The pencil hardness test, also referred to as the Wolff-Wilborn test, uses the varying hardness values of graphite pencils to evaluate a coating’s hardness.
The pencil hardness test is perhaps the simplest form of hardness test. Pencils are pushed into the sample and the coating hardness is identified by the trace generated.
There are two scales to grade the hardness of a pencil's graphite core. The first is a numerical scale; the higher the number the harder the marking core. As the core becomes softer in the lower numbers it leaves more graphite on the material and a darker mark. The second scale is the HB graphite scale; the "H" represents the hardness while the "B" indicates blackness.