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From: warinner@typhoon.xnet.com (Robert Warinner)
Newsgroups: alt.folklore.urban
Subject: Re: Glass Flows!
Date: 7 Apr 1997 15:24:22 GMT

Michael Craft (jsi@idiom.com) wrote:
: Were the glass making techniques of the 1920's really that
: primitive as to not be able to produce clear window glass
: and were the contractors who installed the glass were too
: careless whether some of their windows flowed and some were
: clear?

First point: how do you know that the clear glass and the wavy glass are the same age?

Second point: yes, the glass making techniques used in the 1920's produced wavy glass. Chances are that glass mannufactured in the 1920's was manufactured by rolling which left the waves you now see. There were basically five methods of making plate (window) glass:

• By casting: molten glass is poured into a mold and later polished. The glass produced is often not very transparent and contains many defects. This method was used by the Romans and survived well into the nineteenth century.
• By the "crown glass" method: molten glass is blown into a large sphere. The sphere is spun and a small hole is opened in the bottom of the sphere. The sphere opens into a disk of relatively uniform thickness.
• By the "cylinder" or "broad" method: molten glass was blown into a cylinder which was cut open to produce a glass plate.
• By mechanical rolling: molten glass was pressed between rollers much as sheet metal is produced. Some imperfections were still present in the glass.
• By the "float glass" method: molten glass is poured from the furnace onto a vat of molten metal. This allows the glass to distribute itself evenly resulting in perfectly flat glass. The glass is free of the defects produced by the previous mechanical processes. The float glass method was invented 1959 by Alistair Pilkington of the Pilkington Glass Company.

Here's a short time line showing when these methods were used:

4000 BC Glass manufactured in Near East.
1668 Broad glass method invented in St. Gobain, France.
1870 Rolled glass produced by Chance Brothers
1898 Wired glass first made
1910 Mechanical cylinder drawing machine
1913 Sheet glass drawn mechanically by Fourcault machine straight from the furnace
1918 Bicheroux process for casting, grinding and polishing of plate glass
1937 Pilkington twin grinding and polishing of plate glass
1959 Float glass method invented by Pilkington Bros.

My house, which was built in 1929, has wavy glass. Some of the waves are horizontal, some are vertical, and some are diagonal. This is just an artifact of their manufacture, not due to "flow".

References:

http://www.pfg.co.za/fltline.htm - Float glass manufacturing process.
http://www.pfg.co.za/history.htm - History of glass.
http://www.britglass.co.uk/news/mglass/making1.html - History of glass.
http://www.britglass.co.uk/news/mglass/making5.html - History of glass.
http://www.trp.dundee.ac.uk/research/glossary/glass.html - Glass glossary.

Andrew "MEGO" Warinner