We are all familiar with a No. 2 pencil for taking those ‘fill in the circle’ tests. The’ No. 2′ is a measure of the hardness of the graphite (and subsequently how dark the writing is). There are many different hardness scales and each are used for different materials and for different purposes. Some of the more popular scales used in the surface finishing industry include the following:
MOHS Scale – Used for non-metallic materials (minerals) and is a measure of the scratch resistance by another material. On this scale diamond is 10 (hardest material) while glass is 5.5 and a fingernail is about 2.5.
Rockwell Hardness – There are a variety of Rockwell scales (all indicated by a letter). The value is determined by the amount of indentation by a penetrator cone into the surface. The B scale (HRB) and the C scale (HRC) are most common. Hardened steel is about a HRC 60 which is very roughly equivalent to 5.5 MOHS.
Shore Hardness – Measures the elasticity of a material. A diamond hammer is used to strike a surface and the ‘rebound’ of the hammer is measured. The harder the material, the higher the rebound.
There are many other hardness scales and each has it’s own test methods and parameters. The best way to determine the true measure of hardness is to compare results for different materials across multiple scales.