K9 glass

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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]


  1. ^ Zhang, Chenglong; Cong, Weilong; Feng, Pingfa; Pei, Zhijian J. (2014). "Rotary ultrasonic machining of optical K9 glass using compressed air as coolant: a feasibility study". Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. 228 (4): 504. doi:10.1177/0954405413506195. hdl:2097/17898. S2CID 53380862.
  2. ^ Hong Hua Tan (13 August 2010). Applied Mechanics And Mechanical Engineering. Trans Tech Publications Ltd. p. 49. ISBN 978-3-03813-388-9.
  3. ^ a b "K9(BK7) Yinxing Optical". Retrieved May 29, 2023.
  4. ^ "Refraction (The Physics Hypertextbook)". Retrieved May 29, 2023.
  5. ^ "What Is K9 Crystal?". Reference.com.