shards of glass are really sharp and pretty dangerous.  Tempered glass solves both of these problems simultaneously.  Glass is much stronger in compression than tension.tempered.glass

Tempered Glass Process

If you can cause the surface of the glass to become compressed relative to the interior of it, you can harden it by a factor of up to 10.  There are a couple of ways to do this.  One is to heat the glass and then cool it very quickly.  The surface of the glass will cool much more rapidly than the interior.  The slow cooling of the interior causes it to want to contract more than the surface, placing the surface under considerable compression.   This strengthens the glass and makes it more scratch-resistant and heat-resistant in the bargain.  Another method is chemical tempering, in which sodium atoms on the surface of the glass are replaced with potassium atoms, which are significantly larger.  This also puts the surface in compression, and can be done with glass of complicated shapes that would not survive heat tempering.

One interesting effect of the tempering process is that tempered glass doesn’t just crack.  When tempered glass encounters a big enough stress, it shatters into small granules.  If the integrity of the surface of the glass becomes compromised, the interior, which is under huge tension, will disintegrate.  This is much safer than big dangerous shards, but does make the glass suddenly an awful lot harder to see through.  This is one reason why the windshield of your car is not made with tempered glass, but laminated glass.  Laminated glass is made by bonding two or more layers of glass with an ‘interlayer’ of plastic film which will hold the pieces together if the glass should crack.

willis-tower-ledge-shatter

Chicago’s Willis (aka Sears) Tower 103rd floor Skydeck had one of its famous glass ledges shatter an interlayer May 2014

Tempered glass is an extremely useful material, but it does demand some planning.  Because of tempered glass’ all-or-nothing breakage, it must have been already cut to the size, shape, and already have any holes cut out before the tempering process.  There’s no cutting the glass down to fit afterwards. replace.sears.willis.tower.glass.ledge

Tempering as an industrial process started in the 20th century, but it was a party trick far before that.  One of the first examples of tempered glass is something called Prince Rupert’s drops (or balls) , named after the Bavarian prince who brought it to the attention of the court.  If you let a blob of molten glass drip into a bucket of water, it will form an extended teardrop shape with interesting properties.  The bulbous end of the drop is tempered and can withstand extreme force, such as hitting it with a hammer.  The tail, however, is very delicate, and if broken, the whole thing will shatter into tiny pieces. 

When you think about it the stuff is a bit odd, but that’s glass for you.  It’s odd stuff.

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One thought on ““All New” Glass Fun Facts: Part 2

  1. Pingback: “All New” Glass Fun Facts: Part 1 | Washington Glass Studio

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