How to measure hardness.
The Mohs scale of hardness was devised in 1812 by German mineralogist Friedrich Mohs (1773-1839). The Mohs scale places materials on a scale from soft to hard by whether they can or cannot scratch each other. The idea of measuring hardness of materials by how scratch resistant they are in comparison to other materials was mentioned by Theophrastus already in about 300BC. The Mohs scale is still used today as a way of rating the hardness of materials.
The Mohs scale consists of ten standard minerals, the higher the number, the harder the mineral.
When Mohs devised the scale, diamonds were the hardest known naturally occurring material and therefore measure 10 on the scale. A hardened steel file measure about 7 on the Mohs scale. Window glass measure about 6. Human teeth measure about 5 and finger nails between 2 and 3. Gold and silver also end up between 2 and 3. The graphite in a pencil measure about 1 on the Mohs scale.
The Mohs scale is strictly relative. For example is diamond, which is hardness 10 on the scale, four times harder than corundum, which is hardness 9, and six times harder than topaz, which is hardness 8. The Mohs scale uses half numbers for in-between hardnesses.
Friedrich Mohs was born in Gernrode in Germany. He studied chemistry, mathematics and physics at the University of Halle. He also studied at the Mining Academy in Freiberg. In 1812 he became professor in Graz, in 1818 professor in Freiberg and in 1826 professor in Vienna.
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