flint glass

Heavy crystal glass, containing lead, highly suitable for cutting and engraving. It was first introduced c.1675, and so called because early examples were made with powdered flint instead of the more usual sand. It has a higher refractive index than crown glass (without lead), and thus played an important part in making corrected compound lenses.

Flint glass is an optical glass that has relatively high refractive index and low Abbe number. A concave lens of flint glass is commonly combined with a convex lens of crown glass to produce an achromatic doublet lens because of their compensating optical properties.

With respect to glass, the term "flint" derives from the flint nodules found in the chalk deposits of southeast England that were used as a source of high purity silica by George Ravenscroft, circa 1662, to produce a potash lead glass that was the predecessor to English lead crystal.

Traditionally, flint glasses contain around 4%—60% lead oxide, however the manufacture and disposal of these glasses are sources of pollution.