K9 glass, sometimes referred to as K9 crystal, is a variety of optical borosilicate crown glass. The letter "K" is a reference to the German word for "crown" (Krone) and the number 9 refers to the lead oxide content of the glass (9%). K9 glass has high optical clarity, and is used in many contexts requiring this characteristic. It has been regarded as a difficult-to-cut material because of its superior mechanical properties.^[1] It has an elastic modulus per pascal of 8x10¹⁰, a Poisson's ratio of 0.21, and a density of 2510 kilograms per cubic meter (less dense than most other leaded glass).^[2] Its refractive index is 1.517 at 590 nm wavelength^[3] (compared to 1.523 for crown glass (soda-lime) and 1.618 for 45% leaded glass)^[4] Its clarity (90% transmission at 590 nm^[3]), combined with low cost have made it desirable for chandeliers, lasers, telescopes, etc.

K9 is produced in large quantities by China, which sells it at a price far below higher-quality well-known glass manufacturers such as Swarovski. A similar product to K9 (but without lead) is BK7.^[5]

References