crown glass

crown glass

noun
1.
an optical glass of low dispersion and generally low index of refraction.
2.
an old form of window glass formed by blowing a globe and whirling it into a disk.

Origin:
1700–10

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
crown glass
 
n
1.  another name for optical crown
2.  an old form of window glass made by blowing a globe and spinning it until it formed a flat disc

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Encyclopedia Britannica
Encyclopedia

crown glass

handmade glass of soda-lime composition for domestic glazing or optical uses. The technique of crown glass remained standard from the earliest times: a bubble of glass, blown into a pear shape and flattened, was transferred to the glassmaker's pontil (a solid iron rod), reheated and rotated at speed, until centrifugal force formed a large circular plate of up to 60 inches in diameter. The finished "table" of glass was thin, lustrous, highly polished (by "fire-polish"), and had concentric ripple lines, the result of spinning; crown glass was slightly convex, and in the centre of the crown was the bull's eye, a thickened part where the pontil was attached. This was often cut out as a defect, but later it came to be prized as evidence of antiquity. Nevertheless, and despite the availability of cheaper cylinder glass (cast and rolled glass had been invented in the 17th century), crown glass was particularly popular for its superior quality and clarity. The crown process, which may have been Syrian in origin, was in use in Europe since at least the 14th century, when the industry was centred in Normandy, where a few families of glassblowers monopolized the trade and enjoyed a kind of aristocratic status. From about the mid-17th century the crown glass process was gradually replaced by easier methods of manufacturing larger glass sheets. Window glass of note, however, was made by this method in the U.S. by the Boston Crown Glass Company from 1793 to about 1827.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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