pencil hardness

pencil hardness
   a traditional measure of the hardness of the "leads" (actually made of graphite) in pencils. The hardness scale, from softer to harder, takes the form ..., 3B, 2B, B, HB, F, H, 2H, 3H, 4H, .... The letters stand for Black, Hard, and Firm. (There is no industry standard defining the scale, so there is some variation between manufacturers in how it is applied.) In the U.S., many manufacturers use a numerical scale in which the grades B, HB, F, H, 2H correspond approximately to numbers 1, 2, 2-1/2, 3, and 4, respectively. The pencil hardness scales are not just used for pencils, however. They are used widely to state the durability of paints and other semi-soft coatings. The hardness rating of a coating is the hardness of the hardest pencil that does not penetrate and gouge the coating. This "scratch" hardness scale is analogous to the well-known Mohs hardness scale used in geology to measure the hardness of minerals.

Dictionary of units of measurement. 2015.

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