flint glass

Optics.

an optical glass of high dispersion and a relatively high index of refraction, composed of alkalis, lead oxide, and silica, with or without other bases, sometimes used as the diverging lens component of an achromatic lens.

[1665-75]

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▪ glass

also called  Crystal, or Lead Crystal,  

      heavy and durable glass characterized by its brilliance, clarity, and highly refractive quality. Developed by George Ravenscroft (Ravenscroft, George) (q.v.) in 1675, it ushered in a new style in glassmaking and eventually made England the leading glass producer of the world. Ravenscroft's experimentation was supported by the Worshipful Company of Glass Sellers, a body of English retailers long dissatisfied with the quality of glass from Isola (island) di Murano, Venice.

      The first clear crystal Ravenscroft produced, called flint glass because calcined flint was used as a base, decayed after a period of time. This fault was overcome by adding lead oxide to produce lead crystal. (“Flint glass” thus became a synonymous term for lead crystal, though flint is no longer part of its composition.) The Glass Sellers furnished the designs and set exacting standards for the product. Their earliest examples bear the seal of a raven's head. In time, the adaptation of European traditional styling to the new glass produced a genuine native English style of glassware.

      In the optical glass industry, flint glass is any highly refractive lead-containing glass used to make lenses (lens) and prisms (prism). Because it absorbs most ultraviolet light but comparatively little visible light, it is also used for telescope lenses. The light-dispersive power of flint glass can be made twice as high as that of crown glass (of conventional soda-lime composition), and the two complementary types of glass are cemented together to make lenses corrected for chromatic aberration. In the container glass industry, flint glass is any clear glass free of colouring.

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