K9 glass
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" and the number 9 refers to the lead content of the glass. 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. 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
