Glass Press

   Glass, when stressed, shows the property of double refraction. When placed between crossed polarizing filters, bands of color may be seen, with the spacing of the bands being correlated to the amount of stress present at a particular point: small spacing indicates a large stress. 

   The stress can be produced by heating a sample and then cooling it rapidly. Such samples are likely to shatter, with the extreme case being Prince Rupert's Drops, made by melting small pieces of glass into a water bath. A safer technique is to start with an annealed piece of glass and stress it mechanically with a glass press.

   The press at the right, from Colby College, is used for long bars of glass. It was made by Duboscq of Paris.

 


   This glass press is in the collection of Hobart and William Smith Colleges in Geneva, New York.

   It was bought about 1925 from Max Kohl of Chemnitz, Germany, and is described as "Fresnel's Press for showing that pressure renders glass doubly refracting."

   The price was 44 Reichsmarks.
   These glass presses are used to stress rectangular glass specimens. They were both made by Duboscq of Paris. 

   The upper example is in the collection of the National Museum of American History of the Smithsonian Institution in Washington, D.C., while the lower one is in the collection of Dartmouth College in Hannover, New Hampshire.

   However, these are hardly museum pieces. I usually do demonstrations of stressed plastic with the aid of an overhead projector and two crossed sheets of polarizing film. One sheet is placed on the top of the projector and the other is held about 10 cm above it on a ring stand. The polarization specimen is then held between the two polarizing filters 
   Here is the pair of Duboscq glass presses at Washington and Lee University in Lexington, Virginia.
 

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